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1, Gary L. Bennett, Richard M. Thallman, Warren M. Snelling, John W. Keele, Harvey C. Freetly, and Larry A. Kuehn USDA, Agricultural Research Service, U.S. Meat Animal Research Center, Clay Center, NE 68933-0166 ABSTRACT: One approach to reducing calving (n = 204) were not different from steers in con- difficulty is to select heifers with higher breeding trol lines (n = 91) for hot carcass weight but had value for calving ease. Calving ease is often asso- significantly greater fat depth. Two production ciated with lower birth weight and that may result systems were compared considering the seven in other possible effects on lifetime productivity. populations as replicates. The systems differed Females from experimental select and control in selection history of females (select and con- calving ease lines within each of the seven popu- trol lines) and the use of bulls within their lines lations were compared. Random samples of 720 as young cows, but used the same bulls in both heifers from lines selected for better calving ease lines as older cows. Cows were culled after single breeding values and 190 heifers from control unsuccessful breeding and kept for up to four lines selected for average birth weights were fol- parities. Select line cows tended (P ≤ 0.10) to lowed through four parities. Select and control wean more calves and stay in the herd longer. lines within the same population were selected They were assisted significantly fewer times to achieve similar yearling weight breeding at calving and had greater calf weight gain to values. Weights of sampled heifers in select lines weaning when evaluated over their herd life. were 2.6 kg (P < 0.01) lighter at birth but not Mature weights were lighter in select lines, but different from control lines at weaning. Select marketable cow weight from the systems was lines had significantly shorter hip height, lighter nearly identical. Control lines did have more mature weight, and greater calving success at marketable young cow weight and select lines second parity. Their calves were born signifi- older cow weight. Weaned calf weight per heifer cantly earlier with lighter weights and less as- starting the system was significantly greater for sistance. Significant interactions with parity the select heifer system due to greater survival of showed fewer calves assisted and greater calf calves from heifers and greater calving success at survival to weaning as heifers but negligible dif- second parity. No important unfavorable effects ferences with control lines in later parities. Steer of genetic differences in calving ease were iden- progeny sampled from these dams in select lines tified in this experiment. Key words: beef cattle, calving difficulty, cow productivity, mature size, production systems Published by Oxford University Press on behalf of the American Society of Animal Science 2021. This work is written by (a) US Government employee(s) and is in the public domain in the US. Transl. Anim. Sci. 2021.5:1-10 doi: 10.1093/tas/txab009 on intensity, heritabilities, and genetic correlations. INTRODUCTION Other traits also can change depending on correl- Traits targeted for selection are expected to ations with selection criteria, but these changes change in intended and beneficial directions based may not be beneficial. One approach to predict- ing changes in other traits is to estimate genetic correlations with selection criteria. This works Corresponding author: firstname.lastname@example.org well when all combinations of traits have been Received September 23, 2020. measured in structured populations, relationships Accepted January 19, 2021. 1 Bennett et al. among traits are linear, and heritabilities are at least Research Center Institutional and Animal Care moderate. Alternatively, selecting and then meas- Committee in accordance with the 1988 Guide uring responses in targeted and nontargeted traits for the Care and Use of Agricultural Animals in can estimate responses in low heritability traits and Agricultural Research and Teaching. identify nonlinear associations if enough change is made. Experimental Design Improved calving ease in heifers combined with The experiment was conducted at the U.S. selection for postnatal growth is a selection strategy Meat Animal Research Center, Clay Center, NE. used by beef cattle breeders. This strategy has re- Experimental design, selection methods, manage- sulted in breed trends over the last 25 years of large ment of cows and calves, and breeding value trends increases in yearling weight EPD and modest to for traits measured up to yearling age were de- moderate decreases in birth weight EPD (highly scribed by Bennett (2008). Briefly, four purebreds correlated to calving difficulty) thus achieving (Angus, Charolais, Gelbvieh, and Hereford) and anticipated changes in targeted and closely cor- three composite populations (MARC I, MARC II, related traits (Kuehn and Thallman, 2017). The and MARC III) of cattle were each split into a se- potential effects of these changes on cow product- lect line (about 135 cows) and a control line (about ivity raise two areas of concern. One is the nega- 35 cows). Cattle in both lines were selected based tive direct−maternal genetic correlation for calving on multitrait EBV calculated from calving difficulty ease and birth weight (Bennett and Gregory, 2001). score in heifers, birth weight, weaning weight, and Direct and maternal calving ease were negatively postweaning gain to yearling age measured at the correlated in Simmental sired crossbred calves, research center within each population. An excep- Piedmontese, Asturiana de los Valles beef cattle, tion was industry sires initially screened into the and Angus ranging from −0.93 to −0.22 (Burfening four purebred populations using industry EPD for et al., 1981; Cubas et al., 1991; Carnier et al., 2000; birth and yearling weights. Birth weight EPD was Gutiérrez et al., 2007). Another possible change is used as a proxy for calving difficulty in industry increased calf mortality of calves born with both sires from Angus, Charolais, and Hereford breeds higher and lower birth weights (Morris et al., 1986; because these breeds did not calculate EPD for Azzam et al., 1993). calving ease at that time. Select lines were selected The objective of this experiment was to es- for lower calving difficulty score EBV and control timate differences in cow traits and productivity lines for birth weight EBV change proportional between lines selected for improved heifer calving to yearling weight EBV change. Both lines were ease and growth and their control lines (Bennett, selected so that yearling weights were not expected 2008; Bennett et al., 2008). These control lines were to change (composite populations) or select and selected so that they had the same yearling weights control lines would increase by the same amount as their corresponding select lines. The hypothesis (purebred populations). Calves were born from tested was that genetically improving heifer calving 1993 through 1999 and the selection goals were ease in the absence of differences in yearling weights achieved (Bennett et al., 2008). Select lines had re- affects cow productivity. duced calving difficulty scores and similar growth to yearling age compared to control lines. MATERIALS AND METHODS Heifers born in 1996 and 1997 were randomly Research protocols were approved and moni- sampled within sire (Table 1) and retained in the ex- tored by the USDA, ARS, U.S. Meat Animal perimental herd until weaning their fourth calf at Table 1. Number of heifers sampled Population Select Control 1996 1997 Angus 100 30 65 65 Charolais 99 31 64 66 Gelbvieh 108 21 63 66 Hereford 100 30 66 64 MARC I 106 25 67 64 MARC II 107 23 65 65 MARC III 100 30 64 66 Total 720 190 454 456 Translate basic science to industry innovation Changes in cows selected for calving ease about 5.5 yr of age unless culled sooner. Cows were Weights of calves were recorded at birth on culled after being open one time, were unhealthy, or pasture and upon entering the feedlot at weaning. were otherwise unlikely to have and wean another calf. Calving difficulty was subjectively assessed by Heifers remained in the selection herd through 1999 trained field staff and given scores with increas- and thus heifers and cows in select lines were bred to ing difficulty from 1 (no assistance) to 7 (cesarean selected bulls and those in control lines were bred to birth). Abnormal presentations were given a score control bulls. Select and control line cows were bred of 8 but were considered separate from the 1 to 7 to the same bulls beginning in 1999 for calves born continuum of difficulty. Calf data for this experi- in 2000. Heifers born in 1996 and 1997 completed ment were considered complete at weaning, except the experiment in 2001 and 2002, respectively. Within for sampled steers born in 1998 and 1999 which populations and parity groupings, select and control were fed a diet based on corn and corn silage until lines were managed as a single group except when sep- slaughter at about 15 mo of age. Carcass weight, fat arated into similar breeding pastures. Hereford cows depth, longissimus area, estimated internal (kidney, born in 1997 were used for other purposes following heart, and pelvic) fat %, and marbling score were weaning in 2001 resulting in a maximum of three recorded at a commercial abattoir, and Yield Grade calvings for that replicate. was calculated. Steers born in 1998 and 1999 to the females sampled in 1996 and 1997 were randomly sampled Statistical Analyses within sire and dam and fed for slaughter. All steers within a population and year were slaughtered on Individual animal analyses. Three types of statis- the same day. tical analyses were used on individual animal traits. A nonlinear procedure PROC NLMIXED in SAS Cow and Calf Management (Version 9.4, SAS Institute Inc., Cary, NC) was used to fit modified Brody curves (Brody, 1945) to Matings were made within select and control herd-life weights and hip heights of cows and test lines to produce calves born in the last 2 yr of se- for selection differences. Other individual traits were lection (1998 and 1999) and managed as described fitted to linear models using PROC GLIMMIX in by Bennett (2008). Cows from both lines within a SAS with an identity link for continuous traits and population were bred by natural service or AI to a logit link for binary traits. Only selection and con- the same bulls for calves born in 2000, 2001, and trol means or differences and their significant inter - 2002. Breeding lasted for 9 wk by natural service actions are reported for cow and calf measurements. or for 3 wk of AI followed by 6 wk natural ser- Cow measurement analyses. The Brody equa- vice in individual sire pastures, beginning May 27, tion for postinflection growth was modified with 1999; May 30, 2000; and June 11, 2001, respectively. multiplicative factors (ME ) to account for the dif- Bulls were selected from within a population for use ferences in nutritional and physiological status at across both lines in the population. The selection precalving (ME ) and prebreeding (ME ) weights of these bulls used the same criteria as select lines. 2 3 from those at pregnancy palpation (ME = 1.0). Hereford population calves born in 2000 and 2001 The remaining Brody curve parameters for ma- were an exception. All cows were bred by AI to a ture value (A), equation extrapolated proportion single MARC II bull each year followed by natural of A remaining at 0 wk of age (B) and maturing service to Angus bulls. rate (k) were augmented to allow differences in After 1999, cow and calf management were cow’s birth year (BY = 1996, 1997), selection goal similar to pre-1999 management as described by (SC = control or select), population (PO = 1 to 7), Bennett (2008). Cows were maintained on pas- f g age of cow at each measurement in weeks (AW ), ture with limited additional corn silage and alfalfa a random effect of the cow for the A parameter haylage fed from November until April to offset (cow ), and a random residual (e ). The resulting reduced forage availability and winter weather con- j z nonlinear model for cow weight was ditions. Cows were measured three times each year. Cow weights, hip heights, and cow condition scores Cow weight =(A + cow ) c,d,f ,g,j,z c,f ,g j (1 to 10) were recorded in January or February be- × [1 − ME × B × exp(−k × AW )] d c,f ,g c,f ,g z fore calving began (precalving), in May or June be- + e . fore breeding started (prebreeding), and in October The model used for cow hip heights was re- up to 3 wk after weaning when palpated for preg- duced because the temporal effects of ME on bone nancy (palpation). Translate basic science to industry innovation Bennett et al. growth are not expected. The nonlinear model for jack through cesarean with scores of 5, 6, or 7) were hip height was analyzed with a logit link. In this model parity (PA ) was defined as two classes, heifer (first parity) Cow height =(A + cow ) c,f ,g,j,z c,f ,g j or cow, because of high survival and low calving as- × [1 − B × exp(−k × AW )] c,f ,g c,f ,g z sistance rates in calves born to cows. The logit-link × + e . model for binary calf traits was A linear model for 8,949 cow condition scores Binary calf trait = μ + BY + SC + PO + PA c,f ,g,j,o,p,z c f g o across 13 herd-life measurement events (HE ; pre- + MF + SC × PO + SC p f g f calving, prebreeding, and palpation from the first × PA + SC × MF + cow + e . o f p j z palpation through the fth palpa fi tion) included random effects for cow (cow ), sire of cow (sire ), j i Carcass traits for steers born in 1998 and 1999 and residual (e ) and interactions including selec- z were analyzed with a linear model including calf tion goal with HE. Thus, the linear model for cow birth year (CY ), dam parity nested within a year condition score was (first parity in both years and second parity in the second year), and slaughter age (SA ) as a covari- Condition score = μ + BY + SC + PO c,f ,g,h,i,j,z c f g ate, resulting in + HE + SC × HE + BY h f h c × HE + PO × HE + sire h g h i Carcass trait = μ + SC + PO + CY + PA (CY ) f ,g,o,q,z f g q o q + cow + e . + MF + SC × CY + b · SA + e . j z p f q z z Reproductive success or failure within each System-level analyses. System traits were accu- parity was fitted to a linear model using a logit mulated across the four parities for the 28 combin- link function. Data consisted of either heifers that ations of select and control lines sampled in 1996 or began the experiment or cows present at the pre- 1997 in the seven populations. No adjustments were vious pregnancy palpation. The model used for re- made to data but select and control lines within a productive success was sampling year and population were managed the same except for the sires of calves born in 1998 Success = μ + BY + SC + PO . c,f ,g c f g and 1999. System traits for each of the 28 combin- Calf measurement analyses. Calf birth date was ations were standardized by dividing by the original analyzed with the following linear model includ- number of heifers sampled. Differences in stand- ing differences in sire lines (SB ) nested within the ardized traits between select and control lines were Hereford population and interaction of selection calculated within populations and sampling years. with cow’s birth year, population, parity (PA , 1 to Differences for the two sampling years were weighted 4), and sex of calf (MF ): by expected variances of the differences based on the original numbers of heifers sampled to calculate a Birth date = μ + BY + SC + PO c,f ,g,i,j,n,o,p,z c f g weighted average of differences within each popula- + SB (PO )+ PA + MF + SC n g o p f tion. A t-test was used on the resulting seven popu- × BY + SC × PO + SC × PA c f g f o lation values to determine whether select and control + SC × MF + BY × PA + PO f p c o g lines were significantly different. Herefords sampled × PA + sire + cow + e . o i j z in 1997 only completed three parities. The Hereford difference used was a weighted average of traits ac- The following model was used for calf cumulated through four parities for heifers sampled birth weight: in 1996 and through three parities for heifers sam- Birth weight = μ + BY + SC + PO + SB (PO ) c,f ,g,i,j,n,o,p,z c f g n g pled in 1997. + PA + MF + PO × BY + SC o p g c f × PO + SC × PA + SC × MF g f o f p RESULTS AND DISCUSSION + BY × PA + PO × PA + PO c o g o g × MF + sire + cow + e . p i j z Heifer Sampling Calf weaning weight used the same model as Heifers in select lines sampled for this experi- the birth weight with the addition of a covariate ment (Table 1) were the progeny of 172 sires and 629 for weaning age and interaction of parity with sex. dams. Those in control lines were the progeny of 93 Binary calf traits of weaning survival, calving as- sires and 177 dams. They reflected differences in the sistance (calving difficulty score > 1), and incidence overall selection experiment (Bennett et al., 2008). of difficult calving (moderate difficulty with a calf Birth weights of control lines exceeded those of select Translate basic science to industry innovation Changes in cows selected for calving ease lines but weaning weights did not differ (Table 2). 1959) with the same genetic goal as a constrained Population differences are not reported for these or phenotypic index (Brascamp, 1984). The −3.3% any other traits because small numbers in each con- difference in observed mature weight is between trol line result in unreliable within-population dif- the −7% difference in birth weight and <−1% dif- ferences. However, the combined statistical power ference in yearling weights of the heifers. However, of seven small control lines is adequate to estimate responses to selection for a single weight or gain overall differences between select and control lines. period tend to be partially maintained throughout a cow’s lifetime (Archer et al., 1998). Unlike re- sponses in weight, select lines in this experiment were shorter as yearlings (Bennett et al., 2008) Cow Measurements and at every subsequent measurement. Taken with Modified Brody curves showed substantial and Meyer’s (1995) suggestion that cannon bone length significant differences between select and control at birth could be an early indicator of mature size, lines for mature measurements. Control lines ex- skeletal measures at younger ages seem to offer a ceeded select lines by 3.3% for mature weight and means of manipulating mature size that is some- 1.6% for mature height (Table 3). Control lines al- what independent of early weights. ready exceeded select lines for height as yearlings Calving success was calculated as either a per- (Bennett et al., 2008) and that is reflected in a sig- centage of heifers starting the experiment or cows nificantly greater proportion of mature height ex- bred the previous year. Differences were significant trapolated to 0 weeks of age (parameter B). These only for second parity when select lines exceeded con- patterns are illustrated in Figs. 1 and 2. Condition trol lines. Differences in third and fourth parity calving score had repeatability of 0.34 and did not differ success were not significant but tended to be greater between lines nor did line differences interact with for select lines as a percentage of original heifers only measurement events (P = 0.28). The differences in because the once open culling policy reduced the the development of height and weight may indicate number of control cows after second parity. Calving that the smaller frame size of select line cows limit difficulty in first parity heifers has been reported to weight at maturity but not earlier growth. be associated with delayed and reduced conception at Estimates of genetic correlations between ma- their second parity (Brinks et al., 1973; Laster et al., ture weight and other weights from birth through 1973) and there was more calving difficulty in con- maturity are usually positive and moderate to high. trol line heifers. However, second parity cows in select Bullock et al. (1993) reported mature weight genetic lines experiencing no, moderate, or substantial dif- fi correlations increasing from 0.64 for birth weight culty as heifers had calving success rates of 89%, 87%, to 0.89 for yearling weight with corresponding and 86 %, respectively. Corresponding success rates heritabilities of 0.49 and 0.30. Portes et al. (2020) in control lines were 83%, 82%, and 83 % respectively. estimated somewhat greater genetic correlations Calving difficulty difference as heifers does not ap- between yearling weights and 5-yr-old weights but pear to explain the line difference in calving success at higher heritabilities of birth and 5-yr-old weights. second parity. Doornbos et al. (1984) reported that a Meyer (1995) analyzed herds of Hereford and shorter duration of labor was associated with earlier Wokalup cattle and concluded that genetic correl- return to estrus and higher pregnancy rates at palpa- ations between cannon bone length measured at tion. This is a possible explanation of the results seen birth and mature weight were 0.6 to 0.7. Further, in this experiment, but the duration of labor was not animals with shorter bone length at birth tended to measured. approach mature weights more quickly. In the select lines described here, breeding Calf Measurements values for calving ease (strongly influenced by birth weight breeding values) and yearling weight were Calves of select line dams were born 3.3 days selected against their genetic correlation (Cockrem, earlier (P < 0.001) than those with control line Table 2. Average differences in birth and weaning weights between randomly sampled select and control heifers Trait Average Select—control 1996–1997 Birth weight, kg 37.3 −2.6 ± 0.5** 0.2 ± 0.4 Weaning weight, kg 217.5 0.2 ± 1.5 −6.9 ± 2.6* Translate basic science to industry innovation Bennett et al. Table 3. Individual cow traits Trait and parameter Average Select—Control SED P Cow mature weight (parameter A), kg 621.4 −20.7 4.95 <0.001 Cow weight parameter B 0.852 −0.009 0.013 0.48 −1 Cow weight maturing rate k, week 0.0112 0.0003 0.0003 0.33 Cow mature height (parameter A), cm 136.5 −2.3 0.4 <0.001 Cow height parameter B 0.112 −0.018 0.004 <0.001 −1 Cow height maturing rate k, week 0.0105 −0.0001 0.0005 0.89 Cow condition score 5.98 −0.04 0.04 0.30 Calves per cow bred First parity, % 85.9 1.7 2.8 0.55 Second parity, % 85.5 6.1 3.0 0.04 Third parity, % 87.9 0.5 3.1 0.88 Fourth parity, % 81.2 −0.7 4.1 0.86 Calves per original heifer First parity, % 85.9 1.6 2.8 0.58 Second parity, % 74.0 7.3 3.5 0.04 Third parity, % 66.9 5.9 3.8 0.12 Fourth parity, % 56.6 4.4 4.2 0.30 Equation extrapolated proportion of A remaining at 0 wk of age. The interaction with herd-life measurement events was not significant (P = 0.28). Repeatability across measurement events was estimated to be 0.34. Significant average differences showed that calves from heifers and cows in select lines had lighter birth weights and fewer births were assisted (Table 4). However, differences for calf birth weight, survival to weaning, and percentage assisted at birth significantly depended on the parity of their dam. Table 5 shows the parity × line means for these traits and percentages of difficult births (P = 0.06 for interaction). Parity is confounded with sire line in these data with younger cows bred to sires within Figure 1. Cow weights at herd-life management events for select their line and older cows of both lines bred to the (circles) and control lines (triangles). Solutions for select (solid line) same sires. The difference in birth weights between and control (dashed line) lines from modified Brody equations are lines at third and fourth parity is about half of the shown. difference at first parity and illustrates the contri- bution of both sire and dam to calf birth weights. Calves from first parity dams in select lines were born with substantially less calving difficulty and increased survival to weaning (Table 5). Older cows have much less calving difficulty and greater sur - vival to weaning and differences between lines are negligible. Breeding values for heifer calving ease were es- timated from a multitrait model including calving difficulty scores on heifers (only) and birth weight, weaning weight, and postweaning gain on all ani- Figure 2. Cow hip heights at herd-life management events for se- lect (circles) and control lines (triangles). Solutions for select (solid mals (Bennett, 2008). Most information for the line) and control (dashed line) lines from modified Brody equations calving ease breeding value was supplied by birth are shown. weights because there were many more birth weights than heifer calving scores, birth weight dams. This could reflect greater fertility, shorter had high heritability and genetic correlation with postpartum interval, or shorter gestation length of direct calving ease scores (Bennett and Gregory, select line fetuses and dams. 2001). Additionally, industry sires were screened Translate basic science to industry innovation Changes in cows selected for calving ease Table 4. Selection line effects and interactions for calf traits Calf trait Mean Select—control SED Selection P Selection × population P Selection × sex P Selection × parity P 85.0 −3.3 0.84 <0.001 0.76 0.38 0.57 Birth date, d Birth weight, kg 38.1 −3.3 0.31 <0.001 0.78 0.09 <0.001 Weaned wt., kg 218.1 −2.2 1.5 0.16 0.17 0.16 0.58 Survival , % 92.0 1.3 1.5 0.40 0.31 0.61 <0.001 Assisted calving , % 10.5 −7.0 1.8 <0.001 0.95 0.96 0.03 Difficult calving , % 1.22 −0.26 0.74 0.73 0.46 0.90 0.06 Parities 2, 3, and 4 were analyzed as a single parity for survival and calving traits. Table 5. Interaction of parity and selection line for calf traits at birth and survival to weaning Line Parity N Birth weight, kg Survival, % Assisted calving, % Difficult calving, % Select 1 615 33.2 86.8 16.4 2.6 2 552 35.7 3 492 38.5 4 388 38.5 2, 3, 4 552 93.8 2.8 0.5 Control 1 160 37.6 70.9 39.3 8.7 2 130 40.1 3 118 40.2 4 95 41.1 2, 3, 4 130 94.6 3.9 0.2 into purebred herds based on birth weight EPD be- 81%, 81%, and 70%, respectively. Birth weight of cause calving ease EPD was not uniformly available the lighter control calves averaged 33.3 kg and se- in national genetic evaluations of purebreds at that lect line middleweight calves averaged 33.0 kg. The time. Birth weight is known to affect calf survival in phenotypic relationship between low birth weight a curvilinear fashion. Heavy calves are more likely and reduced survival to weaning did not predict the to have difficult calvings and light calves also ex- higher survival of calves when birth weights were perience some calving and perinatal complications reduced by genetic selection for heifer calving ease. (Eriksson et al., 2004). Dystocia and other compli- cations can lead to being stillborn or subsequent Steer Carcass Measurements death. As a result, researchers have examined rela- tionships between birth weight and survival (Morris Steers born in 1997 and 1998 were sampled from et al., 1986; Johanson and Berger, 2003). 112 sires and 197 dams in select lines (204 steers) In preceding generations of the populations and 45 sires and 82 dams in control lines (91 steers). used in this experiment, Gregory et al. (1991) Carcass weights adjusted for age were nearly equal found that dystocia incidence in heifers increased and the only significant difference was adjusted fat linearly with birth weight but survival to weaning thickness (Table 6). Steers from select lines had 11% decreased at both ends of the phenotypic distribu- greater fat depth. Similar carcass weights but 3.3% tion. A sharp decrease (84% to 70%) occurred for smaller cow mature weights in select lines were ob- calves that were over 1.5 SD below the mean com- served (Table 3); therefore, select line steers were pared to those between 0.5 and 1.5 SD below the slaughtered at a greater proportion of cow mature mean. The 1.13 SD difference in birth weight be- weight. Greater fat depth is consistent with greater tween select and control line calves born to heifers compositional maturity. If fed to similar fat depth, would be enough to push many calves in the select select steers would have been harvested at lighter line into the low birth weight, lower survival cat- carcass weights than control steers. egory. However, 197 lighter, 218 middle, and 200 heavier birth weight single-born calves relative to System Traits their select line means all had weaning survival rates of 89%. Within control lines, 52 lighter, 58 System inputs. System traits were calculated middle, and 50 heavier calves had survival rates of from totals of unadjusted measurements for each Translate basic science to industry innovation Bennett et al. combination of population, line, and the year be- Some traits differed between individual animal fore dividing by the original number of heifers as- or system comparisons of the select and control signed to it. Differences between lines within line lines. Calving assistance in select lines was less in and population were averaged across the two sam- both comparisons. However, weight gain of cows pling years. The SE of average differences across from weaning until culled or sold and metabolic the seven populations was used to test the overall weight maintained of select and control line cows difference between select and control lines. were similar in the comparison of the system but Herd life traits showed trends of about 10% mature weights of the select lines were less when more calves weaned (P = 0.07) and about 5% longer compared as individual cows. Individually, calf herd life (P = 0.10) for heifers starting the experi- weaning weights were similar but calves in the select ment in select lines (Table 7). Dividing calf traits lines suckled longer and gained more weight. These by a number of calves born or weaned (Table 8) differences are caused by trends in longer herd life resulted in differences for birth date and weight and greater survival of calves born to first-calf heif- similar to adjusted data. Differences in calving suc- ers in select lines (Table 7). cess in second parity (Table 3) and survival of calves System outputs. System-level traits related to born to heifers (Table 5) contribute to these trends. outputs and the value of outputs are also shown System-level traits related to inputs and cost of in Table 9. The last weights of cows that were alive inputs were calculated (Table 8). Weights measured when culled or completing the experiment were throughout a heifer’s herd life following weaning summed by age class and standardized to heifers were used to estimate weight gain until the last beginning the experiment. Herd-life weaning weight measurement before death, being culled, or com- was calculated from the total weight of weaned pleting the experiment. These weights and the days calves or from the total weight of weaned calves between weights were used to estimate total meta- minus weaning weight of the heifer to account for bolic weight maintained until death, culling, or com- the cost of replacement heifers (net weaned calf pletion. Additional measures related to inputs were weight). Control lines produced a more market- fetal growth (total birth weight), lactation days (total able weight of heifers and cows less than 32-mo weaning age), and suckled calf gain (total weaning of age because more were culled after being open weight minus birth weight) were calculated over a after breeding for second parity (Table 3) and be- heifer’s herd life. Herd-life total incidences of calving cause weights of control cows were starting to in- assistance, difficult calvings, and malpresentations crease relative to select lines at that age (Fig. 1). were calculated for heifers beginning the experiment. Increased outputs of the calf and older cow weights Table 6. Carcass traits in steer progeny born in 1998 and 1999 Carcass trait Select Control Difference SED P Hot carcass weight, kg 368.9 370.0 −1.1 3.8 0.77 Loin muscle area, cm 88.52 88.54 −0.02 0.92 0.98 Adjusted fat thickness, cm 0.898 0.805 0.093 0.044 0.04 Internal KPH , % 2.10 2.07 0.04 0.05 0.48 Marbling score 521.5 509.0 12.5 7.9 0.11 Yield grade 2.505 2.414 0.091 0.071 0.20 204 91 Estimated internal kidney, pelvic, and heart fat %. b 00 00 00 Marbling scores of 400 = slight , 500 = small , 600 = modest , etc. (USDA, 2020). Yield grade = 2.50 + (0.9843 × adjusted fat thickness, cm) + (0.2 × kidney, heart, and pelvic fat %) + (0.00838 × hot carcass weight, kg) ˗ (0.0496 × longissimus area, cm ). Table 7. Cow herd life and number of calves born and weaned Trait, per heifer Average Select—control SE t value P Calves born 2.82 0.188 0.106 1.77 0.13 Calves weaned 2.58 0.250 0.116 2.16 0.07 Herd lifetime, mo 51.7 2.44 1.23 1.98 0.10 Calved every year 0.55 0.043 0.031 1.39 0.21 Translate basic science to industry innovation Changes in cows selected for calving ease Table 8. Calf traits from birth to weaning Trait, per calf Average Select—control SE t value P Birth date, d − −2.98 0.88 −3.39 0.01 Birth weight, kg 38.1 −2.79 0.41 −6.78 0.001 Weaning age, d 185.9 2.95 1.76 1.68 0.14 Weaning weight, kg 212.5 1.40 3.47 0.40 0.70 Table 9. Inputs and outputs differences in select and control line systems standardized to a single sampled heifer at the beginning of the experiment Trait standardized to a single original heifer Average Select—control SE P Inputs Weight gain of cow, kg 329.8 −2.6 9.1 0.79 a 0.75 Metabolic weight maintained , d × kg 132,749 5,499 4,459 0.26 Assisted calvings per herd life, score 2 to 8 0.321 −0.184 0.032 0.001 Difficult calvings per herd life, score 5 to 7 0.059 −0.048 0.025 0.11 Malpresentations per herd life, score 8 0.085 −0.016 0.021 0.46 Lactation time, d 490.5 53.99 18.59 0.03 Fetal growth (birth weight), kg 105.7 −1.95 4.15 0.66 Weight gain of suckled calves, kg 453.0 55.1 19.3 0.03 Outputs Marketable heifer weight , ≤ 32-mo-old, kg 108.7 −34.5 8.1 0.005 Marketable cow weight , > 32-mo-old, kg 425.6 32.0 13.4 0.05 Weaned calf weight, kg 557.5 54.6 22.7 0.05 Net weaned calf weight, kg 340.0 54.4 22.7 0.05 Summation of daily metabolic weight of heifers beginning at weaning and ending on the last weigh date before death, culling, or termination of the experiment. All cows alive at culling or at termination of the experiment are included in marketable cow weight. from select lines were significant or trending. Calf culling policies, and use cattle that often differ in- weaned weights exceeded control lines by 10% and versely in both calving ease and growth. Direct ex- net weaned weight by 17%. Increased marketable perimental evaluation of systems has limitations weights of younger cows from control lines were in statistical power and the number of variations nearly offset by increased marketable weights of compared. In this experiment, replication of experi- older cows from select lines. However, the value of mental selection lines provided an unusual oppor- younger cow weight is higher than older cow weight. tunity with some level of statistical power. Calving Systems experimentation. The systems com- ease effects on the system could be isolated because pared are 1) use of heifers selected for calving breeding values for growth through yearling age in ease and then bred to calving ease bulls as young control and selection lines within each population cows, and 2) use of heifers with the same growth were manipulated to be the same (Bennett, 2008). potential and no history of calving ease selection bred to similar bulls. Both lines within a popula- Conclusion tion were bred to the same bulls as females aged, further isolating system differences to calving ease The principal question addressed by this re- genetic effects. Both systems were terminated after search is whether a genetic difference in calving ease, four parities and females were culled after being independent of growth to yearling age, had negative open once. The third system of interest is breeding effects on cow productivity. Component trait effects young females in control lines to bulls from select show cows that as heifers had calves with fewer and lines, but additional experimental resources were easier assists that then survived better to weaning; in not available. their second parity had better calving success; and The systems compared are not the same as com- were shorter and lighter at maturity. Experimental mercial production systems but they do capture evaluation of a system estimated that the average some dynamics of commercial production which heifer with better calving ease would produce 10% usually keep cows much longer, may have different more weaned calf weight with no difference in cow Translate basic science to industry innovation Bennett et al. Carnier, P., A. Albera, R. Dal Zotto, A. F. Groen, M. Bona, and weight gain or metabolic weight maintained. Herd- G. Bittante. 2000. Genetic parameters for direct and ma- life productivity of heifers selected for calving ease ternal calving ability over parities in Piedmontese cattle. and growth was found to be greater than those J. Anim. Sci. 78:2532–2539. doi:10.2527/2000.78102532x selected only for the same level of growth. Cockrem, F. 1959. Selection for relationships opposite to those predicted by the genetic correlation between two traits in ACKNOWLEDGMENTS the house mouse (Mus musculus). Nature 183:342–343. doi:10.1038/183342a0 Mention of the trade name, proprietary Cubas, A. C., P. J. Berger, and M. H. Healey. 1991. Genetic product, or specific equipment does not constitute parameters for calving ease and survival at birth in Angus field data. J. Anim. Sci. 69:3952–3958. a guarantee or warranty by the U.S. Department of doi:10.2527/1991.69103952x Agriculture (USDA) and does not imply approval Doornbos, D. E., R. A. Bellows, P. J. Burfening, and to the exclusion of other products that may be suit- B. W. Knapp. 1984. Effects of dam age, prepartum nutri- able. The USDA is an equal opportunity provider tion and duration of labor on productivity and postpar- and employer. tum reproduction in beef females. J. Anim. Sci. 59:1–10. doi:10.2527/jas1984.5911 Conflict of interest statement. The authors de- Eriksson, S., A. Näisholm, K. Johansson, and J. Philipsson. 2004. clare that they have no competing interests. Genetic parameters for calving difficulty, stillbirth, and birth weight for Hereford and Charolais at first and later parities. LITERATURE CITED J. Anim. Sci. 82:375–383. doi:10.2527/2004.822375x Gregory, K. E., L. V. Cundiff, and R. M. Koch. 1991. Breed Archer, J. A., R. M. Herd, P. F. Arthur, and P. F. 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Translational Animal Science – Oxford University Press
Published: Jan 27, 2021
Keywords: beef cattle; calving difficulty; cow productivity; mature size; production systems
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