Genetic parameters of feed efficiency traits and their relationships with egg quality traits in laying period of ducks

Genetic parameters of feed efficiency traits and their relationships with egg quality traits in... Abstract The objective of this study was to estimate genetic parameters for feed efficiency and relevant traits in 2 laying duck breeds, and to determine the relationship of residual feed intake (RFI) with feed efficiency and egg quality traits. Phenotypic records on 3,000 female laying ducks (1,500 Shaoxing ducks and 1,500 Jinyun ducks) from a random mating population were used to estimate genetic parameters for RFI, feed conversion ratio (FCR), feed intake (FI), BW, BW gain (BWG), and egg mass laid (EML) at 42 to 46 wk of age. The heritability estimates for EML, FCR, FI, and RFI were 0.22, 0.19, 0.22, and 0.27 in Shaoxing ducks and 0.14, 0.19, 0.24, and 0.24 for Jinyun ducks, respectively. RFI showed high and positive genetic correlations with FCR (0.47 in Shaoxing ducks and 0.63 in Jinyun ducks) and FI (0.79 in Shaoxing ducks and 0.86 in Jinyun ducks). No correlations were found in RFI with BW, BWG, or EML at either genetic or phenotypic level. FCR was strongly and negatively correlated with EML (−0.81 and −0.68) but inconsistently correlated with FI (0.02 and 0.17), suggesting that EML was the main influence on FCR. In addition, no significant differences were found between low RFI (LRFI) and high RFI (HRFI) ducks in egg shape index, shell thickness, shell strength, yolk color, albumen height, or Haugh unit (HU). The results indicate that selection for LRFI could improve feed efficiency and reduce FI without significant changes in EML or egg quality. INTRODUCTION Feed represents two-thirds of the total costs of poultry production, especially in developing countries. Improvement in feed efficiency would reduce the amount of feed required for production (growth or laying), the production cost, and the amount of nitrogenous waste. The most commonly used measures are feed conversion ratio (FCR) and residual feed intake (RFI) (Koch et al., 1963; Case et al., 2012). FCR is defined as the ratio of feed intake (FI) per unit of egg mass in egg-type ducks. However, selection for FCR can lead to unexpected consequences because the genetic improvement is complex and cannot accommodate the required differential economic weighting between FI and egg mass (Luiting and Urff, 1991a; Lin and Aggrey, 2013). RFI represents the amount of feed consumed that is not accounted for by the expected requirements of production (e.g., milk and egg production or body weight gain) and body weight maintenance (Kennedy et al., 1993; Willems et al., 2013). RFI is a fraction of total FI phenotypically unlinked to maintenance and production requirements. RFI is a heritable feed efficiency trait that allows an animal to be ranked based on its individual FI, which is independent of its production traits (Herd and Arthur, 2009). In particular, selection for low RFI animals might be helpful to reduce feed cost and nitrogenous waste, and minimize the environmental footprint of animal production (Moore et al., 2009). The effectiveness of selection for RFI has been fully demonstrated in mammals (Cruz et al., 2010; Durunna, et al., 2011) and avian breeds (Gilbert et al., 2007; Aggrey et al., 2010; Berry and Crowley, 2012). In egg-type poultry, more estimates for RFI can be found in the literature, especially for laying chickens (Schulman et al., 1994; Yuan et al., 2015). These studies showed that heritabilities for RFI were moderate to high, ranging from 0.25 to 0.60. As for laying ducks, 2 papers have been found on RFI estimates so far (Basso et al., 2012; Zeng et al., 2016). The heritabilities for RFI were 0.24 and 0.26, respectively. However, the experiment sample numbers in these studies were few. A greater sample size conducts a more comprehensive data analysis, including estimation of heritability, and genetic correlations are necessary for selection for RFI in laying ducks. Therefore, the objective of the current study was to estimate genetic parameters for feed efficiency and relevant traits in 2 laying duck breeds and to determine the relationship of RFI with feed efficiency and egg quality traits. MATERIALS AND METHODS The animal care and use protocol was approved by the Institutional Animal Care and Use Committee of the Zhejiang Academy of Agricultural Sciences and performed in accordance with the Regulations for the Administration of Affairs Concerning Experimental Animals. Population Studies and Management Shaoxing duck is a Chinese dominant layer breed that is recognized for its high ratio of blue eggs, whereas Jinyun duck is an indigenous breed that is recognized for its prematurity. Thirty-five F1 males and 380 F1 females from Shaoxing ducks and 40 F1 males and 395 F1 females from Jinyun ducks were randomly selected to produce the F2 generation, yielding a total of about 3,000 female laying ducks, including 1,500 Shaoxing ducks and 1,500 Jinyun ducks. All F2 birds were raised at Hubei ShenDan health food co., LTD. At 12 wk of age, ducks were placed in individual cages with ad libitum food and water. The feeding trial was conducted from 42 to 46 wk of age, since this corresponds to the peak period of egg production and is the standard time period used to assess feed efficiency. During this period, birds were fed standard commercial diets, as shown in Table 1. Table 1. Composition and main characteristics of the basal diet. Ingredients  Content (g/kg)  Nutrient  Content (g/kg)  Maize grain  400  Metabolizable energy  11.2b  Wheat  290  Crude protein  16.5  Soybean meal  120  Total phosphorus  0.70  Wheat bran  90  Total calcium  3.35  Calcium hydrophosphate  12  Total lysine  0.79  Stone powder  80  Total methionine  0.40  Salt  3  Ether extract  29.0  Premixa  5      Ingredients  Content (g/kg)  Nutrient  Content (g/kg)  Maize grain  400  Metabolizable energy  11.2b  Wheat  290  Crude protein  16.5  Soybean meal  120  Total phosphorus  0.70  Wheat bran  90  Total calcium  3.35  Calcium hydrophosphate  12  Total lysine  0.79  Stone powder  80  Total methionine  0.40  Salt  3  Ether extract  29.0  Premixa  5      aSupplied per kg of diet: vitamin A 1500 U, cholecalciferol 200 U, vitamin E (DL-α-tocopheryl acetate) 10 U, riboflavin 3.5 mg, pantothenic acid 10 mg, niacin 30 mg, cobalamin 10 μg, choline chloride 1000 mg, biotin 0.15 mg, folic acid 0.5 mg, thiamine 1.5 mg, pyridoxine 3.0 mg, Fe 80 mg, Zn 40 mg, Mn 60 mg, I 0.18 mg, Cu 8 mg, Se 0.3 mg. bUnit: MJ/kg. View Large Data Analysis The FI was defined as the amount of distributed feed at the beginning of the wk minus the remaining uneaten feed at the end of the week. Then the total FI for one wk was transformed into the daily FI for each duck in the testing period. Egg mass laid (EML), body weight (BW, average weight in the testing period), and BW gain (BWG, difference between BW at the end of the test period and at the beginning of the test period) were measured. FCR was calculated as a ratio of daily FI and daily egg mass. We estimated the RFI as a linear function of FI and its outputs, such as EML, BW, ΔW, and maintenance requirements (BW0.75) (Luiting and Urff, 1991b):   \begin{equation*} {\rm{FI = \mu + a \times B}}{{\rm{W}}^{{\rm{0}}{\rm{.75}}}}{\rm{ + b \times BWG + c \times EML + d}} \end{equation*} Where: a, b, and c are partial regression coefficients, μ is the intercept. The variable d is the RFI, which is the residual of the previous equation. In addition, poor ducks (egg damaged or food-wasting) were excluded before calculating RFI. Genetic Analyses The genetic parameters for feed efficiency traits were estimated using ASREML software (Gilmour et al., 2009). The model for all traits was:   \begin{equation*}{{\rm{y}}_{{\rm{ij}}}}{\rm{ = \mu + Hatc}}{{\rm{h}}_{\rm{i}}}\,{\rm{ + }}\,{{\rm{a}}_{\rm{j}}}\,{\rm{ + }}\,{{\rm{e}}_{{\rm{ij}}}}\end{equation*} Where: y is the phenotypic value (BW, BWG, EML, FI, FCR, RFI) of the animal; u is the intercept; Hatchi is the fixed effect of hatch; aj represents the random direct additive genetic effect of individual j; and eij is the random residual effect. Heritability estimates were calculated based on a single trait model. As for the genetic correlations among all feed efficiency traits, 3-trait analyses were performed, including the selection criteria as for bivariate analysis. Egg Quality Analysis After the feeding trial, we immediately calculated RFI based on the equations above and then selected 100 high RFI ducks (HRFI) and 100 low RFI ducks (LRFI), which were significantly different in RFI from Shaoxing ducks and Jinyun ducks, respectively. Over the next wk, we collected the eggs of these selected ducks for egg quality analysis. All eggs were individually weighed. The width and length (cm) of each egg were measured using vernier calipers, and the shape index was calculated as the ratio between egg width and egg length, as a percentage. Shell strength (kg/cm2) of non-cracked eggs was measured using an EFG-0503 eggshell strength gauge (Robotmation, Tokyo, Japan). Shell thickness (mm) was measured using an ETG-1061 eggshell thickness gauge (Robotmation). Yolk color, albumen height, and Haugh unit (HU) were used to define internal egg quality. These parameters were measured using an EMT-5200 egg quality gauge (Robotmation). RESULTS Phenotypic Aspects of RFI and its Components Descriptive statistics of feed efficiency traits are summarized in Table 2. It should be noted that lower FCR and RFI indicate greater efficiency. During the test period, RFI values were null 0 ± 16.0 and 0 ± 14.0, and the average FCR were 3.0 ± 0.7 and 2.6 ± 0.3 in Shaoxing ducks and Jinyun ducks, respectively. The mean FI value was 180.4 (172.2) g/d, whereas the EML and BW were approximately 58.9 (67.5) g/d and 1.46 (1.35) kg, respectively. Daily BWG were very small, and eventually close to zero (5.2 and 1.5 g/d). On average, the HRFI group had higher RFI, FI, and FCR, indicating that HRFI ducks consumed significantly more feed to achieve a similar gain as LRFI ducks. Table 2. Descriptive statistics for feed efficiency traits in 2 duck breeds. Breeds  Traitsa  n  Mean ± SD  Shaoxing duck  FI, (g/d)  1464  180.4 ± 23.5    EML, (g/d)  1449  58.9 ± 17.2    BW, (g)  1472  1458.2 ± 235.1    BWG, (g/d)  1464  5.2 ± 4.8    FCR (g/g)  1421  3.0 ± 0.7    RFI, (g/d)  1421  0.0 ± 16.0  Jinyun duck  FI, (g/d)  1468  172.2 ± 15.1    EML, (g/d)  1456  67.5 ± 7.1    BW, (g)  1480  1350.1 ± 123.6    BWG, (g/d)  1460  1.5 ± 3.8    FCR (g/g)  1442  2.6 ± 0.3    RFI, (g/d)  1442  0.0 ± 14.0  Breeds  Traitsa  n  Mean ± SD  Shaoxing duck  FI, (g/d)  1464  180.4 ± 23.5    EML, (g/d)  1449  58.9 ± 17.2    BW, (g)  1472  1458.2 ± 235.1    BWG, (g/d)  1464  5.2 ± 4.8    FCR (g/g)  1421  3.0 ± 0.7    RFI, (g/d)  1421  0.0 ± 16.0  Jinyun duck  FI, (g/d)  1468  172.2 ± 15.1    EML, (g/d)  1456  67.5 ± 7.1    BW, (g)  1480  1350.1 ± 123.6    BWG, (g/d)  1460  1.5 ± 3.8    FCR (g/g)  1442  2.6 ± 0.3    RFI, (g/d)  1442  0.0 ± 14.0  aFI = Feed intake; EML = Daily egg mass; BW = Body weight; BWG = Body weight gain; FCR = Feed conversion ratio; RFI = Residual feed intake. View Large Genetic Parameters Estimated genetic parameters and phenotypic correlations for feed efficiency and egg quality traits are presented in Tables 2, 3 and 4. The heritability estimates for BW, BWG, EML, FCR, FI, and RFI were 0.36, 0.03, 0.22, 0.19, 0.22, and 0.27 in Shaoxing ducks and 0.41, −0.02, 0.14, 0.19, 0.24, and 0.24 for Jinyun ducks, respectively. It was shown that the heritability estimates for feed efficiency traits in 2 duck breeds were similar to each other. The estimated genetic and phenotypic correlations between RFI and FCR in Jinyun ducks (0.63 and 0.59) were slightly higher than those estimated in Shaoxing ducks (0.47 and 0.36). The estimated genetic and phenotypic correlations between RFI and FI showed similar results in these 2 duck breeds. No correlations were found in RFI with BW, BWG, or EML at either genetic or phenotypic level. EML were strongly negative with FCR (−0.73 and −0.63) and positive with FI (0.62 and 0.46) in these 2 laying duck breeds. In addition, there is a positive correlation between FI and BWG. Table 3. Heritabilities (on diagonal), phenotypic (above diagonal), and genetic (below diagonal) correlations, with standard errors (in brackets) for feed efficiency traits in Shaoxing duck. Traitsa  BW  BWG  EML  FCR  FI  RFI  BW  0.36 (0.05)  0.09 (0.04)  0.08 (0.03)  0.01 (0.02)  0.45 (0.07)  −0.09 (0.03)  BWG  −0.04 (0.06)  0.03 (0.01)  0.34 (0.05)  −0.15 (0.03)  0.24 (0.07)  −0.22 (0.13)  EML  0.03 (0.05)  0.51 (0.15)  0.22 (0.06)  −0.73 (0.04)  0.62 (0.06)  0.00 (0.02)  FCR  −0.07 (0.10)  −0.24 (0.09)  −0.81 (0.07)  0.19 (0.05)  −0.06 (0.01)  0.36 (0.07)  FI  0.51 (0.14)  0.16 (0.07)  0.67 (0.14)  0.02 (0.11)  0.22 (0.06)  0.59 (0.10)  RFI  −0.12 (0.10)  −0.21 (0.14)  −0.03 (0.06)  0.47 (0.16)  0.79 (0.05)  0.27 (0.06)  Traitsa  BW  BWG  EML  FCR  FI  RFI  BW  0.36 (0.05)  0.09 (0.04)  0.08 (0.03)  0.01 (0.02)  0.45 (0.07)  −0.09 (0.03)  BWG  −0.04 (0.06)  0.03 (0.01)  0.34 (0.05)  −0.15 (0.03)  0.24 (0.07)  −0.22 (0.13)  EML  0.03 (0.05)  0.51 (0.15)  0.22 (0.06)  −0.73 (0.04)  0.62 (0.06)  0.00 (0.02)  FCR  −0.07 (0.10)  −0.24 (0.09)  −0.81 (0.07)  0.19 (0.05)  −0.06 (0.01)  0.36 (0.07)  FI  0.51 (0.14)  0.16 (0.07)  0.67 (0.14)  0.02 (0.11)  0.22 (0.06)  0.59 (0.10)  RFI  −0.12 (0.10)  −0.21 (0.14)  −0.03 (0.06)  0.47 (0.16)  0.79 (0.05)  0.27 (0.06)  aBW = Body weight; BWG = Body weight gain; EML = Daily egg mass; FCR = Feed conversion ratio; FI = Feed intake; RFI = Residual feed intake. View Large Table 4. Heritabilities (on diagonal), phenotypic (above diagonal), and genetic (below diagonal) correlations, with standard errors (in brackets) for feed efficiency traits in Jinyun duck. Traitsa  BW  BWG  EML  FCR  FI  RFI  BW  0.41 (0.10)  −0.01 (0.02)  0.34 (0.06)  −0.01(0.05)  0.33 (0.07)  −0.11 (0.10)  BWG  0.02 (0.03)  −0.02 (0.03)  −0.06 (0.04)  0.10 (0.03)  0.05 (0.04)  −0.13 (0.09)  EML  0.24 (0.07)  −0.08 (0.03)  0.14 (0.05)  −0.63 (0.06)  0.46 (0.10)  0.06 (0.04)  FCR  −0.04 (0.06)  0.06 (0.05)  −0.68 (0.21)  0.19 (0.07)  0.20 (0.05)  0.59 (0.06)  FI  0.36 (0.12)  0.01 (0.03)  0.51 (0.05)  0.17 (0.13)  0.24 (0.06)  0.89 (0.05)  RFI  −0.10 (0.07)  −0.15 (0.07)  0.02 (0.04)  0.63 (0.06)  0.86 (0.04)  0.24 (0.05)  Traitsa  BW  BWG  EML  FCR  FI  RFI  BW  0.41 (0.10)  −0.01 (0.02)  0.34 (0.06)  −0.01(0.05)  0.33 (0.07)  −0.11 (0.10)  BWG  0.02 (0.03)  −0.02 (0.03)  −0.06 (0.04)  0.10 (0.03)  0.05 (0.04)  −0.13 (0.09)  EML  0.24 (0.07)  −0.08 (0.03)  0.14 (0.05)  −0.63 (0.06)  0.46 (0.10)  0.06 (0.04)  FCR  −0.04 (0.06)  0.06 (0.05)  −0.68 (0.21)  0.19 (0.07)  0.20 (0.05)  0.59 (0.06)  FI  0.36 (0.12)  0.01 (0.03)  0.51 (0.05)  0.17 (0.13)  0.24 (0.06)  0.89 (0.05)  RFI  −0.10 (0.07)  −0.15 (0.07)  0.02 (0.04)  0.63 (0.06)  0.86 (0.04)  0.24 (0.05)  aBW = Body weight; BWG = Body weight gain; EML = Daily egg mass; FCR = Feed conversion ratio; FI = Feed intake; RFI = Residual feed intake. View Large Phenotypic Aspects of Egg Quality To assess the relationship between RFI status and egg quality, external and internal egg characteristics were compared between HRFI and LRFI ducks in 2 duck breeds. The results showed that no remarkable difference in average egg mass were found between these groups (P = 0.69 and 0.73). No significant differences between HRFI and LRFI ducks were observed with respect to measures of external egg quality, such as egg shape index, shell thickness, and shell strength (P > 0.05; Table 5). Likewise, the internal quality parameters of eggs were unaffected by RFI classification, since yolk color (P = 0.78 and 0.37), albumen height (P = 0.15 and 0.20), and HU (P = 0.32 and 0.48) did not differ between HRFI and LRFI ducks. Table 5. Descriptive statistics for egg quality traits in 2 duck breeds. Breeds  Traits  HRFIa  LRFI  P-value  Shaoxing duck  Average egg mass (g)  72.30 ± 5.13  73.07 ± 5.37  0.69    Egg shape index (%)  1.35 ± 0.07  1.31 ± 0.03  0.12    Shell thickness (mm)  0.48 ± 0.05  0.46 ± 0.05  0.96    Shell strength (kg/cm2)  5.00 ± 0.59  4.92 ± 0.73  0.63    Yolk color (level)  11.57 ± 0.54  11.70 ± 0.27  0.78    Albumen height (mm)  6.23 ± 0.73  6.72 ± 1.26  0.15    Haugh unit (HU)  74.57 ± 6.11  76.68 ± 8.17  0.32  Jinyun duck  Average egg mass (g)  73.12 ± 5.76  75.38 ± 6.82  0.73    Egg shape index (%)  1.36 ± 0.08  1.34 ± 0.05  0.21    Shell thickness (mm)  0.52 ± 0.06  0.51 ± 0.08  0.53    Shell strength (kg/cm2)  5.19 ± 0.42  4.93 ± 0.67  0.19    Yolk color (level)  11.88 ± 0.43  11.71 ± 0.33  0.37    Albumen height (mm)  6.42 ± 0.67  6.82 ± 1.19  0.20    Haugh unit (HU)  75.13 ± 4.52  76.92 ± 8.31  0.48  Breeds  Traits  HRFIa  LRFI  P-value  Shaoxing duck  Average egg mass (g)  72.30 ± 5.13  73.07 ± 5.37  0.69    Egg shape index (%)  1.35 ± 0.07  1.31 ± 0.03  0.12    Shell thickness (mm)  0.48 ± 0.05  0.46 ± 0.05  0.96    Shell strength (kg/cm2)  5.00 ± 0.59  4.92 ± 0.73  0.63    Yolk color (level)  11.57 ± 0.54  11.70 ± 0.27  0.78    Albumen height (mm)  6.23 ± 0.73  6.72 ± 1.26  0.15    Haugh unit (HU)  74.57 ± 6.11  76.68 ± 8.17  0.32  Jinyun duck  Average egg mass (g)  73.12 ± 5.76  75.38 ± 6.82  0.73    Egg shape index (%)  1.36 ± 0.08  1.34 ± 0.05  0.21    Shell thickness (mm)  0.52 ± 0.06  0.51 ± 0.08  0.53    Shell strength (kg/cm2)  5.19 ± 0.42  4.93 ± 0.67  0.19    Yolk color (level)  11.88 ± 0.43  11.71 ± 0.33  0.37    Albumen height (mm)  6.42 ± 0.67  6.82 ± 1.19  0.20    Haugh unit (HU)  75.13 ± 4.52  76.92 ± 8.31  0.48  aValues are expressed as mean ± SD. View Large DISCUSSION China owns the biggest duck industry in the world, accounting for approximately 70% of global duck breeding and consumption every year. The common duck is bred mainly to produce eggs for human consumption and as a source of meat when at an old age. Therefore, the selective breeding of these waterfowl focuses on their reproductive traits, such as egg production, egg mass, etc. Recently, there has been rapid development in breeding and production techniques. Additionally, the differences among some species used in agriculture have increased. Differences in selection processes and management conditions lead to differences in production and other variations among animals. Significant advances in feed efficiency have been achieved, as increased feed efficiency represents more profit for the farmers and has the potential to reduce the amount of pollutants released into the environment (Bezerra et al., 2013). Among the number of alternatives, RFI was initially proposed by Koch et al. (1963) as a feed efficiency parameter and has been studied recently for its potential to increase production efficiency. In the current study, we estimate genetic parameters for RFI and relevant traits in 2 laying duck breeds, and to determine the relationship of RFI with feed efficiency and egg quality traits. The heritability estimates for RFI were 0.27 and 0.24, respectively, in our study populations of Shaoxing and Jinyun ducks, which is situated in the large range of published estimations for laying hens (0.20 to 0.30; Aggrey et al., 2010; Yuan et al., 2015). As for laying ducks, 2 papers have been found on RFI estimates so far (Basso et al., 2012; Zeng et al., 2016). The heritabilities for RFI were 0.24 and 0.26, respectively. Our results were consistent with these studies and confirmed that RFI is selectable in laying ducks. The heritability estimates for FI (0.22 and 0.24) in our study agreed well with the previous results in laying chickens and broilers (Gaya et al., 2006; Yuan et al., 2015), but lower than those in laying ducks (Basso et al., 2012), which showed the heritability estimates for FI was 0.34. The high heritability values for BW (0.36 and 0.41), as well as the low heritability values for BWG (0.03 and −0.02), were in agreement with the estimations of Yuan et al., (2015) and Basso et al., (2012). However, our findings diverged with those reported by Case et al., (2012) and Zhang et al., (2017), which showed moderate estimated heritability for BW and BWG in turkeys and Pekin ducks. In addition, our heritability estimations for EML were 0.22 and 0.14 in Shaoxing ducks and Jinyun ducks, which was much lower than those in Rhode Island Red chickens (0.69; Tixier-Boichard et al., 1995). Nevertheless, Luiting and Urff (1991b) showed that heritability estimation for EML in White Leghorn hens was less than 0.30, and decreased with age. The experimental approach utilized in the current study involved ranking ducks by RFI to create distinct groups consisting of low and high RFI ducks. This approach has been used successfully by others, and the spread in RFI values between the LRFI and HRFI groups in the current study was greater than in previous studies (Baker et al., 2006; Nkrumah et al., 2007; Drouilhet et al., 2016). RFI was strongly and positively correlated with FI and FCR, meaning that LRFI ducks consumed less feed and had better feed conversion than did the HRFI ducks. Our results were consistent with previous studies by Yuan et al., (2015) and Zeng et al., (2016). These results show that the more efficient an animal is, the less it eats. Estimation of genetic correlation between RFI and EML were close to zero, indicating that egg production was not a contributor to variation in RFI. That is of great interest, as a selection based on RFI would decrease FI without modifying the genetic level of EML. However, we found that FCR was strongly and negatively correlated with EML but inconsistently correlated with FI, suggesting that EML was the main influence on FCR. Selection for decreased FCR could result in increased EML, which may have a negative effect in egg sales. RFI was weakly negatively correlated with BWG, which is consistent with previous studies by Yuan et al., (2015) and Basso et al., (2012), indicating that some ducks may consume their BG for laying eggs during the peak of the laying period. Egg quality plays an important role in the egg industry both for producers and consumers. Some shell quality traits such as shell strength and shell thickness have an importance value from an economic point of view, since broken eggs (6 to 8%) are discarded, resulting in money loss (Coucke et al., 1999; Ahammed et al., 2014). Shell properties are crucial also for shelf life and safety of eggs and egg products (Reu de et al., 2005). Meanwhile, albumen properties have an influence on the preservation performance of eggs and egg product quality. In the current study, there were no differences between LRFI and HRFI ducks in egg shape index, shell thickness, shell strength, yolk color, albumen height, or HU, indicating that selection for RFI could not change duck egg quality. Up to now, we are unaware of any research about the relationship between RFI and egg quality. This study showed for the first time that there were no differences between RFI and egg quality. These results are consistent with several studies that did not find a significant correlation between RFI and carcass merit and meat quality in cattle (Baker et al., 2006; Perkins et al., 2014). However, the relationship between RFI and egg quality is complicated and multifactorial. In the current study, the genetic correlations between RFI and egg quality are not estimate because the small sample of egg and economic loss. Further investigation will be addressed to assess the genetic correlations between RFI and egg quality based on a large sample size. In conclusion, this study evaluated feed efficiency and its component traits in the laying period of ducks and found that the heritability estimate of RFI is moderate and positively correlated with FI and FCR. As found in other studies, this result confirmed that RFI is selectable in laying ducks. Meanwhile, selection for LRFI could reduce FI without significant changes in EML and egg quality. Our results provide valuable insights into genetic evaluation of feed efficiency traits in the laying period and could help in drawing up selection programs for improvement of feed efficiency in egg-type ducks. ACKNOWLEDGEMENTS The authors acknowledge Hubei ShenDan health food co., LTD. staff for their technical work on the experiments. The authors would also like to thank the reviewers for helpful comments and insightful contributions that have led to the final version of this paper. This work was sponsored by the earmarked fund for National Waterfowl-industry Technology Research System (CARS-42-6), New Variety Breeding of Livestock and Poultry (2016C02054-12) and The Open Project of Key Laboratory of Animal Embryo Engineering and Molecular Breeding of Hubei Province (KLAEMB-2016-04). REFERENCES Aggrey S. E., Karnuah A. B., Sebastian B., Anthony N. B.. 2010. Genetic properties of feed efficiency parameters in meat-type chickens. Genet. Sel. Evol . 42: 25. 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Genetic parameters of feed efficiency traits and their relationships with egg quality traits in laying period of ducks

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© 2018 Poultry Science Association Inc.
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Abstract

Abstract The objective of this study was to estimate genetic parameters for feed efficiency and relevant traits in 2 laying duck breeds, and to determine the relationship of residual feed intake (RFI) with feed efficiency and egg quality traits. Phenotypic records on 3,000 female laying ducks (1,500 Shaoxing ducks and 1,500 Jinyun ducks) from a random mating population were used to estimate genetic parameters for RFI, feed conversion ratio (FCR), feed intake (FI), BW, BW gain (BWG), and egg mass laid (EML) at 42 to 46 wk of age. The heritability estimates for EML, FCR, FI, and RFI were 0.22, 0.19, 0.22, and 0.27 in Shaoxing ducks and 0.14, 0.19, 0.24, and 0.24 for Jinyun ducks, respectively. RFI showed high and positive genetic correlations with FCR (0.47 in Shaoxing ducks and 0.63 in Jinyun ducks) and FI (0.79 in Shaoxing ducks and 0.86 in Jinyun ducks). No correlations were found in RFI with BW, BWG, or EML at either genetic or phenotypic level. FCR was strongly and negatively correlated with EML (−0.81 and −0.68) but inconsistently correlated with FI (0.02 and 0.17), suggesting that EML was the main influence on FCR. In addition, no significant differences were found between low RFI (LRFI) and high RFI (HRFI) ducks in egg shape index, shell thickness, shell strength, yolk color, albumen height, or Haugh unit (HU). The results indicate that selection for LRFI could improve feed efficiency and reduce FI without significant changes in EML or egg quality. INTRODUCTION Feed represents two-thirds of the total costs of poultry production, especially in developing countries. Improvement in feed efficiency would reduce the amount of feed required for production (growth or laying), the production cost, and the amount of nitrogenous waste. The most commonly used measures are feed conversion ratio (FCR) and residual feed intake (RFI) (Koch et al., 1963; Case et al., 2012). FCR is defined as the ratio of feed intake (FI) per unit of egg mass in egg-type ducks. However, selection for FCR can lead to unexpected consequences because the genetic improvement is complex and cannot accommodate the required differential economic weighting between FI and egg mass (Luiting and Urff, 1991a; Lin and Aggrey, 2013). RFI represents the amount of feed consumed that is not accounted for by the expected requirements of production (e.g., milk and egg production or body weight gain) and body weight maintenance (Kennedy et al., 1993; Willems et al., 2013). RFI is a fraction of total FI phenotypically unlinked to maintenance and production requirements. RFI is a heritable feed efficiency trait that allows an animal to be ranked based on its individual FI, which is independent of its production traits (Herd and Arthur, 2009). In particular, selection for low RFI animals might be helpful to reduce feed cost and nitrogenous waste, and minimize the environmental footprint of animal production (Moore et al., 2009). The effectiveness of selection for RFI has been fully demonstrated in mammals (Cruz et al., 2010; Durunna, et al., 2011) and avian breeds (Gilbert et al., 2007; Aggrey et al., 2010; Berry and Crowley, 2012). In egg-type poultry, more estimates for RFI can be found in the literature, especially for laying chickens (Schulman et al., 1994; Yuan et al., 2015). These studies showed that heritabilities for RFI were moderate to high, ranging from 0.25 to 0.60. As for laying ducks, 2 papers have been found on RFI estimates so far (Basso et al., 2012; Zeng et al., 2016). The heritabilities for RFI were 0.24 and 0.26, respectively. However, the experiment sample numbers in these studies were few. A greater sample size conducts a more comprehensive data analysis, including estimation of heritability, and genetic correlations are necessary for selection for RFI in laying ducks. Therefore, the objective of the current study was to estimate genetic parameters for feed efficiency and relevant traits in 2 laying duck breeds and to determine the relationship of RFI with feed efficiency and egg quality traits. MATERIALS AND METHODS The animal care and use protocol was approved by the Institutional Animal Care and Use Committee of the Zhejiang Academy of Agricultural Sciences and performed in accordance with the Regulations for the Administration of Affairs Concerning Experimental Animals. Population Studies and Management Shaoxing duck is a Chinese dominant layer breed that is recognized for its high ratio of blue eggs, whereas Jinyun duck is an indigenous breed that is recognized for its prematurity. Thirty-five F1 males and 380 F1 females from Shaoxing ducks and 40 F1 males and 395 F1 females from Jinyun ducks were randomly selected to produce the F2 generation, yielding a total of about 3,000 female laying ducks, including 1,500 Shaoxing ducks and 1,500 Jinyun ducks. All F2 birds were raised at Hubei ShenDan health food co., LTD. At 12 wk of age, ducks were placed in individual cages with ad libitum food and water. The feeding trial was conducted from 42 to 46 wk of age, since this corresponds to the peak period of egg production and is the standard time period used to assess feed efficiency. During this period, birds were fed standard commercial diets, as shown in Table 1. Table 1. Composition and main characteristics of the basal diet. Ingredients  Content (g/kg)  Nutrient  Content (g/kg)  Maize grain  400  Metabolizable energy  11.2b  Wheat  290  Crude protein  16.5  Soybean meal  120  Total phosphorus  0.70  Wheat bran  90  Total calcium  3.35  Calcium hydrophosphate  12  Total lysine  0.79  Stone powder  80  Total methionine  0.40  Salt  3  Ether extract  29.0  Premixa  5      Ingredients  Content (g/kg)  Nutrient  Content (g/kg)  Maize grain  400  Metabolizable energy  11.2b  Wheat  290  Crude protein  16.5  Soybean meal  120  Total phosphorus  0.70  Wheat bran  90  Total calcium  3.35  Calcium hydrophosphate  12  Total lysine  0.79  Stone powder  80  Total methionine  0.40  Salt  3  Ether extract  29.0  Premixa  5      aSupplied per kg of diet: vitamin A 1500 U, cholecalciferol 200 U, vitamin E (DL-α-tocopheryl acetate) 10 U, riboflavin 3.5 mg, pantothenic acid 10 mg, niacin 30 mg, cobalamin 10 μg, choline chloride 1000 mg, biotin 0.15 mg, folic acid 0.5 mg, thiamine 1.5 mg, pyridoxine 3.0 mg, Fe 80 mg, Zn 40 mg, Mn 60 mg, I 0.18 mg, Cu 8 mg, Se 0.3 mg. bUnit: MJ/kg. View Large Data Analysis The FI was defined as the amount of distributed feed at the beginning of the wk minus the remaining uneaten feed at the end of the week. Then the total FI for one wk was transformed into the daily FI for each duck in the testing period. Egg mass laid (EML), body weight (BW, average weight in the testing period), and BW gain (BWG, difference between BW at the end of the test period and at the beginning of the test period) were measured. FCR was calculated as a ratio of daily FI and daily egg mass. We estimated the RFI as a linear function of FI and its outputs, such as EML, BW, ΔW, and maintenance requirements (BW0.75) (Luiting and Urff, 1991b):   \begin{equation*} {\rm{FI = \mu + a \times B}}{{\rm{W}}^{{\rm{0}}{\rm{.75}}}}{\rm{ + b \times BWG + c \times EML + d}} \end{equation*} Where: a, b, and c are partial regression coefficients, μ is the intercept. The variable d is the RFI, which is the residual of the previous equation. In addition, poor ducks (egg damaged or food-wasting) were excluded before calculating RFI. Genetic Analyses The genetic parameters for feed efficiency traits were estimated using ASREML software (Gilmour et al., 2009). The model for all traits was:   \begin{equation*}{{\rm{y}}_{{\rm{ij}}}}{\rm{ = \mu + Hatc}}{{\rm{h}}_{\rm{i}}}\,{\rm{ + }}\,{{\rm{a}}_{\rm{j}}}\,{\rm{ + }}\,{{\rm{e}}_{{\rm{ij}}}}\end{equation*} Where: y is the phenotypic value (BW, BWG, EML, FI, FCR, RFI) of the animal; u is the intercept; Hatchi is the fixed effect of hatch; aj represents the random direct additive genetic effect of individual j; and eij is the random residual effect. Heritability estimates were calculated based on a single trait model. As for the genetic correlations among all feed efficiency traits, 3-trait analyses were performed, including the selection criteria as for bivariate analysis. Egg Quality Analysis After the feeding trial, we immediately calculated RFI based on the equations above and then selected 100 high RFI ducks (HRFI) and 100 low RFI ducks (LRFI), which were significantly different in RFI from Shaoxing ducks and Jinyun ducks, respectively. Over the next wk, we collected the eggs of these selected ducks for egg quality analysis. All eggs were individually weighed. The width and length (cm) of each egg were measured using vernier calipers, and the shape index was calculated as the ratio between egg width and egg length, as a percentage. Shell strength (kg/cm2) of non-cracked eggs was measured using an EFG-0503 eggshell strength gauge (Robotmation, Tokyo, Japan). Shell thickness (mm) was measured using an ETG-1061 eggshell thickness gauge (Robotmation). Yolk color, albumen height, and Haugh unit (HU) were used to define internal egg quality. These parameters were measured using an EMT-5200 egg quality gauge (Robotmation). RESULTS Phenotypic Aspects of RFI and its Components Descriptive statistics of feed efficiency traits are summarized in Table 2. It should be noted that lower FCR and RFI indicate greater efficiency. During the test period, RFI values were null 0 ± 16.0 and 0 ± 14.0, and the average FCR were 3.0 ± 0.7 and 2.6 ± 0.3 in Shaoxing ducks and Jinyun ducks, respectively. The mean FI value was 180.4 (172.2) g/d, whereas the EML and BW were approximately 58.9 (67.5) g/d and 1.46 (1.35) kg, respectively. Daily BWG were very small, and eventually close to zero (5.2 and 1.5 g/d). On average, the HRFI group had higher RFI, FI, and FCR, indicating that HRFI ducks consumed significantly more feed to achieve a similar gain as LRFI ducks. Table 2. Descriptive statistics for feed efficiency traits in 2 duck breeds. Breeds  Traitsa  n  Mean ± SD  Shaoxing duck  FI, (g/d)  1464  180.4 ± 23.5    EML, (g/d)  1449  58.9 ± 17.2    BW, (g)  1472  1458.2 ± 235.1    BWG, (g/d)  1464  5.2 ± 4.8    FCR (g/g)  1421  3.0 ± 0.7    RFI, (g/d)  1421  0.0 ± 16.0  Jinyun duck  FI, (g/d)  1468  172.2 ± 15.1    EML, (g/d)  1456  67.5 ± 7.1    BW, (g)  1480  1350.1 ± 123.6    BWG, (g/d)  1460  1.5 ± 3.8    FCR (g/g)  1442  2.6 ± 0.3    RFI, (g/d)  1442  0.0 ± 14.0  Breeds  Traitsa  n  Mean ± SD  Shaoxing duck  FI, (g/d)  1464  180.4 ± 23.5    EML, (g/d)  1449  58.9 ± 17.2    BW, (g)  1472  1458.2 ± 235.1    BWG, (g/d)  1464  5.2 ± 4.8    FCR (g/g)  1421  3.0 ± 0.7    RFI, (g/d)  1421  0.0 ± 16.0  Jinyun duck  FI, (g/d)  1468  172.2 ± 15.1    EML, (g/d)  1456  67.5 ± 7.1    BW, (g)  1480  1350.1 ± 123.6    BWG, (g/d)  1460  1.5 ± 3.8    FCR (g/g)  1442  2.6 ± 0.3    RFI, (g/d)  1442  0.0 ± 14.0  aFI = Feed intake; EML = Daily egg mass; BW = Body weight; BWG = Body weight gain; FCR = Feed conversion ratio; RFI = Residual feed intake. View Large Genetic Parameters Estimated genetic parameters and phenotypic correlations for feed efficiency and egg quality traits are presented in Tables 2, 3 and 4. The heritability estimates for BW, BWG, EML, FCR, FI, and RFI were 0.36, 0.03, 0.22, 0.19, 0.22, and 0.27 in Shaoxing ducks and 0.41, −0.02, 0.14, 0.19, 0.24, and 0.24 for Jinyun ducks, respectively. It was shown that the heritability estimates for feed efficiency traits in 2 duck breeds were similar to each other. The estimated genetic and phenotypic correlations between RFI and FCR in Jinyun ducks (0.63 and 0.59) were slightly higher than those estimated in Shaoxing ducks (0.47 and 0.36). The estimated genetic and phenotypic correlations between RFI and FI showed similar results in these 2 duck breeds. No correlations were found in RFI with BW, BWG, or EML at either genetic or phenotypic level. EML were strongly negative with FCR (−0.73 and −0.63) and positive with FI (0.62 and 0.46) in these 2 laying duck breeds. In addition, there is a positive correlation between FI and BWG. Table 3. Heritabilities (on diagonal), phenotypic (above diagonal), and genetic (below diagonal) correlations, with standard errors (in brackets) for feed efficiency traits in Shaoxing duck. Traitsa  BW  BWG  EML  FCR  FI  RFI  BW  0.36 (0.05)  0.09 (0.04)  0.08 (0.03)  0.01 (0.02)  0.45 (0.07)  −0.09 (0.03)  BWG  −0.04 (0.06)  0.03 (0.01)  0.34 (0.05)  −0.15 (0.03)  0.24 (0.07)  −0.22 (0.13)  EML  0.03 (0.05)  0.51 (0.15)  0.22 (0.06)  −0.73 (0.04)  0.62 (0.06)  0.00 (0.02)  FCR  −0.07 (0.10)  −0.24 (0.09)  −0.81 (0.07)  0.19 (0.05)  −0.06 (0.01)  0.36 (0.07)  FI  0.51 (0.14)  0.16 (0.07)  0.67 (0.14)  0.02 (0.11)  0.22 (0.06)  0.59 (0.10)  RFI  −0.12 (0.10)  −0.21 (0.14)  −0.03 (0.06)  0.47 (0.16)  0.79 (0.05)  0.27 (0.06)  Traitsa  BW  BWG  EML  FCR  FI  RFI  BW  0.36 (0.05)  0.09 (0.04)  0.08 (0.03)  0.01 (0.02)  0.45 (0.07)  −0.09 (0.03)  BWG  −0.04 (0.06)  0.03 (0.01)  0.34 (0.05)  −0.15 (0.03)  0.24 (0.07)  −0.22 (0.13)  EML  0.03 (0.05)  0.51 (0.15)  0.22 (0.06)  −0.73 (0.04)  0.62 (0.06)  0.00 (0.02)  FCR  −0.07 (0.10)  −0.24 (0.09)  −0.81 (0.07)  0.19 (0.05)  −0.06 (0.01)  0.36 (0.07)  FI  0.51 (0.14)  0.16 (0.07)  0.67 (0.14)  0.02 (0.11)  0.22 (0.06)  0.59 (0.10)  RFI  −0.12 (0.10)  −0.21 (0.14)  −0.03 (0.06)  0.47 (0.16)  0.79 (0.05)  0.27 (0.06)  aBW = Body weight; BWG = Body weight gain; EML = Daily egg mass; FCR = Feed conversion ratio; FI = Feed intake; RFI = Residual feed intake. View Large Table 4. Heritabilities (on diagonal), phenotypic (above diagonal), and genetic (below diagonal) correlations, with standard errors (in brackets) for feed efficiency traits in Jinyun duck. Traitsa  BW  BWG  EML  FCR  FI  RFI  BW  0.41 (0.10)  −0.01 (0.02)  0.34 (0.06)  −0.01(0.05)  0.33 (0.07)  −0.11 (0.10)  BWG  0.02 (0.03)  −0.02 (0.03)  −0.06 (0.04)  0.10 (0.03)  0.05 (0.04)  −0.13 (0.09)  EML  0.24 (0.07)  −0.08 (0.03)  0.14 (0.05)  −0.63 (0.06)  0.46 (0.10)  0.06 (0.04)  FCR  −0.04 (0.06)  0.06 (0.05)  −0.68 (0.21)  0.19 (0.07)  0.20 (0.05)  0.59 (0.06)  FI  0.36 (0.12)  0.01 (0.03)  0.51 (0.05)  0.17 (0.13)  0.24 (0.06)  0.89 (0.05)  RFI  −0.10 (0.07)  −0.15 (0.07)  0.02 (0.04)  0.63 (0.06)  0.86 (0.04)  0.24 (0.05)  Traitsa  BW  BWG  EML  FCR  FI  RFI  BW  0.41 (0.10)  −0.01 (0.02)  0.34 (0.06)  −0.01(0.05)  0.33 (0.07)  −0.11 (0.10)  BWG  0.02 (0.03)  −0.02 (0.03)  −0.06 (0.04)  0.10 (0.03)  0.05 (0.04)  −0.13 (0.09)  EML  0.24 (0.07)  −0.08 (0.03)  0.14 (0.05)  −0.63 (0.06)  0.46 (0.10)  0.06 (0.04)  FCR  −0.04 (0.06)  0.06 (0.05)  −0.68 (0.21)  0.19 (0.07)  0.20 (0.05)  0.59 (0.06)  FI  0.36 (0.12)  0.01 (0.03)  0.51 (0.05)  0.17 (0.13)  0.24 (0.06)  0.89 (0.05)  RFI  −0.10 (0.07)  −0.15 (0.07)  0.02 (0.04)  0.63 (0.06)  0.86 (0.04)  0.24 (0.05)  aBW = Body weight; BWG = Body weight gain; EML = Daily egg mass; FCR = Feed conversion ratio; FI = Feed intake; RFI = Residual feed intake. View Large Phenotypic Aspects of Egg Quality To assess the relationship between RFI status and egg quality, external and internal egg characteristics were compared between HRFI and LRFI ducks in 2 duck breeds. The results showed that no remarkable difference in average egg mass were found between these groups (P = 0.69 and 0.73). No significant differences between HRFI and LRFI ducks were observed with respect to measures of external egg quality, such as egg shape index, shell thickness, and shell strength (P > 0.05; Table 5). Likewise, the internal quality parameters of eggs were unaffected by RFI classification, since yolk color (P = 0.78 and 0.37), albumen height (P = 0.15 and 0.20), and HU (P = 0.32 and 0.48) did not differ between HRFI and LRFI ducks. Table 5. Descriptive statistics for egg quality traits in 2 duck breeds. Breeds  Traits  HRFIa  LRFI  P-value  Shaoxing duck  Average egg mass (g)  72.30 ± 5.13  73.07 ± 5.37  0.69    Egg shape index (%)  1.35 ± 0.07  1.31 ± 0.03  0.12    Shell thickness (mm)  0.48 ± 0.05  0.46 ± 0.05  0.96    Shell strength (kg/cm2)  5.00 ± 0.59  4.92 ± 0.73  0.63    Yolk color (level)  11.57 ± 0.54  11.70 ± 0.27  0.78    Albumen height (mm)  6.23 ± 0.73  6.72 ± 1.26  0.15    Haugh unit (HU)  74.57 ± 6.11  76.68 ± 8.17  0.32  Jinyun duck  Average egg mass (g)  73.12 ± 5.76  75.38 ± 6.82  0.73    Egg shape index (%)  1.36 ± 0.08  1.34 ± 0.05  0.21    Shell thickness (mm)  0.52 ± 0.06  0.51 ± 0.08  0.53    Shell strength (kg/cm2)  5.19 ± 0.42  4.93 ± 0.67  0.19    Yolk color (level)  11.88 ± 0.43  11.71 ± 0.33  0.37    Albumen height (mm)  6.42 ± 0.67  6.82 ± 1.19  0.20    Haugh unit (HU)  75.13 ± 4.52  76.92 ± 8.31  0.48  Breeds  Traits  HRFIa  LRFI  P-value  Shaoxing duck  Average egg mass (g)  72.30 ± 5.13  73.07 ± 5.37  0.69    Egg shape index (%)  1.35 ± 0.07  1.31 ± 0.03  0.12    Shell thickness (mm)  0.48 ± 0.05  0.46 ± 0.05  0.96    Shell strength (kg/cm2)  5.00 ± 0.59  4.92 ± 0.73  0.63    Yolk color (level)  11.57 ± 0.54  11.70 ± 0.27  0.78    Albumen height (mm)  6.23 ± 0.73  6.72 ± 1.26  0.15    Haugh unit (HU)  74.57 ± 6.11  76.68 ± 8.17  0.32  Jinyun duck  Average egg mass (g)  73.12 ± 5.76  75.38 ± 6.82  0.73    Egg shape index (%)  1.36 ± 0.08  1.34 ± 0.05  0.21    Shell thickness (mm)  0.52 ± 0.06  0.51 ± 0.08  0.53    Shell strength (kg/cm2)  5.19 ± 0.42  4.93 ± 0.67  0.19    Yolk color (level)  11.88 ± 0.43  11.71 ± 0.33  0.37    Albumen height (mm)  6.42 ± 0.67  6.82 ± 1.19  0.20    Haugh unit (HU)  75.13 ± 4.52  76.92 ± 8.31  0.48  aValues are expressed as mean ± SD. View Large DISCUSSION China owns the biggest duck industry in the world, accounting for approximately 70% of global duck breeding and consumption every year. The common duck is bred mainly to produce eggs for human consumption and as a source of meat when at an old age. Therefore, the selective breeding of these waterfowl focuses on their reproductive traits, such as egg production, egg mass, etc. Recently, there has been rapid development in breeding and production techniques. Additionally, the differences among some species used in agriculture have increased. Differences in selection processes and management conditions lead to differences in production and other variations among animals. Significant advances in feed efficiency have been achieved, as increased feed efficiency represents more profit for the farmers and has the potential to reduce the amount of pollutants released into the environment (Bezerra et al., 2013). Among the number of alternatives, RFI was initially proposed by Koch et al. (1963) as a feed efficiency parameter and has been studied recently for its potential to increase production efficiency. In the current study, we estimate genetic parameters for RFI and relevant traits in 2 laying duck breeds, and to determine the relationship of RFI with feed efficiency and egg quality traits. The heritability estimates for RFI were 0.27 and 0.24, respectively, in our study populations of Shaoxing and Jinyun ducks, which is situated in the large range of published estimations for laying hens (0.20 to 0.30; Aggrey et al., 2010; Yuan et al., 2015). As for laying ducks, 2 papers have been found on RFI estimates so far (Basso et al., 2012; Zeng et al., 2016). The heritabilities for RFI were 0.24 and 0.26, respectively. Our results were consistent with these studies and confirmed that RFI is selectable in laying ducks. The heritability estimates for FI (0.22 and 0.24) in our study agreed well with the previous results in laying chickens and broilers (Gaya et al., 2006; Yuan et al., 2015), but lower than those in laying ducks (Basso et al., 2012), which showed the heritability estimates for FI was 0.34. The high heritability values for BW (0.36 and 0.41), as well as the low heritability values for BWG (0.03 and −0.02), were in agreement with the estimations of Yuan et al., (2015) and Basso et al., (2012). However, our findings diverged with those reported by Case et al., (2012) and Zhang et al., (2017), which showed moderate estimated heritability for BW and BWG in turkeys and Pekin ducks. In addition, our heritability estimations for EML were 0.22 and 0.14 in Shaoxing ducks and Jinyun ducks, which was much lower than those in Rhode Island Red chickens (0.69; Tixier-Boichard et al., 1995). Nevertheless, Luiting and Urff (1991b) showed that heritability estimation for EML in White Leghorn hens was less than 0.30, and decreased with age. The experimental approach utilized in the current study involved ranking ducks by RFI to create distinct groups consisting of low and high RFI ducks. This approach has been used successfully by others, and the spread in RFI values between the LRFI and HRFI groups in the current study was greater than in previous studies (Baker et al., 2006; Nkrumah et al., 2007; Drouilhet et al., 2016). RFI was strongly and positively correlated with FI and FCR, meaning that LRFI ducks consumed less feed and had better feed conversion than did the HRFI ducks. Our results were consistent with previous studies by Yuan et al., (2015) and Zeng et al., (2016). These results show that the more efficient an animal is, the less it eats. Estimation of genetic correlation between RFI and EML were close to zero, indicating that egg production was not a contributor to variation in RFI. That is of great interest, as a selection based on RFI would decrease FI without modifying the genetic level of EML. However, we found that FCR was strongly and negatively correlated with EML but inconsistently correlated with FI, suggesting that EML was the main influence on FCR. Selection for decreased FCR could result in increased EML, which may have a negative effect in egg sales. RFI was weakly negatively correlated with BWG, which is consistent with previous studies by Yuan et al., (2015) and Basso et al., (2012), indicating that some ducks may consume their BG for laying eggs during the peak of the laying period. Egg quality plays an important role in the egg industry both for producers and consumers. Some shell quality traits such as shell strength and shell thickness have an importance value from an economic point of view, since broken eggs (6 to 8%) are discarded, resulting in money loss (Coucke et al., 1999; Ahammed et al., 2014). Shell properties are crucial also for shelf life and safety of eggs and egg products (Reu de et al., 2005). Meanwhile, albumen properties have an influence on the preservation performance of eggs and egg product quality. In the current study, there were no differences between LRFI and HRFI ducks in egg shape index, shell thickness, shell strength, yolk color, albumen height, or HU, indicating that selection for RFI could not change duck egg quality. Up to now, we are unaware of any research about the relationship between RFI and egg quality. This study showed for the first time that there were no differences between RFI and egg quality. These results are consistent with several studies that did not find a significant correlation between RFI and carcass merit and meat quality in cattle (Baker et al., 2006; Perkins et al., 2014). However, the relationship between RFI and egg quality is complicated and multifactorial. In the current study, the genetic correlations between RFI and egg quality are not estimate because the small sample of egg and economic loss. Further investigation will be addressed to assess the genetic correlations between RFI and egg quality based on a large sample size. In conclusion, this study evaluated feed efficiency and its component traits in the laying period of ducks and found that the heritability estimate of RFI is moderate and positively correlated with FI and FCR. As found in other studies, this result confirmed that RFI is selectable in laying ducks. Meanwhile, selection for LRFI could reduce FI without significant changes in EML and egg quality. 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Poultry ScienceOxford University Press

Published: Mar 1, 2018

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