Variance components were estimated for lamb weight at birth, 50 d, and 100 d of age. Data from the Canadian flock recording program for lambs born in 1977 to 1991 for Hampshires (n = 6,395) and Polled Dorsets (n = 29,204) and 1982 to 1991 for Romanovs (n = 3,432) were studied. Observed weights were pre-adjusted for the effects of age of dam, sex of lamb, birth-rearing type, month or quarter of year of birth, parity-lambing interval, and age of dam at first lambing, using estimates derived from a fixed effects model including contemporary groups plus these factors. Pre-adjusting for nuisance variables reduced the number of equations in the model for variance component estimation. A single-trait animal model with derivative-free restricted maximum-likelihood procedures was used. Random effects were additive direct and maternal genetic, litter (common environmental), and error. An alternate model excluded maternal genetic effects. Estimates of litter variance as a proportion of phenotypic variance were of moderate size (.12 to .43) and consistent across breeds and models. The mean correlation between direct and maternal genetic effects, across traits and breeds, weighted by the number of animals, was -.40 (SE = .15). The maternal genetic variance or direct-maternal genetic covariance component, or both, was different from zero (P < .05) for all traits in Hampshires and Polled Dorsets, suggesting that maternal effects were important for weight of lambs even at 100 d of age. Estimates of direct heritability ranged from .05 to .45, varying across traits, breeds, and models.(ABSTRACT TRUNCATED AT 250 WORDS)

Estimation of variance components for lamb weights in three sheep populations.

Variance components were estimated from an animal model using a restricted maximum likelihood procedure which allowed for unequal design matrices and missing observations (VCE). Data sets containing: (i) 15 275 records of linear type classifications on heifers, (ii) 3399 live weight and condition scores measured at calving and (iii) 1157 records of yield, dry-matter intake, average live weight and condition score during the first 26 weeks of lactation; were analysed jointly.Heritability estimates for dry-matter intake, live weight and condition score in the largest data set were 0·44, 0·44 and 0·35 respectively and the genetic correlation between condition score and the yield traits ranged from −0·29 to −0·46. The genetic correlation between milk yield and average live weight was negative (−0·09) but after adjusting for the genetic variation in condition score this correlation was positive (0·29). Genetic correlations between live weight and stature, chest width, body depth and rump width were consistently high (0·52 to 0·64; 0·75 to 0·86; 0·59 to 0·81; 0·56 to 0·74, respectively). Chest width and body depth were little to moderately correlated with dry-matter intake (0·25 to 0·28 and 0·20 to 0·34 respectively), and angularity (−0·47 to −0·77) and chest width (0·32 to 0·73) appeared to be good predictors of condition score. These correlations showed that (i) the relative value of live weight compared with food intake capacity determines the optimum direction of selection for stature, chest width, body depth and angularity, and consequently the optimum size of the dairy cow, and that (ii) live weight, condition score and food intake can be predicted from the type traits with little loss in accuracy. A restricted index which maintains condition score at its current level was predicted to reduce overall (economic) genetic gain by 5%.

Genetic correlations between linear type traits, food intake, live weight and condition score in Holstein Friesian dairy cattle

The Baluchi breed is the most common native breed of Iran adapted to harsh environments in the eastern parts of the country. The data used in the present study, collected from two research flocks at the Abbasabad sheep breeding station in north-east Iran, included 20 534 animals descended from 363 sires, 5992 dams, 282 maternal grandsires, and 2865 maternal granddams during the period 1966 to 1989. The traits recorded were: birth weight (BW), weaning weight (WW), weight at 6 months (W6), weight at 12 months (YW), pre-weaning gain (WG), post-weaning gain (PWG), lamb fleece weight (LFW), ewe fleece weight sheared before first joining (FW1) and adult ewe fleece weight (FW). Genetic parameters, estimated with restricted maximum likelihood and a two-trait animal model, were similar in the two flocks. Direct heritabilities for the various body weight traits were moderate and varied between 0·13 and 0·32, while the maternal heritabilities were low and varied between 0·01 and 0·12. Direct and maternal genetic correlations between WW and weights at later ages were moderate to high (0·59 to 0·96). Direct heritabilities of weight gain measures varied between 0·12 and 0·19, while no significant maternal influence on either of these weight gain measures could be detected. The estimates of direct genetic correlation between WG and PWG were positive and varied between 0·54 and 0·74, while negative maternal genetic correlation (0·17 on average) between WG and PWG was detected. For LFW, direct heritability was low and no maternal heritability could be shown. For FW1, both direct and maternal genetic influences were demonstrated (0·07 to 0·26). Direct genetic correlation between LFW and FW1 was very low and close to zero, while maternal genetic correlation was positive and relatively high (0·72 on average). The relative contributions to phenotypic variance from variance components due to common environmental effects ranged from 0·01 to 0·15 for all traits. The repeatability of FW was low (0·03 to 0·12).

Genetic parameters for lamb weight at different ages and wool production in Baluchi 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.

/pdf/responses-to-selection-for-lean-growth-in-sheep-2qzn52841f.pdf

Responses to selection for lean growth in sheep

An evaluation was carried out over a 5-year period in 10 commercial flocks of Scottish Blackface, Scottish Half-bred and Mule ewes to evaluate 10 sire breeds: Border Leicester, Dorset Down, Hampshire Down, He de France, North Country Cheviot, Oxford Down, Southdown, Suffolk, Texel and Wensleydale. An average of 43 sires was used per sire breed.The analysis involved a total of 3360 lambs of which one-third had the left side dissected. Sire breeds were compared when their progeny were slaughtered at the same estimated carcass subcutaneous fat proportion (approx. 120 g/kg).Carcass weights were related to the adult body size of the sire breeds with a range of 4 kg between Southdown and Wensleydale crosses. Crosses by the conventional meat sire breeds tended to have higher daily carcass-weight and lean-tissue gains than those by Border Leicester and Wensleydale sires. Texel and Suffolk crosses did not differ significantly in carcass weight, daily carcass-weight gain, daily lean gain or age at slaughter (P > 0·05).Significant sire-breed × dam-breed interactions were recorded for daily carcass-weight gain and daily lean gain (P < 0·05). Dorset Down and Southdown crosses tended to grow relatively faster when from Mule dams; Texel crosses grew relatively faster when from Blackface dams.There was relatively little difference between sire breeds in visually assessed carcass conformation; all the means were within two points on a 15-point scale. Texel crosses had a higher carcass lean proportion than other crosses: their advantage over Suffolk crosses was 22 g/kg.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0003356100028099

Growth and carcass characteristics of crossbred lambs by ten sire breeds, compared at the same estimated carcass subcutaneous fat proportion

Data were available on crossbred meat lambs for 10 live-weight and growth traits (2585 lambs), 5 carcass traits (1884 lambs) and 10 dissection traits (944 lambs). These were the progeny born on an experimental farm over 5 years, from 102, 79 and 65 sires respectively for the three groups of traits. Genetic parameters were estimated from the sire components in a nested analysis of variance within breed of sire and year subclass, adjusting for sex, dam age, rearing type and other factors.The heritabilities of the live-weight and growth traits were all low, the highest being average daily gain to slaughter (0·10, s.e. 0·06), and none was statistically significant. The heritabilities of the traits of carcass composition were much higher: percentage lean (0·41, s.e. 0·13), percentage fat (0·37, s.e. 0·13) and percentage bone (0·16, s.e. 0·10). Intermediate values were found for most other traits: killing-out percentage (0·16, s.e. 0·07), fat depth (0·21, s.e. 0·11), eye-muscle area (0·14, s.e. 0·10) and lean/bone ratio (0·13, s.e. 0·09).Phenotypic and genetic correlations among the traits were also estimated. No major incompatibilities were found among the traits. The results are discussed in relation to the genetic improvement of lean meat production in sheep.

Genetic parameters of growth and carcass composition in crossbred lambs

Data on 2120 lambs produced over 7 years in the Animal Breeding Research Organization synthetic Dam Line (49% Finn, 22% East Friesian, 16 % Border Leicester and 13 % Dorset Horn) were analysed by least squares. The effects of sex, age of dam, year of birth, birthrearing class, and age of lamb on weights at birth and at 4, 8, 12 and 16 weeks of age were all appreciable. However, pooled within-year correction factors should be adequate in adjusting records for selection. Heritability estimates for weights at the five ages ranged from 0·17 to 0·24 by the sire component method and from 0·18 to 0·28 by regression of offspring on dam. Previous reports of differences in heritability in singles and in twins were not confirmed. Estimates of the genetic correlations among the various lamb weights were high (0·62 to 1·04) indicating that selection for weight at one age should result in increased weights at all ages.

Phenotypic and genetic parameters for lamb weights in a synthetic line of sheep

Performance of crossbred sheep from a synthetic Dam Line

Comparison of crossbred ewes from five crossing sire breeds.

SUMMARY Data on 902 ewes (1755 records) bom over 7 years in the synthetic ABRO Dam Line were analysed by least squares. Reproductive traits of the ewe were not affected by whether she was a single or a twin or by the age of her dam. Ewe age had major effects on all reproductive traits. Litter weight traits were affected by the sex distribution and the age of the litter when weighed. Heritability estimates, both by half sib and regression methods, were low for litter size, low to moderate for litter weights, and higher for ewe and fleece weights. Genetic correlations among the litter-weight traits were high. Together with the heritability estimates, they indicated that selection on litter weight at birth (and perhaps other traits) might give a greater change in total litter weight at weaning, the main measure of ewe productivity and the objective in improvement, than would direct selection.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0021859600031920

Phenotypic and genetic parameters for reproductive performance in a synthetic line of sheep.

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