Terminal Sire Sheep Breeds Research Articles

Genetic parameters for various growth, carcass and meat quality traits in a New Zealand sheep population

Meat quality and carcass characteristics are becoming more relevant for the sheep breeding programs because of their economic value. Understanding the genetic basis for these traits and its relationship with standard traits is necessary for this to be accomplished. Genetic and phenotypic parameters were estimated for 33 growth, carcass and meat quality traits using a large and unique data set from a variety of terminal sire sheep breeds and composites. This is the most comprehensive study to date of genetic parameter estimates for carcass and meat quality traits in New Zealand sheep and includes many traits that are difficult or expensive to measure and novel traits such as number of rib pairs.Heritability estimates ranged from 0.01 for meat redness at 168h after display to 0.44 for ultrasonic eye muscle depth. Most genetic correlations among growth and carcass traits were favourable and moderate to high. However, some genetic antagonisms such as those between carcass fatness and carcass weight were observed indicating that selection to produce heavier carcasses would also result in fatter carcasses. The genetic correlations among meat quality traits were quite variable. Marbling and tenderness were favourably but weakly genetically correlated, indicating that indirect selection gains by selecting only for traditional traits (i.e. growth and carcass traits) would be small and it is recommended that both be included in a breeding program in order to make greater genetic progress in these traits. The genetic correlations among growth/carcass and eating quality traits were moderate to low; however, some genetic antagonisms were observed, such as carcass fatness with marbling and meat redness, indicating that a stronger emphasis on selection for leanness could affect meat quality traits and consequently consumer eating satisfaction.The heritability estimates and phenotypic variances for the carcass and meat quality traits of New Zealand sheep populations suggest that most traits, apart from pH and meat yellowness, have sufficient phenotypic and genetic variation to enable substantial genetic gains to be achieved through selection. The genetic parameters presented in this study provide a valuable reference to design and/or update a terminal sire breeding program emphasizing meat quality traits. It is important to note that unfavourable genetic correlations identified in this study were low to moderate in magnitude, making it practical to select for favourable genetic progress in all traits or at least to maintain commercially acceptable levels for some traits, if they are measured and balanced in a selection index.

Effects of birth-rearing type on weaning weights in meat sheep are systematically associated with differences in mean performance among flocks.

BackgroundAdjustment of body weights for systematic environmental effects such as dam age and litter size is essential for accurate prediction of breeding values in meat sheep and often accomplished by pre-adjusting records using simple multiplicative adjustment factors, which are derived as ratios of least-squares means of weights of lambs in target and reference classes. However, increasing use of multibreed genetic evaluations that incorporate data from both purebred and commercial flocks has generated concerns regarding the ability of simple additive or multiplicative adjustment factors to properly correct for environmental effects in flocks that differ widely in mean performance. Thus, consistency of adjustment factors across flocks and systematic effects of the level of flock performance on these factors were evaluated using data from the US National Sheep Improvement Program.MethodsWe used birth and weaning weights of lambs from 29 flocks that had at least 500 records per flock and represented several terminal-sire sheep breeds. Effects of lamb sex, dam age class and litter size on birth weights, and of dam age class and combined effects of type of birth and rearing on weaning weights were evaluated. Interactions between these effects and flock were assessed. Bias associated with different adjustment protocols was evaluated for high- and low-performance flocks.ResultsEffects of litter size and differences between yearling and adult dams varied (P < 0.001) among flocks. For weaning weights, additive adjustment factors were not associated with the level of flock performance, but multiplicative adjustment factors were strongly and inversely related to flock means for weaning weights (W). Flock-specific adjustment factors (F = αWβ) reduced bias in adjusted weaning weights associated with differences in flock performance. By contrast, simple multiplicative adjustment factors were appropriate to adjust birth weights.ConclusionsDifferences in weaning weights among single, twin, and triplet lambs were inversely related to the level of flock performance. Use of simple multiplicative adjustment factors led to adjustment bias when applied across flocks with large differences in mean performance. This bias was reduced by using additive adjustment factors or multiplicative factors that were derived as simple exponential functions of flock means for weaning weight.

Estimation of genetic parameters for lambing ease, birthweight and gestation length in Australian sheep

This study presents estimates of genetic parameters for lambing ease (LE), birthweight (BW) and gestation length (GL) in Australian terminal sire sheep breeds using data from the Sheep Genetics LAMBPLAN database. LE was scored on lambs on a 1–5 scale, with 1 being no assistance and 5 being other such as special veterinary assistance. The full dataset consisted of 43 448 records on LE and its two subsets (the single and twin subsets) based on the birth type of the litter were analysed. Four models with different combinations of random effects consisting of direct genetic, maternal genetic and maternal permanent environmental effects were compared. All traits were analysed using linear animal models and linear sire models with LE further analysed by threshold sire models for all datasets to evaluate the influence of datasets and models on the estimation of genetic parameters. The results showed that multiple-born lambs had shorter GL, less BW and less lambing difficulty than single-born lambs. Lambing difficulty decreased with the increase of dam age from 1.5 to 4.5 years, and then increased afterwards. Genetic parameters using linear animal models were similar to those using linear sire models for all traits. Phenotypic variance and direct heritability were higher for single-born lambs compared with twin-born lambs. No significant maternal permanent environmental effect was detected for LE. Based on results using linear animal models with the full dataset, the direct heritabilities were 0.06 ± 0.01, 0.15 ± 0.01 and 0.52 ± 0.02, the maternal heritabilities were 0.03 ± 0.01, 0.15 ± 0.01 and 0.13 ± 0.02 for LE, BW and GL, respectively. The proportions of maternal permanent environmental effects to the total variances were 0.13 ± 0.01 for BW. Low to moderate direct genetic correlations of 0.31 ± 0.09 (LE and BW), 0.24 ± 0.11 (LE and GL) and 0.08 ± 0.08 (BW and GL) were estimated using tri-variate analysis from the full dataset, indicating the trend that lambs with greater BW and longer GL would result in more lambing difficulty.

Ability of sire breeding values to predict progeny bodyweight, fat and muscle using various transformations across environments in terminal sire sheep breeds

Data used for the genetic evaluation of the terminal sire sheep breeds in Australia originate from a large range of genotypes and environments. This means there are large differences in the level of production and therefore contemporary group means and variances within the data. This study examined four transformations to account for the heterogeneity of variance in the observed data and their effect on the ability of estimated breeding values of sires (sire EBV) to predict progeny performance. This predictive ability was described by regressing offspring performance on sire EBV. The expected value of this regression is 0.5, which indicates that half of the sire EBV differences can be expected in the progeny. The transformations of observed data were investigated in low, medium and high production environments for weight and ultrasound scan traits (fat and muscle) in terminal sire sheep breeds. There were records from over 300 000 sheep in the LAMBPLAN terminal sire dataset, predominately from Poll Dorset, Texel, Suffolk and White Suffolk breeds. The transformation methods applied to the observed data were: traits expressed as a percentage of the contemporary group mean; traits re-scaled to a common contemporary group mean in units of measurement; a logarithmic transformation; and a square root transformation. The heritabilities and other variance ratios estimated from the transformed traits were not significantly different from those using the observed data. Phenotypes transformed to a proportion of the contemporary group mean, either as a percentage or in units of measurement, resulted in the most consistent EBV across all production environments for weight and fat traits, with little effect of transformations for muscle traits. The transformation of data to the contemporary mean in units of measurement for weight and fat traits has been implemented in the Sheep Genetics evaluation system. The consistency of the progeny–sire EBV regressions around 0.5 in the data from these purebred industry flocks is heartening for terminal sire evaluation.

Genetic parameters for live weight traits in South African terminal sire sheep breeds

Genetic parameter estimates for live weight traits were derived using Residual Maximum Likelihood (REML) procedures for the South African Dormer, Ile de France and Merino Landsheep terminal sire sheep breeds. Birth weight and weaning weight records were available for all breeds. However, pre-weaning weights were available for only the Ile de France and Merino Landsheep breeds, while post-weaning weights were available only for the Dormer breed. Direct heritability estimates (h2) derived using single-trait analyses were 0.25, 0.28 and 0.37 for birth weight, weaning weight and post-weaning weight respectively for the Dormer breed. Corresponding h2 estimates for the Ile de France and Merino Landsheep breeds were respectively 0.13, 0.53 and 0.14 and 0.23, 0.36 and 0.17 for birth weight, pre-weaning weight and weaning weight. The haphazard data structure of the respective populations resulted in complications in the partitioning of maternal effects into maternal genetic (m2) and maternal permanent environmental (c2) components. Low to moderate maternal heritability (m2) and dam permanent environmental (c2) estimates were derived for the Dormer and Ile de France breeds. Genetic, phenotypic and environmental correlations were estimated using three-trait analysis and were found to be moderate to high for live weight traits in the Dormer and Ile de France breeds. The moderate genetic parameter estimates obtained in this study led to the conclusion that direct selection for live weight traits in the terminal sire breeds will result in improved growth in the terminal sire sheep breeds.

Predicting carcass composition of terminal sire sheep using X-ray computed tomography

AbstractThe best means to utilize X-ray computed tomography (CT) and ultrasound to predict carcass lean, fat and bone weights in vivo in terminal sire sheep were tested. Data on 160 lambs from three breeds were considered: 50 Suffolk males, 50 Suffolk females, 40 Texel males and 20 Charollais males. One-fifth of the lambs within each breed and sex group were slaughtered at each of 14, 18 and 22 weeks of age and the remaining two-fifths at 26 weeks. Carcasses were dissected into lean, fat and bone weights. Prior to slaughter all lambs were CT scanned, with cross-sectional scans taken at seven sites along the body (ischium, hip, mid shaft of femur, 2nd and 5th lumbar vertebrae and 6th and 8th thoracic vertebrae), and ultrasound scanned at the 3rd lumbar vertebra and 13th rib.A set of three CT scans that reliably predicted carcass lean, fat and bone weights was identified which included a scan in each of the three main carcass regions: ischium in the hind leg, 5th lumbar vertebra in the loin and 8th thoracic vertebra in the shoulder. Breed and sex affected the intercepts of the prediction equations but not their slopes. Therefore, a minimal set of equations is likely to be sufficient to predict tissue weights, at least within terminal sire sheep breeds. Equations derived showed high degrees of fit to the data with R2values of 0·924, 0·978 and 0·830 for lean, fat and bone weights, respectively, when predicted using CT alone, and 0·589 and 0·857 for lean and fat weights, respectively, when predicted using ultrasound alone. Using live weight in addition to CT information only improved prediction accuracy slightly for lean (0·966) and fat (0·986) although more substantially for bone (0·925). Where live and tissue weights are considered contemporaneously in genetic evaluations, excluding live weight from prediction may therefore be preferable to avoid colinearity among weight measures.

QTL Detection in the UK Suffolk and Texel Sheep Sire Referencing Schemes

Genomic research and the detection of quantitative trait loci (QTL) provide tools to enhance genetic progress and improve understanding of the biology of commercially important traits. The large sire reference schemes in UK terminal sire sheep breeds provide a unique opportunity to investigate QTL segregation within commercial populations. This study aims to identify QTL for performance traits in commercial Suffolk and Texel sheep.

A selection index for lean meat production in sheep

It is generally accepted that leanness of terminal sire sheep breeds should be increased. However, it is very difficult to define formally a selection goal for these breeds. Put loosely, at a given age, we probably require terminal sires with as much extra lean meat as possible, whilst minimising any increase in fat weight. In the past, selection has been mainly on weight for age, but this may actually increase fatness at the age of selection.Intuitively we expect some in vivo measurements of carcass composition to help in achieving the selection goal. In practice, in vivo measurements of carcass composition in sheep are often imprecise, though the treatment of the animals can affect the relative precision. Ad libitum feeding on a high energy, high protein feed appears to increase the variation in carcass composition of ram lambs compared to those on a lower plane of nutrition (Simm et al, 1985 compared to Cuthbertson et al, 1983). Under these conditions we were able to predict carcass composition reasonably precisely from live weight and ultrasonic measurements (means and original and residual s.d.s for carcass lean weight: 18.30, 1.60 and l.00 kg and for fat weight 13.74, 2.70 and 0.96 kg).

Selection for improved carcass composition in terminal sire sheep breeds

Selection for improved carcass composition in terminal sire sheep breeds

Selection indices for lean meat production in sheep

Selection indices for terminal sire sheep breeds were examined. The selection goal comprised carcass lean and fat weight at 150 days of age, and index measurements examined were live weight (LW), ultrasonic fat depth (UFD) and ultrasonic muscle depth (UMD) at 150 days of age. Relative economic values (REV) for lean weight ranged from + 1 to +5 at an REV of −1 for fat weight. Literature or unpublished estimates of phenotypic and genetic parameters were used. The expected annual responses to selection on different indices were calculated, assuming an average ratio of selection intensity:generation interval, for the two sexes, of 0.60. Responses ranged from +16 g lean, −222 g fat year −1 (REVS +1:−1) to +211 g lean, +132 g fat year −1 (REVS +5:−1). The theoretical maximum responses were also calculated, assuming perfect precision of predicting carcass composition. These ranged from + 107 g lean, − 270 g fat year −1 (REVS + 1: −1) to + 282 g lean, +57 g fat year −1 (REVS +5:−1). Further increases in the response in lean weight of 4–39 g year −1 were expected from selection on indices including measurements on half-sibs. REVS of +3 and −1 for the index based on LW, UFD and UMD were thought to be most appropriate, since the expected response in lean weight was approaching its maximum value, whilst the increase in fat was still relatively small. Expected responses in lean and fat weight were 94 and 26% of those expected from selection on LW alone. With perfect precision of prediction up to 90 g year −1 extra lean weight could be achieved, with a reduction in fat weight at 150 days of age. The index with REVS of +3 and −1 is being used to select performance-tested rams in the Edinburgh School of Agriculture Suffolk flock.

An economic selection index for lean meat production in New Zealand sheep

ABSTRACTAn economic selection index was derived for terminal sire sheep breeds or strains, using physical and financial results from New Zealand export lamb-producing flocks. The aggregate breeding value of the index comprised carcass lean weight and carcass total fat weight. Index measurements were live weight, ultrasonic fat depth and ultrasonic muscle depth. Economic values used for lean and fat weights (NZ$5·65 and NZ$–1·2 per kg, respectively) were the change in profit per kg increase in lean or fat weight from that in the average carcass of lambs slaughtered in 1984/85. The s.d. of the aggregate breeding value was NZS3·20, and the correlation between the full index and the aggregate breeding value was 0·23. This correlation fell to 0·18, 0·15 and 0·07, respectively, when muscle depth or fat depth or muscle and fat depths were omitted from the index. Selection on the full index, or on the index omitting muscle depth was expected to lead to an increase in carcass lean weight and a slight decrease in carcass fat weight. Selection on the index omitting fat depth, or selection on live weight alone, was expected to lead to an increase in both fat and lean weights.

Suffolk Sheep

Genetic parameter estimates for live weight traits were derived using Residual Maximum Likelihood (REML) procedures for the South African Dormer, Ile de France and Merino Landsheep terminal sire sheep breeds. Birth weight and weaning weight records were available for all breeds. However, pre-weaning weights were available for only the Ile de France and Merino Landsheep breeds, while post-weaning weights were available only for the Dormer breed. Direct heritability estimates (h2) derived using single-trait analyses were 0.25, 0.28 and 0.37 for birth weight, weaning weight and post-weaning weight respectively for the Dormer breed. Corresponding h2 estimates for the Ile de France and Merino Landsheep breeds were respectively 0.13, 0.53 and 0.14 and 0.23, 0.36 and 0.17 for birth weight, pre-weaning weight and weaning weight. The haphazard data structure of the respective populations resulted in complications in the partitioning of maternal effects into maternal genetic (m2) and maternal permanent environmental (c2) components. Low to moderate maternal heritability (m2) and dam permanent environmental (c2) estimates were derived for the Dormer and Ile de France breeds. Genetic, phenotypic and environmental correlations were estimated using three-trait analysis and were found to be moderate to high for live weight traits in the Dormer and Ile de France breeds. The moderate genetic parameter estimates obtained in this study led to the conclusion that direct selection for live weight traits in the terminal sire breeds will result in improved growth in the terminal sire sheep breeds.