Model definition for genetic evaluation of purebred and crossbred lambs including heterosis

Authors: VARGAS JURADO, NAPOLEON - University Of Nebraska; NOTTER, DAVID - Virginia Tech; Taylor, Joshua - Bret; BROWN, DANIEL - University Of New England; Mousel, Michelle; LEWIS, RONALD - University Of Nebraska

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2022
Publication Date: 6/3/2022
Citation: Vargas Jurado, N., Notter, D.R., Taylor, J.B., Brown, D.J., Mousel, M.R., Lewis, R.M. 2022. Model definition for genetic evaluation of purebred and crossbred lambs including heterosis. Journal of Animal Science. 100:1-14. https://doi.org/10.1093/jas/skac188.
DOI: https://doi.org/10.1093/jas/skac188

Interpretive Summary: Genetic evaluation of sheep breeds in the U.S. is conducted through the National Sheep Improvement Program (NSIP). Producers across the U.S. submit performance data from their sheep flocks to NSIP, and in return, estimates of genetic merit, termed estimated breeding values (EBV), are calculated for traits of interest. Producers then use EBV to select sheep of higher genetic merit in their production system. Accuracy of EBV depend on the volume of data collected from genetically-linked flocks. As the volume of data increase, EBV accuracy increase. Currently, EBV are obtained only using data collected on purebred animals, which are then used to select purebred animals within that same breed. This is unfortunate because crossbreeding is commonly used to increase the production performance of sheep flocks in the U.S. Therefore, there are hundreds of thousands of crossbred sheep generating millions of performance measurements each year that are not currently used in genetic evaluation. As a key first step to incorporate these data, researchers at the USDA, U.S. Sheep Experiment Station (Dubois, ID), University of Nebraska (Lincoln, NE), Virginia Tech (Blacksburg, VA), and University of New England (Armidale, NSW, Australia) joined efforts to determine the advantage in weight gains in lambs that were themselves crossbred. Crossbred lambs weighed between 5% (at birth) and 10% (post-weaning) more than their purebred counterparts. The specific breeds combined in the cross also impacted the extent of that advantage. Those benefits will need to be accounted for to include the performance of crossbred offspring when estimating the genetic merit of their purebred parents.

Technical Abstract: Crossbreeding is common practice among commercial sheep producers given improvements in animal performance. However, genetic evaluation of U.S. sheep is performed within breed type (terminal sire, semi-prolific, western range). While incorporating crossbred records may improve assessment of purebreds, it requires accounting for heterotic and breed effects in the evaluation. The objectives of this study were to: i) describe the development of a paternal composite (PC) line, ii) determine the effect of direct and maternal heterosis on growth traits of crossbred lambs, iii) estimate (co)variance components for direct and maternal additive, and uncorrelated maternal environmental, effects, and iv) provide an interpretation of the estimates of random effects of genetic groups, and to use those solutions to compare the genetic merit of founding breed subpopulations. Data included purebred and crossbred records on birth weight (BN; n = 14395), pre-weaning weight measured at 39 or 84 d (WN; n = 9298) depending on year, weaning weight measured at 123 d (WW; n = 9230), and post-weaning weight measured at 252 d (PW; n = 1593). Mean (SD) BW were 5.3 (1.1), 22.2 (8.7), 39.1 (7.2), and 54.2 (8.7) kg for BN, WN, WW, and PW, respectively. Breeds evaluated in the study included Siremax, Suffolk, Texel, Polypay, Columbia, Rambouillet, and Targhee. Estimates of heterotic effects and covariance components were obtained using a multiple trait animal model. Genetic effects based on founders’ breeds were significant and included in the model. Estimates of direct heterosis were 2.89 +/- 0.61, 2.60 +/- 0.65, 4.24 +/- 0.56, and 6.09 +/- 0.86%, and estimates of maternal heterosis were 1.92 +/- 0.87, 4.64 +/- 0.80, 3.95 +/- 0.66, and 4.04 +/- 0.91%, for BN, WN, WW, and PW, respectively. Correspondingly, direct heritability estimates were 0.17 ± 0.02, 0.13 ± 0.02, 0.17 ± 0.02, and 0.46 ± 0.04 for BN, WN, WW, and PW. Additive maternal effects defined trivial variation in PW. For BN, WN, and WW, respectively, maternal heritability estimates were 0.16 +/- 0.02, 0.10 +/- 0.02, and 0.07 +/- 0.02. Uncorrelated maternal effects defined little variation in any trait. Direct and maternal heterosis had considerable impact on growth traits, emphasizing the value of crossbreeding and the need to account for heterosis, in addition to breed effects, if crossbred lamb information is included in genetic evaluation.