Effects of low oxygen during chorioallantoic membrane development on post-hatch growing performance of broiler chickens

Effects of low oxygen during chorioallantoic membrane development on post-hatch growing... ABSTRACT The prenatal circulatory system is adaptive and capable of plasticity designed for the needs of the growing tissue. When a broiler embryo is faced with hypoxic stress, the process of angiogenesis in tissues begins. Exposure to hypoxic conditions of 17% oxygen during the chorioallantoic membrane (CAM) development (E5 to E12) affected the circulatory system and contributed to an increase in the blood oxygen carrying capacity. The present study aimed to evaluate the effects of hypoxic exposure during CAM development on post-hatch performance of broilers and to examine whether hypoxic exposure improved sustainability of birds exposed to acute heat stress. Two consecutive trials, with male broilers from each of the incubation treatments—optimal conditions and exposure to hypoxia of 15 or 17% oxygen, for 12 h/day, during CAM development—were conducted. In experiment 1, 60 male chicks from each group were raised in individual cages. In experiment 2, 160 male chicks from each group were raised in 40-chick pens until marketing. On d 35, 20 birds from each group were transferred to individual cages kept at a temperature of 23°C for 72 h, and then birds were exposed to 35°C for 5 hours. Body temperatures were measured at 0, 2, and 5 h of the heat exposure. In both experiments BW, feed intake, and FCR were recorded. At marketing, chicks were slaughtered, and relative weights of breast muscle, abdominal fat pad, heart, and liver were calculated. Hypoxia treatment resulted in a FCR advantage. Food intake was similar in all treatments, but groups exposed to hypoxia grew better than controls until the age of 35 days. Hypoxia-treated groups had higher relative breast, heart, and liver weights than controls. Body temperatures of hypoxia-treated chickens remained lower during heat stress exposure, and their mortality rate was lower as well. Intermittent exposure to moderate hypoxia during CAM development confers advantages to broilers in feed utilization efficiency and in coping with heat stress. It may be considered as a mitigating step in incubation to facilitate broilers in achieving their full growth potential. INTRODUCTION Fast-growing broilers have a high oxygen demand for intensive growth rate. As long as the optimal environmental conditions enable consistent growth of energy supply, the rapid growth potential provided by the chickens’ genetics can be realized. However, while the optimum conditions improve manufacturing efficiency, sub-optimal environmental conditions induce higher oxygen demand, which is beyond the capacity of the cardiovascular system (Druyan et al., 2008; Druyan et al., 2009). Inability to efficiently supply oxygen demand has a deleterious effect on growth rate (Jonker et al., 2015). Industrial farming that has been consistently providing optimal growth conditions for generations has affected a selection process that actually reduced the ability of the birds to perform under sub-optimal conditions and to maintain a dynamic balance between the different body systems (Havenstein et al., 2003). As a result, growth rates are impaired, the genetic potential of poultry for rapid growth is not exhausted, and growth and food utilization are compromised. In order to reduce the extent of the financial loss involved in prolonging the growth period, various approaches to provide optimal growth conditions have been adopted. These include temperature control using expensive technologies. However, as the cultivation outlasts genetic growth, the physiological oxygen utilization capacity of the chickens may not suffice for their metabolic demands (Druyan et al., 2012). Taken together, under current management conditions, the full genetic potential cannot be exploited, and it is thus necessary to invest in temperature control, adequate nutrition, density, and environmental conditions to take advantage of the full genetic potential. The prenatal circulatory system is adaptive and capable of plasticity designed to meet the needs of the growing tissue. When the embryo is faced with hypoxic stress, the process of angiogenesis in tissues begins (Druyan and Levi, 2012). In our laboratory, when embryos were exposed to hypoxic conditions of 17% oxygen during the period of the development of the chorioallantoic membrane (CAM) (from embryonic d E5 to E12), an exposure effect was shown on the circulatory system: The CAM grew, the number of blood vessels increased, and hematocrit (red blood cell content) and hemoglobin levels rose. These changes contributed to an increase in the blood-oxygen-carrying capacity of those embryos and raised the level of body metabolism expressed as yolk sac higher consumption compared to the control (Druyan and Levi, 2012; Druyan et al., 2012). Next, we sought to understand the effect of exposure to hypoxic conditions and adaptation of circulatory system variables related to the transportation of oxygen on post-hatch performance. Developmental changes induced by environmental or incubation conditions may play a role in post-hatch performance, affecting growth and metabolism (Decuypere, 2002). It has been shown that incubation under chronic hypoxic conditions (as exist at 2,000 m above sea level) led to initial growth depression resulting in a significantly lower body weight (BW) up to 14 d post hatch. However, at later stages, no significant effect on BW was found (Hassanzadeh et al., 2004). Piestun et al. (2008a,b, 2011) have demonstrated that thermal manipulation during the development and maturation of the hypothalamus-hypophysis-thyroid axis had a long-lasting effect on post-hatching thermos tolerance of broilers. This effect persisted until an early marketing age of 35 d, exhibiting no effect of thermal manipulation (TM) on BW of male birds and a negative effect of TM on female BW (Piestun et al., 2013). At 44 d, birds that were incubated at high temperatures weighed almost 100 g less than birds incubated under standard temperature (Hulet et al., 2007). The objectives of the current study were: 1) To evaluate the effects of hypoxia exposure at levels of 15 or 17% during CAM development on the growth rate and feed conversion of broilers up to the age of poultry marketing under standard growth conditions, and 2) to examine whether hypoxia exposure confers a performance advantage to birds under an acute heat stress. MATERIALS AND METHODS Experimental Design All the procedures in this study were carried out in accordance with the accepted ethical and welfare standards of the Israel Ethics Committee (IL-581/15). Experiment 1 Nine hundred fertilized Cobb strain broiler chicken (Gallus domesticus) eggs with an average weight of 64.0 ± 2.5 g were obtained from a breeder flock of hens during their optimal period of egg production (35 wk old). All eggs were numbered and weighed individually prior to incubation. They were incubated in a Danki Medium-Size Incubator for 2,500 eggs (Danki ApS, Ikast, Denmark) under standard incubation conditions of 37.8°C and 56% RH, and were turned once per hour. At E5, eggs were randomly divided into 3 groups of 300 eggs: 1 – Control; 2 – 17% O2—in which the eggs were exposed to hypoxia—17% oxygen during the 8 d of CAM development (E5-E12); and 3 – 15% O2—in which the eggs were exposed to hypoxia—15% oxygen during the 8 d of CAM development (E5-E12). Exposure to hypoxic conditions was accomplished by transferring the eggs of the 2 treatment groups from the control incubator into a medium-sized incubator (Danki ApS, Ikast, Denmark) equipped with a Model 2BGA-SP-MA O2 and CO2 Control System (Emproco Ltd, Ashkelon, Israel) for 12 hours. The 2BGA-SP-MA control system includes an infrared CO2 detector with a sensitivity range of 0 to 5 ± 0.01%, and an electrochemical cell for oxygen measurement that covered 0 to 25 ± 0.1%. In response to the CO2 sensor readings, an electronically controlled pump infused ambient air into the incubator to maintain CO2 concentration at the standard level of 0.03%. The O2 sensor activated an electronically controlled pump that infused N2 into the incubator to maintain the desired oxygen concentrations of 17 ± 0.2% or 15± 0.2%. After hatching, 60 male chicks were chosen from each group. Each chick was individually tagged and weighed; the chicks from each group were divided into groups, each comprised of 10 chicks and raised together in battery cages until the age of 14 d (6 × 10 battery cages for each incubation treatment). On d 14, chicks were transferred to individual cages with individual feeders where each individual chick was housed on its own (60 individual cages for each incubation treatment). Water and feed in mash form were available for ad libitum consumption. The diet was designed to meet or exceed NRC (1994) recommendations. The birds were held under the recommended temperature regime (starting from 34°C on d of hatch to 24°C from d 21 onwards) with 55% Rh and 20:4 h of light. Each chick was weighed every wk, and its individual weekly food intake was calculated. At the end of the experiment, at the age of 40 d, the chickens were individually weighed, and the feed was removed for 12 h before slaughter. Breast muscle, abdominal fat pad, heart, and liver were removed and weighed and their weights calculated relative to their live BW. Experiment 2 Part a: Twelve hundred fertilized Cobb eggs were incubated until E5 and randomly divided into 3 groups of 400 eggs each as described above, followed by hypoxia and control treatments as in experiment 1. One-hundred-sixty male hatched chicks with a similar average weight from each of the 3 treatment groups were chosen; thus, the entire experiment included 480 chicks. At the end of the hatching process, chicks from each treatment group were distributed into 4 equal growing pens, 40 chicks per pen (total 12 pens) at a density of approximately 12 chicks per m2. Each chick was tagged and weighed individually. Chicks were raised according to Cobb recommendations for environmental conditions and grown until the age of 39 days. The location of each growing pen was randomly assigned within the chicken house. Water and feed in mash form were available for ad libitum consumption. The diet was designed to meet or exceed NRC (1994) recommendations. At weekly intervals, BW was individually recorded, feed consumption was measured by pen, and unadjusted FCR (kg of feed consumed/kg of live BW) was calculated for each pen. At 39 d of age, chickens were individually weighed, and the feed was removed for 12 h prior to slaughter. Breast muscle, abdominal fat pad, heart, and liver were removed and weighed and their relative weights calculated based on live BW. Part b: At d 35 after weighing, 5 chickens from each yard were selected (chickens had an average representative weight of the population). A total 20 birds from each treatment group was transferred (3 × 20 = 60 birds) to individual avian cages at room temperature of 23°C for 72 h for adaptation. At the end of the adjustment period, chickens were exposed to acute heat stress that was induced within a 30 min time period from 23 to 35°C. Following 5 h of heat stress, the temperature declined back to 23°C within 30 minutes. Each bird was individually weighed at the beginning and end of the heat exposure, and their body temperatures were measured at the beginning, after 2 h, and at the end of the heat exposure. Measurement of body temperature was done with a digital thermometer (Super Speed Digital Thermometer, Procare Measure Technology Co. Ltd., San Chung City, Taipei 241, Taiwan, accurate to ±0.1°C). Statistics All data were processed statistically using one-way Analysis of Variance (ANOVA). Values that differed (at a level of P < 0.05) were considered statistically significant. In addition, the Tukey–Kramer test was conducted, comparing the averages of the treatments at the exposure point. Mortality data were subjected to chi squared analysis. RESULTS Experiment 1 The growth of the hatchlings up to age of 35 d clearly indicates that exposure to 17% O2 for 12 h/d during the CAM development did not negatively affected broilers’ BW post hatch. The 17% O2 treated chicks were found to be significantly heavier than control chicks on d 7, d 14, and d 28 (1652±21.8 g vs. 1567±21.4 on d 28 for the 17% chicks compared to the control chicks, respectively). On the other hand, no significant differences in BW were found by the end of the experiment (Table 1). Table 1. Effect of daily exposure to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on body weight (g) and weight gain (g/day) of broilers (males) from hatching to marketing age.     Experiment 1  Experiment 2  Variable  Age (d)  Control  17% O2  15% O2  Control  17% O2  15% O2  BW (g)  0  48.5±0.39  48.2±0.40  48.7±0.40  48.5±0.40  48.2±0.40  48.7±0.40    7  180.3±2.22b  187.2±2.26a  178.8±2.25b  164.4±2.86a  163.2±2.87a  153.8±2.86b    14  503.6±6.67b  523.5±6.79a  505.9±6.80b  486.3±3.36a  481.7±3.38a  468.0±3.34b    21  929.0±15.02  965.5±15.45  961.8±15.30  1034.8±6.08a  1015.0±6.10a,b  992.9±6.04b    28  1567.3±21.43b  1653.0±22.25a  1642.3±21.83a  1712.5±9.77a  1705.0±9.80a  1666.3±9.70b    35  2267.1±27.60  2336.3±28.40  2337.0±27.86  2493.6±13.66a  2503.6±13.66a  2446.5±13.48b  DWG (g/d)  0 to 7  18.83±0.30b  19.86±0.31a  18.55±0.31b  17.05±0.41a  16.90±0.41a  15.51±0.41b    7 to 14  46.19±0.73  48.04±0.74  46.73±0.74  45.99±0.33  45.46±0.33  44.89±0.32    14 to 21  61.00±1.47  63.14±1.51  65.24±1.50  78.35±0.64a  78.20±0.64a  74.99±0.63b    21 to 28  91.17±1.67b  98.50±1.73a  97.21±1.70a  96.81±1.34  98.57±1.35  96.19±1.33    28 to 35  99.03±2.52  98.94±2.56  99.24±2.52  111.29±1.79  114.01±1.80  111.45±1.77      Experiment 1  Experiment 2  Variable  Age (d)  Control  17% O2  15% O2  Control  17% O2  15% O2  BW (g)  0  48.5±0.39  48.2±0.40  48.7±0.40  48.5±0.40  48.2±0.40  48.7±0.40    7  180.3±2.22b  187.2±2.26a  178.8±2.25b  164.4±2.86a  163.2±2.87a  153.8±2.86b    14  503.6±6.67b  523.5±6.79a  505.9±6.80b  486.3±3.36a  481.7±3.38a  468.0±3.34b    21  929.0±15.02  965.5±15.45  961.8±15.30  1034.8±6.08a  1015.0±6.10a,b  992.9±6.04b    28  1567.3±21.43b  1653.0±22.25a  1642.3±21.83a  1712.5±9.77a  1705.0±9.80a  1666.3±9.70b    35  2267.1±27.60  2336.3±28.40  2337.0±27.86  2493.6±13.66a  2503.6±13.66a  2446.5±13.48b  DWG (g/d)  0 to 7  18.83±0.30b  19.86±0.31a  18.55±0.31b  17.05±0.41a  16.90±0.41a  15.51±0.41b    7 to 14  46.19±0.73  48.04±0.74  46.73±0.74  45.99±0.33  45.46±0.33  44.89±0.32    14 to 21  61.00±1.47  63.14±1.51  65.24±1.50  78.35±0.64a  78.20±0.64a  74.99±0.63b    21 to 28  91.17±1.67b  98.50±1.73a  97.21±1.70a  96.81±1.34  98.57±1.35  96.19±1.33    28 to 35  99.03±2.52  98.94±2.56  99.24±2.52  111.29±1.79  114.01±1.80  111.45±1.77  a,brepresent statistical differences between incubation groups within each experiment (P < 0.05). View Large Table 1. Effect of daily exposure to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on body weight (g) and weight gain (g/day) of broilers (males) from hatching to marketing age.     Experiment 1  Experiment 2  Variable  Age (d)  Control  17% O2  15% O2  Control  17% O2  15% O2  BW (g)  0  48.5±0.39  48.2±0.40  48.7±0.40  48.5±0.40  48.2±0.40  48.7±0.40    7  180.3±2.22b  187.2±2.26a  178.8±2.25b  164.4±2.86a  163.2±2.87a  153.8±2.86b    14  503.6±6.67b  523.5±6.79a  505.9±6.80b  486.3±3.36a  481.7±3.38a  468.0±3.34b    21  929.0±15.02  965.5±15.45  961.8±15.30  1034.8±6.08a  1015.0±6.10a,b  992.9±6.04b    28  1567.3±21.43b  1653.0±22.25a  1642.3±21.83a  1712.5±9.77a  1705.0±9.80a  1666.3±9.70b    35  2267.1±27.60  2336.3±28.40  2337.0±27.86  2493.6±13.66a  2503.6±13.66a  2446.5±13.48b  DWG (g/d)  0 to 7  18.83±0.30b  19.86±0.31a  18.55±0.31b  17.05±0.41a  16.90±0.41a  15.51±0.41b    7 to 14  46.19±0.73  48.04±0.74  46.73±0.74  45.99±0.33  45.46±0.33  44.89±0.32    14 to 21  61.00±1.47  63.14±1.51  65.24±1.50  78.35±0.64a  78.20±0.64a  74.99±0.63b    21 to 28  91.17±1.67b  98.50±1.73a  97.21±1.70a  96.81±1.34  98.57±1.35  96.19±1.33    28 to 35  99.03±2.52  98.94±2.56  99.24±2.52  111.29±1.79  114.01±1.80  111.45±1.77      Experiment 1  Experiment 2  Variable  Age (d)  Control  17% O2  15% O2  Control  17% O2  15% O2  BW (g)  0  48.5±0.39  48.2±0.40  48.7±0.40  48.5±0.40  48.2±0.40  48.7±0.40    7  180.3±2.22b  187.2±2.26a  178.8±2.25b  164.4±2.86a  163.2±2.87a  153.8±2.86b    14  503.6±6.67b  523.5±6.79a  505.9±6.80b  486.3±3.36a  481.7±3.38a  468.0±3.34b    21  929.0±15.02  965.5±15.45  961.8±15.30  1034.8±6.08a  1015.0±6.10a,b  992.9±6.04b    28  1567.3±21.43b  1653.0±22.25a  1642.3±21.83a  1712.5±9.77a  1705.0±9.80a  1666.3±9.70b    35  2267.1±27.60  2336.3±28.40  2337.0±27.86  2493.6±13.66a  2503.6±13.66a  2446.5±13.48b  DWG (g/d)  0 to 7  18.83±0.30b  19.86±0.31a  18.55±0.31b  17.05±0.41a  16.90±0.41a  15.51±0.41b    7 to 14  46.19±0.73  48.04±0.74  46.73±0.74  45.99±0.33  45.46±0.33  44.89±0.32    14 to 21  61.00±1.47  63.14±1.51  65.24±1.50  78.35±0.64a  78.20±0.64a  74.99±0.63b    21 to 28  91.17±1.67b  98.50±1.73a  97.21±1.70a  96.81±1.34  98.57±1.35  96.19±1.33    28 to 35  99.03±2.52  98.94±2.56  99.24±2.52  111.29±1.79  114.01±1.80  111.45±1.77  a,brepresent statistical differences between incubation groups within each experiment (P < 0.05). View Large Broilers prenatally exposed to 15% O2 exhibited similar BW to control during the first 2 wk post hatch; however, at 28 d, BW of 15% treated chicks was similar to that of the 17%, and significantly heavier than control (1642±21.8 and 1567±21.4 for 15% and control, respectively). Evidently, from the calculation of the average daily growth per wk, the chickens treated with 15% O2 and 17% O2 grew at a similar rate as that of the control chicks during the first 2 weeks. However, during the third and fourth wk, their growth rate was higher to significantly higher than the control (98.5±1.7, 97.2±1.7 and 91.17±1.6 g for the 17%, 15%, and control chicks, respectively). During the fifth wk, there was no difference in growth rate among the 3 groups of chicks, and by the end of the experiment, they reached a similar BW (Table 1). Feed intake was similar for all groups throughout the experiment (Figure 1). Figure 1. View largeDownload slide Effect of daily exposures to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on individual weekly average food intake from the beginning of the third wk until the end of the experiment (40 d). Figure 1. View largeDownload slide Effect of daily exposures to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on individual weekly average food intake from the beginning of the third wk until the end of the experiment (40 d). The average individual food efficiency, calculated over 3 wk (d 14 to 35), showed that food consumption efficiency of broilers exposed to 15% O2 or 17% O2 was better (lower FCR value) than that of the control group (1.66, 1.67 vs. 1.70, 17%, 15%, and control chicks, respectively), and as stated, without a negative effect on growth rate or final BW. The treatment of hypoxia resulted in advantageous food conversion at the end of the growth period (Figure 2). In terms of food intake, it was similar among broilers of all treatments, but hypoxia groups grew better compared to the control until the age of 35 days. Figure 2. View largeDownload slide Effect of daily exposure to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on average individual weekly feed conversion rate (FCR) from the beginning of the third wk until the end of the experiment (40 d). On each week of incubation different letters indicate significant differences (P ≤ 0.05) among treatments. Figure 2. View largeDownload slide Effect of daily exposure to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on average individual weekly feed conversion rate (FCR) from the beginning of the third wk until the end of the experiment (40 d). On each week of incubation different letters indicate significant differences (P ≤ 0.05) among treatments. Organ weight data obtained following slaughter indicate that there was no advantage to any of the treatments in terms of the relative breast weight, relative fat weight, or relative liver weight. However, hypoxia treatment conferred an advantage in relative weight of the heart (Table 2). Table 2. Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on slaughter BW and relative breast, abdominal fat, heart, and liver weights (% from slaughter BW) at the age of 40 and 41 d for experiments 1 and 2, respectively. Experiment    Control  17% O2  15% O2  p (F)1  Experiment 1  Body weight 40 d (g)  2789±30.6  2817±31.2  2827±30.7  ns    Slaughter BW (g)  2699±28  2701±29  2709±29  ns    Breast (% of slaughter BW)  21.67±0.24  21.76±0.25  22.11±0.24  ns    Abdominal fat (% of slaughter BW)  1.80±0.051  1.78±0.053  1.81±0.051  ns    Heart (% of slaughter BW)  0.40±0.010b  0.42±0.010a,b  0.43±0.009a  p(F) = 0.0487    Liver (% of slaughter BW)  1.53±0.018  1.51±0.018  1.53±0.017  ns  Experiment 2  Body weight 39 d (g)  2840±67  2929±68  2823±67  ns    Slaughter BW (g)  2788±29a  2798±18a  2703±18b  p(F) = 0.002    Breast (% of slaughter BW)  22.24±0.22b  22.82±0.14a  22.65±0.14a,b  p(F) = 0.049    Abdominal fat (% of slaughter BW)  1.81±0.052a  1.66±0.034b  1.88±0.033a  p(F)<0.0001    Heart (% of slaughter BW)  0.412±0.054a  0.436±0.006a  0.435±0.006a,b  p(F) = 0.046    Liver (% of slaughter BW)  1.55±0.018c  1.60±0.012b  1.64±0.011a  p(F) = 0.0006  Experiment    Control  17% O2  15% O2  p (F)1  Experiment 1  Body weight 40 d (g)  2789±30.6  2817±31.2  2827±30.7  ns    Slaughter BW (g)  2699±28  2701±29  2709±29  ns    Breast (% of slaughter BW)  21.67±0.24  21.76±0.25  22.11±0.24  ns    Abdominal fat (% of slaughter BW)  1.80±0.051  1.78±0.053  1.81±0.051  ns    Heart (% of slaughter BW)  0.40±0.010b  0.42±0.010a,b  0.43±0.009a  p(F) = 0.0487    Liver (% of slaughter BW)  1.53±0.018  1.51±0.018  1.53±0.017  ns  Experiment 2  Body weight 39 d (g)  2840±67  2929±68  2823±67  ns    Slaughter BW (g)  2788±29a  2798±18a  2703±18b  p(F) = 0.002    Breast (% of slaughter BW)  22.24±0.22b  22.82±0.14a  22.65±0.14a,b  p(F) = 0.049    Abdominal fat (% of slaughter BW)  1.81±0.052a  1.66±0.034b  1.88±0.033a  p(F)<0.0001    Heart (% of slaughter BW)  0.412±0.054a  0.436±0.006a  0.435±0.006a,b  p(F) = 0.046    Liver (% of slaughter BW)  1.55±0.018c  1.60±0.012b  1.64±0.011a  p(F) = 0.0006  1Significance level, within each experiment, of the difference between the means of Control, 17% and 15% chicks. a,brepresent statistical differences between incubation groups within each experiment (P < 0.05). View Large Table 2. Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on slaughter BW and relative breast, abdominal fat, heart, and liver weights (% from slaughter BW) at the age of 40 and 41 d for experiments 1 and 2, respectively. Experiment    Control  17% O2  15% O2  p (F)1  Experiment 1  Body weight 40 d (g)  2789±30.6  2817±31.2  2827±30.7  ns    Slaughter BW (g)  2699±28  2701±29  2709±29  ns    Breast (% of slaughter BW)  21.67±0.24  21.76±0.25  22.11±0.24  ns    Abdominal fat (% of slaughter BW)  1.80±0.051  1.78±0.053  1.81±0.051  ns    Heart (% of slaughter BW)  0.40±0.010b  0.42±0.010a,b  0.43±0.009a  p(F) = 0.0487    Liver (% of slaughter BW)  1.53±0.018  1.51±0.018  1.53±0.017  ns  Experiment 2  Body weight 39 d (g)  2840±67  2929±68  2823±67  ns    Slaughter BW (g)  2788±29a  2798±18a  2703±18b  p(F) = 0.002    Breast (% of slaughter BW)  22.24±0.22b  22.82±0.14a  22.65±0.14a,b  p(F) = 0.049    Abdominal fat (% of slaughter BW)  1.81±0.052a  1.66±0.034b  1.88±0.033a  p(F)<0.0001    Heart (% of slaughter BW)  0.412±0.054a  0.436±0.006a  0.435±0.006a,b  p(F) = 0.046    Liver (% of slaughter BW)  1.55±0.018c  1.60±0.012b  1.64±0.011a  p(F) = 0.0006  Experiment    Control  17% O2  15% O2  p (F)1  Experiment 1  Body weight 40 d (g)  2789±30.6  2817±31.2  2827±30.7  ns    Slaughter BW (g)  2699±28  2701±29  2709±29  ns    Breast (% of slaughter BW)  21.67±0.24  21.76±0.25  22.11±0.24  ns    Abdominal fat (% of slaughter BW)  1.80±0.051  1.78±0.053  1.81±0.051  ns    Heart (% of slaughter BW)  0.40±0.010b  0.42±0.010a,b  0.43±0.009a  p(F) = 0.0487    Liver (% of slaughter BW)  1.53±0.018  1.51±0.018  1.53±0.017  ns  Experiment 2  Body weight 39 d (g)  2840±67  2929±68  2823±67  ns    Slaughter BW (g)  2788±29a  2798±18a  2703±18b  p(F) = 0.002    Breast (% of slaughter BW)  22.24±0.22b  22.82±0.14a  22.65±0.14a,b  p(F) = 0.049    Abdominal fat (% of slaughter BW)  1.81±0.052a  1.66±0.034b  1.88±0.033a  p(F)<0.0001    Heart (% of slaughter BW)  0.412±0.054a  0.436±0.006a  0.435±0.006a,b  p(F) = 0.046    Liver (% of slaughter BW)  1.55±0.018c  1.60±0.012b  1.64±0.011a  p(F) = 0.0006  1Significance level, within each experiment, of the difference between the means of Control, 17% and 15% chicks. a,brepresent statistical differences between incubation groups within each experiment (P < 0.05). View Large Experiment 2a The growth of hatchlings up to 35 d clearly indicates that BW of those exposed daily to 17% O2 for 12 h during incubation was not affected (Table 1). Chickens exposed to 17% O2 obtained weight similar to the control group throughout the growing period and even showed a higher weight than controls by about 100 g on average. Broilers prenatally exposed to 15% O2 were of a lower weight throughout the growing period; however, at the end of the experiment on d 39, their BW was similar to that of control (Table 2). Evidently, chickens treated with 15% O2 had a significantly lower growth rate than control and 17% O2 treated groups. Only after the fourth wk, there was no difference among treatments. In contrast, the 17% O2 treated group showed a similar growth rate and even a slight advantage over that of the control group throughout the duration of the experiment (Table 1). Although there were no significant differences in food intake among the groups, at the beginning and at the second wk, there was a trend of lower food consumption by the hypoxia-treated groups compared to the control group (Figure 3). Figure 3. View largeDownload slide Effect of exposures to 17% or 15% oxygen for 12 hours daily during the development of CAM (E5-E12) on average weekly feed intake per pen from placement until the end of the experiment (n=4). Figure 3. View largeDownload slide Effect of exposures to 17% or 15% oxygen for 12 hours daily during the development of CAM (E5-E12) on average weekly feed intake per pen from placement until the end of the experiment (n=4). The feed conversion efficiency of hypoxia-treated chickens was better than that of the control group, with no consistent trend of relative efficiency between the 2 hypoxia treatment groups; neither was consistently better than the other (Figure 4). Figure 4. View largeDownload slide Effect of exposures to 17% or 15% oxygen for 12 hours daily during the development of CAM (E5-E12) on weekly average feed conversion rate (FCR) per pen, from placement until the end of the experiment (n=4). Figure 4. View largeDownload slide Effect of exposures to 17% or 15% oxygen for 12 hours daily during the development of CAM (E5-E12) on weekly average feed conversion rate (FCR) per pen, from placement until the end of the experiment (n=4). Slaughter took place on d 39, following 12 h without feeding. At the time of slaughter, the BW of chickens treated with 17% O2 hypoxia was slightly higher than that of the control group. In contrast, the BW of the 15% O2 group was significantly lower [2,703 g in the 15% group, compared to 2,788 g and 2,798 g in the 17% and control groups, respectively (Table 2)]. Both hypoxia-treated groups had a higher relative breast weight, as well as higher relative heart and liver weights than controls. However, whereas the 17% O2-treated group had a lower relative abdominal fat weight, the relative abdominal fat weight of the 15% O2 group was the same as the control (Table 2). Experiment 2b At the beginning of the exposure to heat stress, body temperature (BT) of the 3 treated chicken groups was similar. After 2.5 h of exposure, all groups developed hyperthermia; however, broilers from the control group had a significantly higher BT than the hypoxia-treated one (43.5, 43.2, and 43.4°C for the control, 17%, and 15% groups, respectively). After 5 h of heat exposure, the differences in BT among the groups were even more pronounced; while the hypoxia-treated broilers kept the temperature at approximately 43°C, the body temperature of the broilers in the control group reached 44°C (Figure 5). The mortality rate in the hypoxia-treated groups was significantly lower than that of the control group. The mortality rate in the 17% O2 treatment group was the lowest, with only a single recorded death (5% mortality n = 1/20); in the 15% O2 group, the death rate was 10% (n = 2/20), while the mortality in the control group at the end of exposure was 25% (n = 5/20; Figure 6). Under standard brooding conditions throughout the experiment, mortality was redundant in all experimental groups. Figure 5. View largeDownload slide Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on body temperature during hot challenge at the age of 35 days. Figure 5. View largeDownload slide Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on body temperature during hot challenge at the age of 35 days. Figure 6. View largeDownload slide Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on mortality during hot challenge at the age of 35 days. Figure 6. View largeDownload slide Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on mortality during hot challenge at the age of 35 days. DISCUSSION In this study, our aim was to evaluate the effect of hypoxia exposure during CAM development on growth rate and FCR of broilers up to marketing age under regular and heat stress conditions. All developing animals have so-called “critical windows” for development (Burggren and Mueller, 2015). Critical windows in chicken embryos have been broadly defined for the sensitivity to hypoxia (Druyan et al., 2012; Haron et al., 2017), for control of ventilation (Ferner and Mortola, 2009), for thermal manipulation (Piestun et al. 2008a,b, 2011), and for metabolic rate (Dzialowski et al., 2002). In a previous study (Druyan and Levi, 2012; Druyan et al., 2012), we found a positive correlation between hypoxic exposure and increase in the CAM vascular area. The critical period for hypoxia exposure during CAM development was found to be E5-E12. Exposure to hypoxia during this period caused the embryos to better adapt by increasing their oxygen-carrying capacity through an increase of blood vessel formation. In the current study, we found that the effects of hypoxic exposure on CAM development were translated into effects on hatchling performance. Exposure to 15 and 17% O2 during embryonic development showed no adverse effect on chicken growth. In fact, by the end of experiment, hypoxia-treated chickens weighed as much as or more than controls. This was in line with the calculated weekly growth rate; treated chicks exhibited an accelerated growth rate compared to controls during the third and fourth wk, with a similar or lower amount of feed intake as controls. Notably, the performance of 17 and 15% broilers grown in cages (experiment 1) was significantly better compared to performance measured when the birds were kept on litter under standard conditions (experiment 2a). This can be partially explained by the differences in broiler motility and locomotion. The birds kept in cages allocated metabolized energy into growth and less into movement. Food conversion rate efficiency of hypoxia-treated chickens up to early marketing age of 35 d was as good as or better than that of controls. Energy consumption in general, and in the domestic fowl in particular, has been shown to be divided between maintenance and production. Therefore, lowering the demand for maintenance energy, while keeping total energy consumption relatively constant, is thought to likely increase the energy available for production. An alternative beneficial response might consist of a combination of reduced maintenance energy demand coupled with an overall reduction in energy consumption. In the present study, the hypoxia-treated chickens had consumed in general less feed while exhibiting a better FCR and an accelerated growth. Combined with the lower body temperature the hypoxia-treated chickens exhibited when exposed to acute heat stress and lower mortality, it can be suggested that hypoxic exposure during CAM development had lowered the demands for maintenance metabolic rates. In terms of relative weights, the breast was larger in treated chickens, with reduced abdominal fat, suggesting better energy balance towards growth rather than accumulation of fat (Piestun et al., 2013). This may indicate that embryonic angiogenesis following hypoxic treatment led to better vascularization and presumably better nutrient delivery to the breast (Hadad Cahaner and Halevy 2014). Thus, the food was utilized for growing tissue rather than fat accumulation. It has recently been shown that acute hypoxic incubation reduces left ventricle function in late-stage chickens by lowering the generation of pressure leading to relaxation; thus, cardiac efficiency is greatly reduced after hypoxic incubation (Jonker et al., 2015). A previous study on adult chickens that had been exposed to permanent hypoxia (15%) during embryonic development (E10-E20) showed that they displayed severe left ventricular dilatation in later life (Tintu et al., 2009). In the present study, 12 h/d exposure to hypoxia (15 and 17%) (E5-E12) caused enlargement of the heart at the age of 5 weeks. This enlargement may potentially confer an advantage under heat stress. P2renatal hypoxic chicken managed to cope with a hot environment better: They showed a significantly lower body temperature following a heat challenge and maintained their relative advantage for 5 hours. Furthermore, although the difference was not significant, treated chickens had lower mortality rates in response to heat stress than controls. It is well known that heat stress exerts a hard load on the cardiovascular system (Olkowski, 2007; Zhang et al., 2017). The increase in heart size that we observed following hypoxic exposure affects stroke volume and improves the delivery of oxygen to tissues. Modern poultry, with relative heart weights not exceeding 5% of overall BW, are the result of 30 yr of selection under the ideal environmental conditions provided in commercial farming. This small proportion of heart weight supports the hypothesis that selective pressures have failed to cause internal systems to adjust to fully meet the metabolic demands of modern poultry. Therefore, we could suggest that in this experiment heart enlargement, because of intermittent prenatal hypoxia, improved only the chicken's capacity to cope with severe environmental conditions. Chronic hypoxia was shown not only to affect the birth weight, growth, and FCR but also slightly affects the survival of broilers (Huang et al., 2017). Similar observations were made by Visschedijk (1985) who studied lowland chickens raised at high altitudes where inadequate O2 exchange resulted in hypoxic syndrome. Hypoxia has been shown to affect the survival rates of both embryos and lowland chickens at high altitudes (Hao et al., 2014). In a later study by the same group, the BW of broilers raised under chronic hypoxic conditions was significantly lower on d 14 (p = 0.04) compared to the control group. The effects of hypoxia on the broilers were progressive, culminating in a significant decrease in the average daily weight gain (p = 0.002) and a considerable increase in FCR on d 14 (p = 0.003) (Huang et al., 2017). No significant difference in the mortality rates of broilers exposed to hypoxia were noted during the rearing period (Huang et al., 2017), though hypoxia is known as a major risk factor for the death of broilers (Zhang et al., 2016). In contrast to those findings from broilers that were continuously under hypoxic conditions at high altitudes, in the present work, exposure of embryos to hypoxia intermittently during a short period of the embryonic development provided advantages in performance and FCR to broilers at the market age. Continuous exposure to hypoxic conditions by incubation at high altitude throughout embryonic development led to initial growth depression, resulting in a significantly lower BW up to 14 days. The difference in BW was observed at 28 and 42 d of age as well. However, no significant incubation effect on BW was found at later ages. Feed intake showed the same pattern of differences as for growth, resulting in no significant differences in FCR, regardless of the incubation altitude or rearing period. (Hassanzadeh et al., 2004). In contrast, we showed better feed utilization by hypoxia-treated chickens. CONCLUSION Our results indicate that intermittent exposure to moderate hypoxia (15 and 17% O2) during CAM development confers advantages to broilers in feed utilization efficiency and in coping with heat stress, and may be considered as a mitigating step for commercial growers to facilitate chickens in achieving their full growth potential. ACKNOWLEDGMENTS This research was funded by grant No. 356-0677 from the Egg and Poultry Board of Israel. The authors wish to thank S. Oblezin and A. Kantor from the Volcani Center Chicken facilities for technical assistance. REFERENCES Burggren W. W., Mueller C. A.. 2015. Developmental critical windows and sensitive periods as three-dimensional constructs in time and space. Physiol. Biochem. Zool . 88: 91– 102. Google Scholar CrossRef Search ADS PubMed  Decuypere E. 2002. Ascites as a multifactorial syndrome of broiler chickens: considerations from a developmental and selection viewpoint. Pages 12– 14 in Proceedings of the 2nd Symposium of World's Poultry Science Association of the Iran Branch . Druyan S., Levi E.. 2012. Reduced O2 concentration during CAM development – Its effect on angiogenesis and gene expression in the broiler embryo CAM. Gene Expr. Patterns  12: 236– 244. Google Scholar CrossRef Search ADS PubMed  Druyan S., Levi E., Shinder D., Stern T.. 2012. Reduced O2 concentration during CAM development–Its effect on physiological parameters of broiler embryos. Poult. Sci . 91: 987– 997. Google Scholar CrossRef Search ADS PubMed  Druyan S., Hadad Y., Cahaner A.. 2008. Growth rate of ascites-resistant versus ascites-susceptible broilers in commercial and experimental lines. Poult. Sci.  87: 904– 911. 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American Journal of Physiology-Regulatory, Integrative and Comparative Physiology . 308: R680– R689. Google Scholar CrossRef Search ADS PubMed  NRC. 1994. Nutrient Requirements of Poultry. 9th rev. ed. National Academy Press, Washington, DC. Olkowski A. A. 2007. Pathophysiology of heart failure in broiler chickens: Structural, biochemical, and molecular characteristics. Poult. Sci.  86: 999– 1005. Google Scholar CrossRef Search ADS PubMed  Piestun Y., Druyan S., Brake J., Yahav S.. 2013. Thermal manipulations during broiler incubation alter performance of broilers to 70 days of age. Poult. Sci.  92: 1155– 1163. Google Scholar CrossRef Search ADS PubMed  Piestun Y., Halevy O., Shinder D., Ruzal M., Druyan S., Yahav S.. 2011. Thermal manipulations during broiler embryogenesis improves post-hatch performance under hot conditions. J. Therm. Biol . 36: 469– 474. Google Scholar CrossRef Search ADS   Piestun Y., Shinder D., Ruzal M., Halevy O., Brake J., Yahav S.. 2008a. Thermal manipulations during broiler embryogenesis: Effect on the acquisition of thermotolerance. Poult. Sci.  87: 1516– 1525. Google Scholar CrossRef Search ADS   Piestun Y., Shinder D., Ruzal M., Halevy O., Yahav S.. 2008b. The effect of thermal manipulations during the development of the thyroid and adrenal axes on in-hatch and post-hatch thermoregulation. J. Therm. Biol . 33: 413– 418. Google Scholar CrossRef Search ADS   Tintu A., Rouwet E., Verlohren S., Brinkmann J., Ahmad S., Crispi F., van Bilsen M., Carmeliet P., Staff A. C., Tjwa M., and others. 2009. Hypoxia induces dilated cardiomyopathy in the chick embryo: Mechanism, intervention, and long-term consequences. PLoS One  4: e5155. Google Scholar CrossRef Search ADS PubMed  Visschedijk A. H. 1985. Gas exchange and hatchability of chicken eggs incubated at simulated high altitude. J. Appl. Physiol.  58: 416– 418. Google Scholar CrossRef Search ADS PubMed  Zhang Q., Gou W., Wang X., Zhang Y., Ma J., Zhang H., Zhang Y., Zhang H.. 2016. Genome resequencing identifies unique adaptations of tibetan chickens to hypoxia and High-Dose ultraviolet radiation in high-altitude environments. Genome Biol Evol . 8: 765– 776. Google Scholar CrossRef Search ADS PubMed  Zhang J., Schmidt C. J., Lamont S. J.. 2017. Transcriptome analysis reveals potential mechanisms underlying differential heart development in fast- and slow-growing broilers under heat stress. BMC Genomics  18: 295. Google Scholar CrossRef Search ADS PubMed  © 2018 Poultry Science Association Inc. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Poultry Science Oxford University Press

Effects of low oxygen during chorioallantoic membrane development on post-hatch growing performance of broiler chickens

Poultry Science , Volume Advance Article (6) – Mar 8, 2018

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© 2018 Poultry Science Association Inc.
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0032-5791
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Abstract

ABSTRACT The prenatal circulatory system is adaptive and capable of plasticity designed for the needs of the growing tissue. When a broiler embryo is faced with hypoxic stress, the process of angiogenesis in tissues begins. Exposure to hypoxic conditions of 17% oxygen during the chorioallantoic membrane (CAM) development (E5 to E12) affected the circulatory system and contributed to an increase in the blood oxygen carrying capacity. The present study aimed to evaluate the effects of hypoxic exposure during CAM development on post-hatch performance of broilers and to examine whether hypoxic exposure improved sustainability of birds exposed to acute heat stress. Two consecutive trials, with male broilers from each of the incubation treatments—optimal conditions and exposure to hypoxia of 15 or 17% oxygen, for 12 h/day, during CAM development—were conducted. In experiment 1, 60 male chicks from each group were raised in individual cages. In experiment 2, 160 male chicks from each group were raised in 40-chick pens until marketing. On d 35, 20 birds from each group were transferred to individual cages kept at a temperature of 23°C for 72 h, and then birds were exposed to 35°C for 5 hours. Body temperatures were measured at 0, 2, and 5 h of the heat exposure. In both experiments BW, feed intake, and FCR were recorded. At marketing, chicks were slaughtered, and relative weights of breast muscle, abdominal fat pad, heart, and liver were calculated. Hypoxia treatment resulted in a FCR advantage. Food intake was similar in all treatments, but groups exposed to hypoxia grew better than controls until the age of 35 days. Hypoxia-treated groups had higher relative breast, heart, and liver weights than controls. Body temperatures of hypoxia-treated chickens remained lower during heat stress exposure, and their mortality rate was lower as well. Intermittent exposure to moderate hypoxia during CAM development confers advantages to broilers in feed utilization efficiency and in coping with heat stress. It may be considered as a mitigating step in incubation to facilitate broilers in achieving their full growth potential. INTRODUCTION Fast-growing broilers have a high oxygen demand for intensive growth rate. As long as the optimal environmental conditions enable consistent growth of energy supply, the rapid growth potential provided by the chickens’ genetics can be realized. However, while the optimum conditions improve manufacturing efficiency, sub-optimal environmental conditions induce higher oxygen demand, which is beyond the capacity of the cardiovascular system (Druyan et al., 2008; Druyan et al., 2009). Inability to efficiently supply oxygen demand has a deleterious effect on growth rate (Jonker et al., 2015). Industrial farming that has been consistently providing optimal growth conditions for generations has affected a selection process that actually reduced the ability of the birds to perform under sub-optimal conditions and to maintain a dynamic balance between the different body systems (Havenstein et al., 2003). As a result, growth rates are impaired, the genetic potential of poultry for rapid growth is not exhausted, and growth and food utilization are compromised. In order to reduce the extent of the financial loss involved in prolonging the growth period, various approaches to provide optimal growth conditions have been adopted. These include temperature control using expensive technologies. However, as the cultivation outlasts genetic growth, the physiological oxygen utilization capacity of the chickens may not suffice for their metabolic demands (Druyan et al., 2012). Taken together, under current management conditions, the full genetic potential cannot be exploited, and it is thus necessary to invest in temperature control, adequate nutrition, density, and environmental conditions to take advantage of the full genetic potential. The prenatal circulatory system is adaptive and capable of plasticity designed to meet the needs of the growing tissue. When the embryo is faced with hypoxic stress, the process of angiogenesis in tissues begins (Druyan and Levi, 2012). In our laboratory, when embryos were exposed to hypoxic conditions of 17% oxygen during the period of the development of the chorioallantoic membrane (CAM) (from embryonic d E5 to E12), an exposure effect was shown on the circulatory system: The CAM grew, the number of blood vessels increased, and hematocrit (red blood cell content) and hemoglobin levels rose. These changes contributed to an increase in the blood-oxygen-carrying capacity of those embryos and raised the level of body metabolism expressed as yolk sac higher consumption compared to the control (Druyan and Levi, 2012; Druyan et al., 2012). Next, we sought to understand the effect of exposure to hypoxic conditions and adaptation of circulatory system variables related to the transportation of oxygen on post-hatch performance. Developmental changes induced by environmental or incubation conditions may play a role in post-hatch performance, affecting growth and metabolism (Decuypere, 2002). It has been shown that incubation under chronic hypoxic conditions (as exist at 2,000 m above sea level) led to initial growth depression resulting in a significantly lower body weight (BW) up to 14 d post hatch. However, at later stages, no significant effect on BW was found (Hassanzadeh et al., 2004). Piestun et al. (2008a,b, 2011) have demonstrated that thermal manipulation during the development and maturation of the hypothalamus-hypophysis-thyroid axis had a long-lasting effect on post-hatching thermos tolerance of broilers. This effect persisted until an early marketing age of 35 d, exhibiting no effect of thermal manipulation (TM) on BW of male birds and a negative effect of TM on female BW (Piestun et al., 2013). At 44 d, birds that were incubated at high temperatures weighed almost 100 g less than birds incubated under standard temperature (Hulet et al., 2007). The objectives of the current study were: 1) To evaluate the effects of hypoxia exposure at levels of 15 or 17% during CAM development on the growth rate and feed conversion of broilers up to the age of poultry marketing under standard growth conditions, and 2) to examine whether hypoxia exposure confers a performance advantage to birds under an acute heat stress. MATERIALS AND METHODS Experimental Design All the procedures in this study were carried out in accordance with the accepted ethical and welfare standards of the Israel Ethics Committee (IL-581/15). Experiment 1 Nine hundred fertilized Cobb strain broiler chicken (Gallus domesticus) eggs with an average weight of 64.0 ± 2.5 g were obtained from a breeder flock of hens during their optimal period of egg production (35 wk old). All eggs were numbered and weighed individually prior to incubation. They were incubated in a Danki Medium-Size Incubator for 2,500 eggs (Danki ApS, Ikast, Denmark) under standard incubation conditions of 37.8°C and 56% RH, and were turned once per hour. At E5, eggs were randomly divided into 3 groups of 300 eggs: 1 – Control; 2 – 17% O2—in which the eggs were exposed to hypoxia—17% oxygen during the 8 d of CAM development (E5-E12); and 3 – 15% O2—in which the eggs were exposed to hypoxia—15% oxygen during the 8 d of CAM development (E5-E12). Exposure to hypoxic conditions was accomplished by transferring the eggs of the 2 treatment groups from the control incubator into a medium-sized incubator (Danki ApS, Ikast, Denmark) equipped with a Model 2BGA-SP-MA O2 and CO2 Control System (Emproco Ltd, Ashkelon, Israel) for 12 hours. The 2BGA-SP-MA control system includes an infrared CO2 detector with a sensitivity range of 0 to 5 ± 0.01%, and an electrochemical cell for oxygen measurement that covered 0 to 25 ± 0.1%. In response to the CO2 sensor readings, an electronically controlled pump infused ambient air into the incubator to maintain CO2 concentration at the standard level of 0.03%. The O2 sensor activated an electronically controlled pump that infused N2 into the incubator to maintain the desired oxygen concentrations of 17 ± 0.2% or 15± 0.2%. After hatching, 60 male chicks were chosen from each group. Each chick was individually tagged and weighed; the chicks from each group were divided into groups, each comprised of 10 chicks and raised together in battery cages until the age of 14 d (6 × 10 battery cages for each incubation treatment). On d 14, chicks were transferred to individual cages with individual feeders where each individual chick was housed on its own (60 individual cages for each incubation treatment). Water and feed in mash form were available for ad libitum consumption. The diet was designed to meet or exceed NRC (1994) recommendations. The birds were held under the recommended temperature regime (starting from 34°C on d of hatch to 24°C from d 21 onwards) with 55% Rh and 20:4 h of light. Each chick was weighed every wk, and its individual weekly food intake was calculated. At the end of the experiment, at the age of 40 d, the chickens were individually weighed, and the feed was removed for 12 h before slaughter. Breast muscle, abdominal fat pad, heart, and liver were removed and weighed and their weights calculated relative to their live BW. Experiment 2 Part a: Twelve hundred fertilized Cobb eggs were incubated until E5 and randomly divided into 3 groups of 400 eggs each as described above, followed by hypoxia and control treatments as in experiment 1. One-hundred-sixty male hatched chicks with a similar average weight from each of the 3 treatment groups were chosen; thus, the entire experiment included 480 chicks. At the end of the hatching process, chicks from each treatment group were distributed into 4 equal growing pens, 40 chicks per pen (total 12 pens) at a density of approximately 12 chicks per m2. Each chick was tagged and weighed individually. Chicks were raised according to Cobb recommendations for environmental conditions and grown until the age of 39 days. The location of each growing pen was randomly assigned within the chicken house. Water and feed in mash form were available for ad libitum consumption. The diet was designed to meet or exceed NRC (1994) recommendations. At weekly intervals, BW was individually recorded, feed consumption was measured by pen, and unadjusted FCR (kg of feed consumed/kg of live BW) was calculated for each pen. At 39 d of age, chickens were individually weighed, and the feed was removed for 12 h prior to slaughter. Breast muscle, abdominal fat pad, heart, and liver were removed and weighed and their relative weights calculated based on live BW. Part b: At d 35 after weighing, 5 chickens from each yard were selected (chickens had an average representative weight of the population). A total 20 birds from each treatment group was transferred (3 × 20 = 60 birds) to individual avian cages at room temperature of 23°C for 72 h for adaptation. At the end of the adjustment period, chickens were exposed to acute heat stress that was induced within a 30 min time period from 23 to 35°C. Following 5 h of heat stress, the temperature declined back to 23°C within 30 minutes. Each bird was individually weighed at the beginning and end of the heat exposure, and their body temperatures were measured at the beginning, after 2 h, and at the end of the heat exposure. Measurement of body temperature was done with a digital thermometer (Super Speed Digital Thermometer, Procare Measure Technology Co. Ltd., San Chung City, Taipei 241, Taiwan, accurate to ±0.1°C). Statistics All data were processed statistically using one-way Analysis of Variance (ANOVA). Values that differed (at a level of P < 0.05) were considered statistically significant. In addition, the Tukey–Kramer test was conducted, comparing the averages of the treatments at the exposure point. Mortality data were subjected to chi squared analysis. RESULTS Experiment 1 The growth of the hatchlings up to age of 35 d clearly indicates that exposure to 17% O2 for 12 h/d during the CAM development did not negatively affected broilers’ BW post hatch. The 17% O2 treated chicks were found to be significantly heavier than control chicks on d 7, d 14, and d 28 (1652±21.8 g vs. 1567±21.4 on d 28 for the 17% chicks compared to the control chicks, respectively). On the other hand, no significant differences in BW were found by the end of the experiment (Table 1). Table 1. Effect of daily exposure to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on body weight (g) and weight gain (g/day) of broilers (males) from hatching to marketing age.     Experiment 1  Experiment 2  Variable  Age (d)  Control  17% O2  15% O2  Control  17% O2  15% O2  BW (g)  0  48.5±0.39  48.2±0.40  48.7±0.40  48.5±0.40  48.2±0.40  48.7±0.40    7  180.3±2.22b  187.2±2.26a  178.8±2.25b  164.4±2.86a  163.2±2.87a  153.8±2.86b    14  503.6±6.67b  523.5±6.79a  505.9±6.80b  486.3±3.36a  481.7±3.38a  468.0±3.34b    21  929.0±15.02  965.5±15.45  961.8±15.30  1034.8±6.08a  1015.0±6.10a,b  992.9±6.04b    28  1567.3±21.43b  1653.0±22.25a  1642.3±21.83a  1712.5±9.77a  1705.0±9.80a  1666.3±9.70b    35  2267.1±27.60  2336.3±28.40  2337.0±27.86  2493.6±13.66a  2503.6±13.66a  2446.5±13.48b  DWG (g/d)  0 to 7  18.83±0.30b  19.86±0.31a  18.55±0.31b  17.05±0.41a  16.90±0.41a  15.51±0.41b    7 to 14  46.19±0.73  48.04±0.74  46.73±0.74  45.99±0.33  45.46±0.33  44.89±0.32    14 to 21  61.00±1.47  63.14±1.51  65.24±1.50  78.35±0.64a  78.20±0.64a  74.99±0.63b    21 to 28  91.17±1.67b  98.50±1.73a  97.21±1.70a  96.81±1.34  98.57±1.35  96.19±1.33    28 to 35  99.03±2.52  98.94±2.56  99.24±2.52  111.29±1.79  114.01±1.80  111.45±1.77      Experiment 1  Experiment 2  Variable  Age (d)  Control  17% O2  15% O2  Control  17% O2  15% O2  BW (g)  0  48.5±0.39  48.2±0.40  48.7±0.40  48.5±0.40  48.2±0.40  48.7±0.40    7  180.3±2.22b  187.2±2.26a  178.8±2.25b  164.4±2.86a  163.2±2.87a  153.8±2.86b    14  503.6±6.67b  523.5±6.79a  505.9±6.80b  486.3±3.36a  481.7±3.38a  468.0±3.34b    21  929.0±15.02  965.5±15.45  961.8±15.30  1034.8±6.08a  1015.0±6.10a,b  992.9±6.04b    28  1567.3±21.43b  1653.0±22.25a  1642.3±21.83a  1712.5±9.77a  1705.0±9.80a  1666.3±9.70b    35  2267.1±27.60  2336.3±28.40  2337.0±27.86  2493.6±13.66a  2503.6±13.66a  2446.5±13.48b  DWG (g/d)  0 to 7  18.83±0.30b  19.86±0.31a  18.55±0.31b  17.05±0.41a  16.90±0.41a  15.51±0.41b    7 to 14  46.19±0.73  48.04±0.74  46.73±0.74  45.99±0.33  45.46±0.33  44.89±0.32    14 to 21  61.00±1.47  63.14±1.51  65.24±1.50  78.35±0.64a  78.20±0.64a  74.99±0.63b    21 to 28  91.17±1.67b  98.50±1.73a  97.21±1.70a  96.81±1.34  98.57±1.35  96.19±1.33    28 to 35  99.03±2.52  98.94±2.56  99.24±2.52  111.29±1.79  114.01±1.80  111.45±1.77  a,brepresent statistical differences between incubation groups within each experiment (P < 0.05). View Large Table 1. Effect of daily exposure to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on body weight (g) and weight gain (g/day) of broilers (males) from hatching to marketing age.     Experiment 1  Experiment 2  Variable  Age (d)  Control  17% O2  15% O2  Control  17% O2  15% O2  BW (g)  0  48.5±0.39  48.2±0.40  48.7±0.40  48.5±0.40  48.2±0.40  48.7±0.40    7  180.3±2.22b  187.2±2.26a  178.8±2.25b  164.4±2.86a  163.2±2.87a  153.8±2.86b    14  503.6±6.67b  523.5±6.79a  505.9±6.80b  486.3±3.36a  481.7±3.38a  468.0±3.34b    21  929.0±15.02  965.5±15.45  961.8±15.30  1034.8±6.08a  1015.0±6.10a,b  992.9±6.04b    28  1567.3±21.43b  1653.0±22.25a  1642.3±21.83a  1712.5±9.77a  1705.0±9.80a  1666.3±9.70b    35  2267.1±27.60  2336.3±28.40  2337.0±27.86  2493.6±13.66a  2503.6±13.66a  2446.5±13.48b  DWG (g/d)  0 to 7  18.83±0.30b  19.86±0.31a  18.55±0.31b  17.05±0.41a  16.90±0.41a  15.51±0.41b    7 to 14  46.19±0.73  48.04±0.74  46.73±0.74  45.99±0.33  45.46±0.33  44.89±0.32    14 to 21  61.00±1.47  63.14±1.51  65.24±1.50  78.35±0.64a  78.20±0.64a  74.99±0.63b    21 to 28  91.17±1.67b  98.50±1.73a  97.21±1.70a  96.81±1.34  98.57±1.35  96.19±1.33    28 to 35  99.03±2.52  98.94±2.56  99.24±2.52  111.29±1.79  114.01±1.80  111.45±1.77      Experiment 1  Experiment 2  Variable  Age (d)  Control  17% O2  15% O2  Control  17% O2  15% O2  BW (g)  0  48.5±0.39  48.2±0.40  48.7±0.40  48.5±0.40  48.2±0.40  48.7±0.40    7  180.3±2.22b  187.2±2.26a  178.8±2.25b  164.4±2.86a  163.2±2.87a  153.8±2.86b    14  503.6±6.67b  523.5±6.79a  505.9±6.80b  486.3±3.36a  481.7±3.38a  468.0±3.34b    21  929.0±15.02  965.5±15.45  961.8±15.30  1034.8±6.08a  1015.0±6.10a,b  992.9±6.04b    28  1567.3±21.43b  1653.0±22.25a  1642.3±21.83a  1712.5±9.77a  1705.0±9.80a  1666.3±9.70b    35  2267.1±27.60  2336.3±28.40  2337.0±27.86  2493.6±13.66a  2503.6±13.66a  2446.5±13.48b  DWG (g/d)  0 to 7  18.83±0.30b  19.86±0.31a  18.55±0.31b  17.05±0.41a  16.90±0.41a  15.51±0.41b    7 to 14  46.19±0.73  48.04±0.74  46.73±0.74  45.99±0.33  45.46±0.33  44.89±0.32    14 to 21  61.00±1.47  63.14±1.51  65.24±1.50  78.35±0.64a  78.20±0.64a  74.99±0.63b    21 to 28  91.17±1.67b  98.50±1.73a  97.21±1.70a  96.81±1.34  98.57±1.35  96.19±1.33    28 to 35  99.03±2.52  98.94±2.56  99.24±2.52  111.29±1.79  114.01±1.80  111.45±1.77  a,brepresent statistical differences between incubation groups within each experiment (P < 0.05). View Large Broilers prenatally exposed to 15% O2 exhibited similar BW to control during the first 2 wk post hatch; however, at 28 d, BW of 15% treated chicks was similar to that of the 17%, and significantly heavier than control (1642±21.8 and 1567±21.4 for 15% and control, respectively). Evidently, from the calculation of the average daily growth per wk, the chickens treated with 15% O2 and 17% O2 grew at a similar rate as that of the control chicks during the first 2 weeks. However, during the third and fourth wk, their growth rate was higher to significantly higher than the control (98.5±1.7, 97.2±1.7 and 91.17±1.6 g for the 17%, 15%, and control chicks, respectively). During the fifth wk, there was no difference in growth rate among the 3 groups of chicks, and by the end of the experiment, they reached a similar BW (Table 1). Feed intake was similar for all groups throughout the experiment (Figure 1). Figure 1. View largeDownload slide Effect of daily exposures to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on individual weekly average food intake from the beginning of the third wk until the end of the experiment (40 d). Figure 1. View largeDownload slide Effect of daily exposures to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on individual weekly average food intake from the beginning of the third wk until the end of the experiment (40 d). The average individual food efficiency, calculated over 3 wk (d 14 to 35), showed that food consumption efficiency of broilers exposed to 15% O2 or 17% O2 was better (lower FCR value) than that of the control group (1.66, 1.67 vs. 1.70, 17%, 15%, and control chicks, respectively), and as stated, without a negative effect on growth rate or final BW. The treatment of hypoxia resulted in advantageous food conversion at the end of the growth period (Figure 2). In terms of food intake, it was similar among broilers of all treatments, but hypoxia groups grew better compared to the control until the age of 35 days. Figure 2. View largeDownload slide Effect of daily exposure to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on average individual weekly feed conversion rate (FCR) from the beginning of the third wk until the end of the experiment (40 d). On each week of incubation different letters indicate significant differences (P ≤ 0.05) among treatments. Figure 2. View largeDownload slide Effect of daily exposure to 17 or 15% oxygen for 12 h during the development of CAM (E5-E12) on average individual weekly feed conversion rate (FCR) from the beginning of the third wk until the end of the experiment (40 d). On each week of incubation different letters indicate significant differences (P ≤ 0.05) among treatments. Organ weight data obtained following slaughter indicate that there was no advantage to any of the treatments in terms of the relative breast weight, relative fat weight, or relative liver weight. However, hypoxia treatment conferred an advantage in relative weight of the heart (Table 2). Table 2. Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on slaughter BW and relative breast, abdominal fat, heart, and liver weights (% from slaughter BW) at the age of 40 and 41 d for experiments 1 and 2, respectively. Experiment    Control  17% O2  15% O2  p (F)1  Experiment 1  Body weight 40 d (g)  2789±30.6  2817±31.2  2827±30.7  ns    Slaughter BW (g)  2699±28  2701±29  2709±29  ns    Breast (% of slaughter BW)  21.67±0.24  21.76±0.25  22.11±0.24  ns    Abdominal fat (% of slaughter BW)  1.80±0.051  1.78±0.053  1.81±0.051  ns    Heart (% of slaughter BW)  0.40±0.010b  0.42±0.010a,b  0.43±0.009a  p(F) = 0.0487    Liver (% of slaughter BW)  1.53±0.018  1.51±0.018  1.53±0.017  ns  Experiment 2  Body weight 39 d (g)  2840±67  2929±68  2823±67  ns    Slaughter BW (g)  2788±29a  2798±18a  2703±18b  p(F) = 0.002    Breast (% of slaughter BW)  22.24±0.22b  22.82±0.14a  22.65±0.14a,b  p(F) = 0.049    Abdominal fat (% of slaughter BW)  1.81±0.052a  1.66±0.034b  1.88±0.033a  p(F)<0.0001    Heart (% of slaughter BW)  0.412±0.054a  0.436±0.006a  0.435±0.006a,b  p(F) = 0.046    Liver (% of slaughter BW)  1.55±0.018c  1.60±0.012b  1.64±0.011a  p(F) = 0.0006  Experiment    Control  17% O2  15% O2  p (F)1  Experiment 1  Body weight 40 d (g)  2789±30.6  2817±31.2  2827±30.7  ns    Slaughter BW (g)  2699±28  2701±29  2709±29  ns    Breast (% of slaughter BW)  21.67±0.24  21.76±0.25  22.11±0.24  ns    Abdominal fat (% of slaughter BW)  1.80±0.051  1.78±0.053  1.81±0.051  ns    Heart (% of slaughter BW)  0.40±0.010b  0.42±0.010a,b  0.43±0.009a  p(F) = 0.0487    Liver (% of slaughter BW)  1.53±0.018  1.51±0.018  1.53±0.017  ns  Experiment 2  Body weight 39 d (g)  2840±67  2929±68  2823±67  ns    Slaughter BW (g)  2788±29a  2798±18a  2703±18b  p(F) = 0.002    Breast (% of slaughter BW)  22.24±0.22b  22.82±0.14a  22.65±0.14a,b  p(F) = 0.049    Abdominal fat (% of slaughter BW)  1.81±0.052a  1.66±0.034b  1.88±0.033a  p(F)<0.0001    Heart (% of slaughter BW)  0.412±0.054a  0.436±0.006a  0.435±0.006a,b  p(F) = 0.046    Liver (% of slaughter BW)  1.55±0.018c  1.60±0.012b  1.64±0.011a  p(F) = 0.0006  1Significance level, within each experiment, of the difference between the means of Control, 17% and 15% chicks. a,brepresent statistical differences between incubation groups within each experiment (P < 0.05). View Large Table 2. Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on slaughter BW and relative breast, abdominal fat, heart, and liver weights (% from slaughter BW) at the age of 40 and 41 d for experiments 1 and 2, respectively. Experiment    Control  17% O2  15% O2  p (F)1  Experiment 1  Body weight 40 d (g)  2789±30.6  2817±31.2  2827±30.7  ns    Slaughter BW (g)  2699±28  2701±29  2709±29  ns    Breast (% of slaughter BW)  21.67±0.24  21.76±0.25  22.11±0.24  ns    Abdominal fat (% of slaughter BW)  1.80±0.051  1.78±0.053  1.81±0.051  ns    Heart (% of slaughter BW)  0.40±0.010b  0.42±0.010a,b  0.43±0.009a  p(F) = 0.0487    Liver (% of slaughter BW)  1.53±0.018  1.51±0.018  1.53±0.017  ns  Experiment 2  Body weight 39 d (g)  2840±67  2929±68  2823±67  ns    Slaughter BW (g)  2788±29a  2798±18a  2703±18b  p(F) = 0.002    Breast (% of slaughter BW)  22.24±0.22b  22.82±0.14a  22.65±0.14a,b  p(F) = 0.049    Abdominal fat (% of slaughter BW)  1.81±0.052a  1.66±0.034b  1.88±0.033a  p(F)<0.0001    Heart (% of slaughter BW)  0.412±0.054a  0.436±0.006a  0.435±0.006a,b  p(F) = 0.046    Liver (% of slaughter BW)  1.55±0.018c  1.60±0.012b  1.64±0.011a  p(F) = 0.0006  Experiment    Control  17% O2  15% O2  p (F)1  Experiment 1  Body weight 40 d (g)  2789±30.6  2817±31.2  2827±30.7  ns    Slaughter BW (g)  2699±28  2701±29  2709±29  ns    Breast (% of slaughter BW)  21.67±0.24  21.76±0.25  22.11±0.24  ns    Abdominal fat (% of slaughter BW)  1.80±0.051  1.78±0.053  1.81±0.051  ns    Heart (% of slaughter BW)  0.40±0.010b  0.42±0.010a,b  0.43±0.009a  p(F) = 0.0487    Liver (% of slaughter BW)  1.53±0.018  1.51±0.018  1.53±0.017  ns  Experiment 2  Body weight 39 d (g)  2840±67  2929±68  2823±67  ns    Slaughter BW (g)  2788±29a  2798±18a  2703±18b  p(F) = 0.002    Breast (% of slaughter BW)  22.24±0.22b  22.82±0.14a  22.65±0.14a,b  p(F) = 0.049    Abdominal fat (% of slaughter BW)  1.81±0.052a  1.66±0.034b  1.88±0.033a  p(F)<0.0001    Heart (% of slaughter BW)  0.412±0.054a  0.436±0.006a  0.435±0.006a,b  p(F) = 0.046    Liver (% of slaughter BW)  1.55±0.018c  1.60±0.012b  1.64±0.011a  p(F) = 0.0006  1Significance level, within each experiment, of the difference between the means of Control, 17% and 15% chicks. a,brepresent statistical differences between incubation groups within each experiment (P < 0.05). View Large Experiment 2a The growth of hatchlings up to 35 d clearly indicates that BW of those exposed daily to 17% O2 for 12 h during incubation was not affected (Table 1). Chickens exposed to 17% O2 obtained weight similar to the control group throughout the growing period and even showed a higher weight than controls by about 100 g on average. Broilers prenatally exposed to 15% O2 were of a lower weight throughout the growing period; however, at the end of the experiment on d 39, their BW was similar to that of control (Table 2). Evidently, chickens treated with 15% O2 had a significantly lower growth rate than control and 17% O2 treated groups. Only after the fourth wk, there was no difference among treatments. In contrast, the 17% O2 treated group showed a similar growth rate and even a slight advantage over that of the control group throughout the duration of the experiment (Table 1). Although there were no significant differences in food intake among the groups, at the beginning and at the second wk, there was a trend of lower food consumption by the hypoxia-treated groups compared to the control group (Figure 3). Figure 3. View largeDownload slide Effect of exposures to 17% or 15% oxygen for 12 hours daily during the development of CAM (E5-E12) on average weekly feed intake per pen from placement until the end of the experiment (n=4). Figure 3. View largeDownload slide Effect of exposures to 17% or 15% oxygen for 12 hours daily during the development of CAM (E5-E12) on average weekly feed intake per pen from placement until the end of the experiment (n=4). The feed conversion efficiency of hypoxia-treated chickens was better than that of the control group, with no consistent trend of relative efficiency between the 2 hypoxia treatment groups; neither was consistently better than the other (Figure 4). Figure 4. View largeDownload slide Effect of exposures to 17% or 15% oxygen for 12 hours daily during the development of CAM (E5-E12) on weekly average feed conversion rate (FCR) per pen, from placement until the end of the experiment (n=4). Figure 4. View largeDownload slide Effect of exposures to 17% or 15% oxygen for 12 hours daily during the development of CAM (E5-E12) on weekly average feed conversion rate (FCR) per pen, from placement until the end of the experiment (n=4). Slaughter took place on d 39, following 12 h without feeding. At the time of slaughter, the BW of chickens treated with 17% O2 hypoxia was slightly higher than that of the control group. In contrast, the BW of the 15% O2 group was significantly lower [2,703 g in the 15% group, compared to 2,788 g and 2,798 g in the 17% and control groups, respectively (Table 2)]. Both hypoxia-treated groups had a higher relative breast weight, as well as higher relative heart and liver weights than controls. However, whereas the 17% O2-treated group had a lower relative abdominal fat weight, the relative abdominal fat weight of the 15% O2 group was the same as the control (Table 2). Experiment 2b At the beginning of the exposure to heat stress, body temperature (BT) of the 3 treated chicken groups was similar. After 2.5 h of exposure, all groups developed hyperthermia; however, broilers from the control group had a significantly higher BT than the hypoxia-treated one (43.5, 43.2, and 43.4°C for the control, 17%, and 15% groups, respectively). After 5 h of heat exposure, the differences in BT among the groups were even more pronounced; while the hypoxia-treated broilers kept the temperature at approximately 43°C, the body temperature of the broilers in the control group reached 44°C (Figure 5). The mortality rate in the hypoxia-treated groups was significantly lower than that of the control group. The mortality rate in the 17% O2 treatment group was the lowest, with only a single recorded death (5% mortality n = 1/20); in the 15% O2 group, the death rate was 10% (n = 2/20), while the mortality in the control group at the end of exposure was 25% (n = 5/20; Figure 6). Under standard brooding conditions throughout the experiment, mortality was redundant in all experimental groups. Figure 5. View largeDownload slide Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on body temperature during hot challenge at the age of 35 days. Figure 5. View largeDownload slide Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on body temperature during hot challenge at the age of 35 days. Figure 6. View largeDownload slide Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on mortality during hot challenge at the age of 35 days. Figure 6. View largeDownload slide Effect of exposure to 17 or 15% oxygen for 12 h daily during the development of CAM (E5-E12) on mortality during hot challenge at the age of 35 days. DISCUSSION In this study, our aim was to evaluate the effect of hypoxia exposure during CAM development on growth rate and FCR of broilers up to marketing age under regular and heat stress conditions. All developing animals have so-called “critical windows” for development (Burggren and Mueller, 2015). Critical windows in chicken embryos have been broadly defined for the sensitivity to hypoxia (Druyan et al., 2012; Haron et al., 2017), for control of ventilation (Ferner and Mortola, 2009), for thermal manipulation (Piestun et al. 2008a,b, 2011), and for metabolic rate (Dzialowski et al., 2002). In a previous study (Druyan and Levi, 2012; Druyan et al., 2012), we found a positive correlation between hypoxic exposure and increase in the CAM vascular area. The critical period for hypoxia exposure during CAM development was found to be E5-E12. Exposure to hypoxia during this period caused the embryos to better adapt by increasing their oxygen-carrying capacity through an increase of blood vessel formation. In the current study, we found that the effects of hypoxic exposure on CAM development were translated into effects on hatchling performance. Exposure to 15 and 17% O2 during embryonic development showed no adverse effect on chicken growth. In fact, by the end of experiment, hypoxia-treated chickens weighed as much as or more than controls. This was in line with the calculated weekly growth rate; treated chicks exhibited an accelerated growth rate compared to controls during the third and fourth wk, with a similar or lower amount of feed intake as controls. Notably, the performance of 17 and 15% broilers grown in cages (experiment 1) was significantly better compared to performance measured when the birds were kept on litter under standard conditions (experiment 2a). This can be partially explained by the differences in broiler motility and locomotion. The birds kept in cages allocated metabolized energy into growth and less into movement. Food conversion rate efficiency of hypoxia-treated chickens up to early marketing age of 35 d was as good as or better than that of controls. Energy consumption in general, and in the domestic fowl in particular, has been shown to be divided between maintenance and production. Therefore, lowering the demand for maintenance energy, while keeping total energy consumption relatively constant, is thought to likely increase the energy available for production. An alternative beneficial response might consist of a combination of reduced maintenance energy demand coupled with an overall reduction in energy consumption. In the present study, the hypoxia-treated chickens had consumed in general less feed while exhibiting a better FCR and an accelerated growth. Combined with the lower body temperature the hypoxia-treated chickens exhibited when exposed to acute heat stress and lower mortality, it can be suggested that hypoxic exposure during CAM development had lowered the demands for maintenance metabolic rates. In terms of relative weights, the breast was larger in treated chickens, with reduced abdominal fat, suggesting better energy balance towards growth rather than accumulation of fat (Piestun et al., 2013). This may indicate that embryonic angiogenesis following hypoxic treatment led to better vascularization and presumably better nutrient delivery to the breast (Hadad Cahaner and Halevy 2014). Thus, the food was utilized for growing tissue rather than fat accumulation. It has recently been shown that acute hypoxic incubation reduces left ventricle function in late-stage chickens by lowering the generation of pressure leading to relaxation; thus, cardiac efficiency is greatly reduced after hypoxic incubation (Jonker et al., 2015). A previous study on adult chickens that had been exposed to permanent hypoxia (15%) during embryonic development (E10-E20) showed that they displayed severe left ventricular dilatation in later life (Tintu et al., 2009). In the present study, 12 h/d exposure to hypoxia (15 and 17%) (E5-E12) caused enlargement of the heart at the age of 5 weeks. This enlargement may potentially confer an advantage under heat stress. P2renatal hypoxic chicken managed to cope with a hot environment better: They showed a significantly lower body temperature following a heat challenge and maintained their relative advantage for 5 hours. Furthermore, although the difference was not significant, treated chickens had lower mortality rates in response to heat stress than controls. It is well known that heat stress exerts a hard load on the cardiovascular system (Olkowski, 2007; Zhang et al., 2017). The increase in heart size that we observed following hypoxic exposure affects stroke volume and improves the delivery of oxygen to tissues. Modern poultry, with relative heart weights not exceeding 5% of overall BW, are the result of 30 yr of selection under the ideal environmental conditions provided in commercial farming. This small proportion of heart weight supports the hypothesis that selective pressures have failed to cause internal systems to adjust to fully meet the metabolic demands of modern poultry. Therefore, we could suggest that in this experiment heart enlargement, because of intermittent prenatal hypoxia, improved only the chicken's capacity to cope with severe environmental conditions. Chronic hypoxia was shown not only to affect the birth weight, growth, and FCR but also slightly affects the survival of broilers (Huang et al., 2017). Similar observations were made by Visschedijk (1985) who studied lowland chickens raised at high altitudes where inadequate O2 exchange resulted in hypoxic syndrome. Hypoxia has been shown to affect the survival rates of both embryos and lowland chickens at high altitudes (Hao et al., 2014). In a later study by the same group, the BW of broilers raised under chronic hypoxic conditions was significantly lower on d 14 (p = 0.04) compared to the control group. The effects of hypoxia on the broilers were progressive, culminating in a significant decrease in the average daily weight gain (p = 0.002) and a considerable increase in FCR on d 14 (p = 0.003) (Huang et al., 2017). No significant difference in the mortality rates of broilers exposed to hypoxia were noted during the rearing period (Huang et al., 2017), though hypoxia is known as a major risk factor for the death of broilers (Zhang et al., 2016). In contrast to those findings from broilers that were continuously under hypoxic conditions at high altitudes, in the present work, exposure of embryos to hypoxia intermittently during a short period of the embryonic development provided advantages in performance and FCR to broilers at the market age. Continuous exposure to hypoxic conditions by incubation at high altitude throughout embryonic development led to initial growth depression, resulting in a significantly lower BW up to 14 days. The difference in BW was observed at 28 and 42 d of age as well. However, no significant incubation effect on BW was found at later ages. Feed intake showed the same pattern of differences as for growth, resulting in no significant differences in FCR, regardless of the incubation altitude or rearing period. (Hassanzadeh et al., 2004). In contrast, we showed better feed utilization by hypoxia-treated chickens. CONCLUSION Our results indicate that intermittent exposure to moderate hypoxia (15 and 17% O2) during CAM development confers advantages to broilers in feed utilization efficiency and in coping with heat stress, and may be considered as a mitigating step for commercial growers to facilitate chickens in achieving their full growth potential. ACKNOWLEDGMENTS This research was funded by grant No. 356-0677 from the Egg and Poultry Board of Israel. The authors wish to thank S. Oblezin and A. Kantor from the Volcani Center Chicken facilities for technical assistance. REFERENCES Burggren W. W., Mueller C. A.. 2015. Developmental critical windows and sensitive periods as three-dimensional constructs in time and space. 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Poultry ScienceOxford University Press

Published: Mar 8, 2018

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