Comparison of models to estimate genetic effects for weaning weight of angus cattle

Weaning weights from nine sets of Angus field data from three regions of th e United States were analyzed. Six animal models were used to compare two a pproaches to account for an environmental dam-offspring covariance and to i nvestigate the effects of sire x herd-year interaction on the genetic param eters. Model 1 included random direct and maternal genetic, maternal perman ent environmental, and residual effects. Age at weaning was a covariate. Ot her fixed effects were age of dam and a herd-year-management-sex combinatio n. Possible influence of a dam's phenotype on her daughter's maternal abili ty was modeled by including a regression on maternal phenotype (f(m)) (Mode l 3) or by fitting grandmaternal genetic and grandmaternal permanent enviro nmental effects (Model 5). Models 2, 4, and 6 were based on Models 1, 3, an d 5, respectively, and additionally included sire x herd-year (SH) interact ion effects. With Model 3, estimates of f(m) ranged from -.003 to .014, and (co)variance estimates were similar to those from Model 1. With Model 5, g randmaternal heritability estimates ranged from .02 to .07. Estimates of ma ternal heritability and direct-maternal correlation (r(am)) increased compa red with Model 1. With models including SH, estimates of the fraction of ph enotypic variance due to SH interaction effects were from .02 to .10. Estim ates of direct and maternal heritability were smaller and estimates of r(am ) were greater than with models without SH interaction effects. Likelihood values showed that SH interaction effects were more important than f(m) and grandmaternal effects. The comparisons of models suggest that r(am) may be biased downward if SH interaction and(or) grandmaternal effects are not in cluded in models for weaning weight.