Effects of index selection on the carcass composition of sheep given either ad libitum or controlled amounts of food

Sheep of a line (S) selected on an index to increase lean weight and decrease fatness at an age, and a control line (C), were given a high quality food at different levels including ad libitum. Live performance was measured from about 21 to 114 kg live weight, The carcasses of each line were analysed for lean, fat and bone at three widely varying weights in both males and females. Level of feeding did not affect the extent to which S was superior to C in either the level of fatness in the carcass (0·86 as much) or the ratio of lean tofat (1·28 as much). The lean to bone ratio was slightly greater in S (1·028 of the value of C; P \u3c 0,05) and was higher on the lowest level of feeding compared with the two higher levels used (P \u3c 0·05 in one experiment on females and P \u3c 0,001 in another on males). On ad libitum feeding the S line grew 1·19 times as fast and was 1·17 times as efficient compared with C. These advantages to S decreased as level of feeding decreased to become virtually zero at the lowest level of feeding used, which allowed C to grow at only 0·53 of the rate seen on ad libitum feeding. On ad libitum feeding growth was well described by a Gompertz growth function of the form W= (Z/B) exp(-exp (Go -8 t)). The maximum growth rate is (Z/e). Line S had a value of 2 that was 1·10 that of C averaged across the two sexes. A Spillman function W= Wo+ (A-WO) (l-exp (-k F)) was used to describe weight, W, in terms of cumulative intake, F. It worked well for ad libitum feeding andfor the two restricted regimes used. The value of the combined parameter (A k) varied across treatments in the same wayas efficiency did

Effects of index selection on the carcass composition of sheep given either ad libitum or controlled amounts of food

Sheep of a line (S) selected on an index to increase lean weight and decrease fatness at an age, and a control line (C), were given a high quality food at different levels including ad libitum. Live performance was measured from about 21 to 114 kg live weight, The carcasses of each line were analysed for lean, fat and bone at three widely varying weights in both males and females. Level of feeding did not affect the extent to which S was superior to C in either the level of fatness in the carcass (0·86 as much) or the ratio of lean tofat (1·28 as much). The lean to bone ratio was slightly greater in S (1·028 of the value of C; P \u3c 0,05) and was higher on the lowest level of feeding compared with the two higher levels used (P \u3c 0·05 in one experiment on females and P \u3c 0,001 in another on males). On ad libitum feeding the S line grew 1·19 times as fast and was 1·17 times as efficient compared with C. These advantages to S decreased as level of feeding decreased to become virtually zero at the lowest level of feeding used, which allowed C to grow at only 0·53 of the rate seen on ad libitum feeding. On ad libitum feeding growth was well described by a Gompertz growth function of the form W= (Z/B) exp(-exp (Go -8 t)). The maximum growth rate is (Z/e). Line S had a value of 2 that was 1·10 that of C averaged across the two sexes. A Spillman function W= Wo+ (A-WO) (l-exp (-k F)) was used to describe weight, W, in terms of cumulative intake, F. It worked well for ad libitum feeding andfor the two restricted regimes used. The value of the combined parameter (A k) varied across treatments in the same wayas efficiency did

Responses to selection for lean growth in sheep

This paper reports the selection responses achieved, and related results, following 9 years of index selection for lean growth in Suffolk sheep. The breeding goal of the index used comprised carcass lean weight and carcass fat weight at a constant age, with relative economic values of + 3 and –1 per kg. The selection criteria were live weight (LWT), ultrasonic fat depth (UFD) and ultrasonic muscle depth (UMD) adjusted to a constant age of 150 days. By year 9, responses in LWT, UFD and UMD in both sexes, as judged by the divergence between selection and control line performance, amounted to 4·88 kg, –1·1 mm and 2·8 mm respectively; these responses are between 7 and 15% of the overall means of the traits concerned. Although selection was originally on index scores based on phenotypic records, the retrospective analyses reported here used the mixed model applications of residual maximum likelihood to estimate parameters and best linear unbiased prediction to predict breeding values. The statistical model comprised fixed effects plus random effects accounting for direct additive, maternal additive and temporary environmental variation. Estimated genetic trends obtained by regressing estimated breeding values on year of birth were similar to annual responses estimated by comparing selection and control line means. Estimates of direct heritabilities were 0·054, 0·177, 0·286, 0·561 and 0·410 for birth weight (BWT), weaning weight (WWT), LWT, UFD and UMD respectively. Corresponding estimates of maternal heritabilities were 0·287, 0·205, 0·160, 0·083 and 0·164. Phenotypic correlations between all pairs of traits were positive and usually moderately high. There were low negative direct additive correlations between BWT and WWT, and between BWT and LWT, but higher positive maternal additive correlations between all other pairs of weight traits

Genetic relationship between longevity and objectively or subjectively assessed performance traits in sheep using linear censored models

Genetic parameters of longevity in crossbred mule ewes, and genetic relationships among longevity, growth, body composition, and subjectively assessed traits on mule lambs and ewes have been estimated using Bayesian linear censored models. Additionally, the genetic associations between longevity and culling reasons were examined. Data comprised 1,797 observations of mule ewes for longevity, culling reasons, growth, body composition, mouth scores, and type traits. Longevity was defined as the time (in years) from 2 yr of age (the age at first lambing of most ewes) to culling or death. Censored data (i.e., observations for which only the lower bound of the true longevity is known, such as when the animals are still alive) comprised 24% of all observations for longevity. Bivariate analyses were used to analyze the longevity of the ewe with each performance trait by fitting linear Bayesian models considering censored observations. Longevity was split into 3 different sub-traits: age at culling due to teeth/mouth conditions, age at culling due to udder conditions, and age at culling due to other culling reasons. These sub-traits and their aggregation into the overall trait of longevity were analyzed in a multiple-trait model. The heritability of longevity was moderate at 0.27, whereas heritabilities of the growth and body composition traits ranged from 0.11 for average of shoulder, loin, and gigot conformation to 0.36 for ewe BW at first premating. Mouth scores and type traits had heritabilities ranging from 0.13 for jaw position to 0.39 for fleece quality. All analyzed traits showed low genetic correlations with longevity, ranging from –0.20 for average conformation scores in live animals to 0.18 for tooth angle. Teeth/mouth conditions resulted in the greatest heritability (0.15) among the sub-traits based on the separate culling reasons. Genetic correlations between separate culling reasons were low to high (0.12 to 0.63 for teeth/mouth conditions with udder conditions and other culling reasons, respectively). Longevity may be preferred as a selection criterion because of (i) its moderate heritability compared with its component sub-traits based on specific culling reasons, and (ii) its moderate to high genetic correlation with these component sub-traits. The moderate heritability for longevity reflects the potential of this trait for genetic improvement, especially when longevity is based on clearly defined culling reasons

Index selection in terminal sires improves early lamb growth

The use of terminal sires (TS) for crossbreeding is integral to the UK sheep industry where approximately 71% of market lambs are sired by TS rams. Early growth of these crossbred lambs affects profitability. The objectives of this study were i) to evaluate the effectiveness of index selection among TS on BW and ADG of their crossbred offspring; and ii) to compare the efficacy of that selection within TS breeds. The most widely used TS breeds in the United Kingdom are Charollais, Suffolk, and Texel. These participated in sire referencing schemes in which they were evaluated on a lean growth index designed to increase carcass lean weight at a given age. From 1999 to 2002, approximately 15 high and 15 low lean growth index rams per breed (93 in total, differing in index on average by 4.6 SD) were selected from within their sire referencing schemes and mated to Welsh and Scottish Mule ewes. Their crossbred offspring were reared commercially on 3 experimental farms in England, Scotland, and Wales. A total of 6,515 lambs were born between 2000 and 2003. Lambs were weighed at birth (BWT), 5 wk (5WT), and 10 wk (10WT), and their ADG from birth to 10 wk was calculated. Lambs sired by high index rams were on average, across breeds, heavier at all ages (P \u3c 0.01) with 0.07 ± 0.03, 0.3 ± 0.1, and 0.4 ± 0.1 kg greater BWT, 5WT, and 10WT, respectively. Their ADG was 5.1 ± 1.9 g/d greater than low-index-sired lambs (P \u3c 0.01). Suffolk-sired lambs were on average heavier at all ages, with greater ADG, whereas Charollais-sired lambs were lightest with smallest ADG. Overall, there was no significant interaction between sire index and sire breed (P \u3e 0.10). Within Suffolk-sired lambs, there was little difference between high and low index sires for the traits studied (P \u3e 0.3). High and low index Charollais-sired lambs differed in BWT (0.09 ± 0.04 kg) and 5WT (0.3 ± 0.1 kg), and Texel-sired lambs differed in 5WT (0.5 ± 0.1 kg), 10WT (0.9 ± 0.2 kg), and ADG (10.2 ± 3.3 g/d; P \u3c 0.01). Lambs from Scottish Mule dams were heavier, with greater ADG, than lambs born to Welsh Mules (P \u3c 0.01). Lambs reared in Scotland were heavier at all ages (P \u3c 0.01). The results suggest that using index selection in TS can improve the growth of their commercial offspring reared on grass

Synthesis of direct and maternal genetic compounds of economically important traits from beef breed-cross evaluations

Published information on relative performance of beef breed crosses was used to derive combined estimates of purebred breed values for predominant temperate beef breeds. The sources of information were largely from the United States, Canada, and New Zealand, although some European estimates were also included. Emphasis was on maternal traits of potential economic importance to the suckler beef production system, but some postweaning traits were also considered. The estimates were taken from comparison studies undertaken in the 1970s, 1980s and 1990s, each with representative samples of beef breeds used in temperate agriculture. Weighting factors for breed-cross estimates were derived using the number of sires and offspring that contributed to that estimate. These weights were then used in a weighted multiple regression analysis to obtain single purebred breed effects. Both direct additive and maternal additive genetic effects were estimated for preweaning traits. Important genetic differences between the breeds were shown for many of the traits. Significant regression coefficients were estimated for the effect of mature weight on calving ease, both maternal and direct additive genetic, survival to weaning direct, and birth weight direct. The breeds with greater mature weight were found to have greater maternal genetic effects for calving ease but negative direct genetic effects on calving ease. A negative effect of mature weight on the direct genetic effect of survival to weaning was observed. A cluster analysis was done using 17 breeds for which information existed on nine maternal traits. Regression was used to predict breed-cross-specific heterosis using genetic distance. Only five traits, birth weight, survival to weaning, cow fertility, and preweaning and postweaning growth rate had enough breed-cross-specific heterosis estimates to develop a prediction model. The breed biological values estimated provide a basis to predict the biological value of crossbred suckler cows and their offspring