Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/20/1996
Publication Date: N/A
Interpretive Summary: Expected progeny differences (EPD) from national cattle evaluations allow beef producers to compare animals from different herds when making selection decisions. Animals in some herds, however, may be unfairly evaluated because procedures used to calculate EPD do not consider different adjustment factors for cattle in different herds or allow heritabilities and genetic correlations to differ between herds. With- in-herd evaluations may use herd-specific adjustment factors and parameter estimates for better evaluation of animals within that herd, but do not allow comparison to cattle in other herds. This study used data from four Miles City Line 1 Hereford herds to determine differ- ences in parameters estimated from each herd and assess the influence of using herd-specific or average adjustment factors on animal evaluations. Results indicated that genetic parameters estimated from individual herds may be different than those estimated with data combined across herds. Adjustment factors for sex, age of dam, and inbreeding were different among herds. Genetic evaluations made with herd-specific factors were different from evaluations that used average adjustments. While incorporating herd-specific parameter estimates may not be feasible, institutions conducting national cattle evaluations should consider means to account for herd-specific differences in adjustment factors. This may be accomplished by estimating effects for individual herds, or including factors such as age of dam in contemporary group definitions. Further research to develop methods that incorporate results of national cattle evaluation into within-herd analyses to obtain EPD that are more applicable to that herd is suggested.
Technical Abstract: Data from four closely related Hereford herds based on Miles City Line 1 breeding were used to estimate variance components and predict genetic merit for birth weight (BW), weaning weight (WW) and postweaning gain (PWG). Effects of inbreeding of calf and dam, sex, age of dam (AOD), and contemporary group were considered fixed in within-herd BW and WW analyses. Direct and maternal additive genetic effects, and permanent environmental effects due to dam were considered random. The model for WW also included age of calf. The model for PWG included inbreeding of calf, contemporary group, and direct additive genetic effects. Across- herd analyses were conducted with different models. The first considered herd-specific inbreeding, sex, and AOD effects. A second model pooled these effects across herds, and a third considered pooled effects but ignored inbreeding. Ranges of within-herd REML heritability estimates were .30 to .55 for direct BW, .03 to .14 for maternal BW, .17 to .30 for direct WW, .11 to .23 for maternal WW and .24 to .48 for PWG. Within-herd estimates of direct-maternal genetic correlations ranged from -.14 to .13 for BW and from -.22 to .13 for WW. With the herd- specific models, across-herd heritability estimates were .36 for direct BW, .12 for maternal BW, .24 for direct WW, .19 for maternal WW and .30 for PWG. Direct-maternal correlation estimates were .12 for BW and -.15 for WW. Estimates from other models were similar. Differences in fixed effects between herds were observed. Genetic evaluations were different when pooled or herd-specific fixed effects were used. Allowance for individual herd differences in fixed effects in across-herd evaluations is suggested.