Combined purebred and crossbred genetic evaluation of Columbia, Suffolk, and crossbred lamb birth and weaning weights: Systematic effects and heterogenous variances

Authors: VARGAS JURADO, N - University Of Nebraska; NOTTER, DAVID - Virginia Tech; Taylor, Joshua - Bret; BROWN, D - University Of New England; Mousel, Michelle; LEWIS, RON - University Of Nebraska

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2023
Publication Date: 12/12/2023
Citation: Vargas Jurado, N., Notter, D.R., Taylor, J.B., Brown, D.J., Mousel, M.R., Lewis, R.M. 2023. Combined purebred and crossbred genetic evaluation of Columbia, Suffolk, and crossbred lamb birth and weaning weights: Systematic effects and heterogenous variances. Journal of Animal Science. 102. Article skad410. https://doi.org/10.1093/jas/skad410.
DOI: https://doi.org/10.1093/jas/skad410

Interpretive Summary: Crossbreeding is an important strategy to improve the efficiency of sheep production systems, especially under extensive conditions. Lambs can benefit from crossbreeding directly or due to their dams also being crossbred. Despite these advantages, crossbred data are not directly incorporated in the U.S. sheep genetic evaluation. Evaluation of purebred animals could benefit from additional information from crossbred lambs due to moderate to strong genetic correlations among purebreds and crossbreds. However, the extent of this benefit would likely depend on the realized linkages (connectedness) among sires and dams of different breeds in the evaluation (across flocks). This study constitutes an additional step towards implementing genetic evaluation including crossbred information by identifying factors, namely differing impacts of systematic effects and heterogenous variances, necessary to consider for reliable prediction. Testing the actual changes in sire estimated breeding values and their accuracy when crossbred progeny are included in a genetic evaluation is left as the next step in our research.

Technical Abstract: Despite the benefits of crossbreeding on animal performance, genetic evaluation of sheep in the U.S. does not directly incorporate records from crossbred lambs. Crossbred animals may be raised in different environments as compared to purebreds. Systemic factors such as age of dam and birth and rearing type may, therefore, affect purebred and crossbred performance differently. Furthermore, crossbred performance may benefit from heterozygosity, and genetic and environmental variances may be heterogenous in different breeds and their crosses. Such issues must be accounted for in a combined (purebred and crossbred) genetic evaluation. The objectives of this study were to i) determine the effect of dam age and birth type on birth weight, and dam age and birth-rearing type on weaning weight, in purebred and crossbred lambs, ii) test for heterogeneous genetic and environmental variances in those weights, and iii) assess the impact of including weights on crossbred progeny on sire estimated breeding values (EBV). Performance records were available on purebred Columbia and Suffolk lambs. Crossbred information was available on lambs sired by Suffolk, Columbia or Texel rams mated to Columbia, Suffolk, or crossbred ewes. A multiple trait animal model was fitted in which weights from Columbia, Suffolk, or crossbred lambs were considered different traits. At birth there were 4160, 2356, and 5273 Columbia, Suffolk, and crossbred records, respectively, with means (SD) of 5.14 (1.04), 5.32 (1.14), and 5.43 (1.23) kg, respectively. At weaning, on average at 122 (12) d, there were 2557, 980, and 3876 Columbia, Suffolk, and crossbred records, respectively, with corresponding means of 39.8 (7.2), 40.3 (7.9), and 39.6 (8.0) kg. Dam age had a large positive effect on birth and weaning weight in pure and crossbred lambs. At birth, however, the predicted effect was larger in crossbred and Suffolk lambs. While an increase in number of lambs born and reared had a strong and negative influence on birth and weaning weight, the size of the effect did not differ across breed types. Environmental variances were homogenous at birth and weaning, but additive variances differed among breed types for both weights. Combining purebred and crossbred information in the evaluation not only improved predictions of genetic merit in purebred sires, but also allowing for direct comparisons of sires of different breeds. Thus, providing breeders with an additional tool for making selection decisions.