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Within-Herd Heritability Estimated with Daughter–Parent Regression for Yield and Somatic Cell Score

C. D. Dechow^*,1 and H. D. Norman

^* Department of Dairy and Animal Science, The Pennsylvania State University, University Park 16802
Animal Improvement Programs Laboratory, ARS, USDA, Beltsville, MD 20705-2350

¹ Corresponding author: cdechow{at}psu.edu

Estimates of heritability within herd (h_WH² ) that were generated with daughter–dam regression, daughter–sire regression, and REML were compared, and effects of adjusting lactation records for within-herd heritability on genetic evaluations were evaluated. Holstein records for milk, fat, and protein yields and somatic cell score (SCS) from the USDA national database represented herds in the US Northeast, Southeast, Midwest, and West. Four data subsets (457 to 499 herds) were randomly selected, and a large-herd subset included the 15 largest herds from the West and 10 largest herds from other regions. Subset heritabilities for yield and SCS were estimated assuming a regression model that included fixed covariates for effects of dam yield or SCS, sire predicted transmitting ability (PTA) for yield or SCS, herd-year-season of calving, and age within parity. Dam records and sire PTA were nested within herd as random covariates to generate within-herd heritability estimates that were regressed toward mean h_WH² for the random subset. Heritabilities were estimated with REML using sire models (REML_SIRE), sire–maternal grandsire models (REML_MGS), and animal models (REML_ANIM) for each herd individually in the large-herd subset. Phenotypic variance for each herd was estimated from herd residual variance after adjusting for effects of year-season and age within parity. Deviations from herd-year-season mean were standardized to constant genetic variance across herds, and records were weighted according to estimated error variance to accommodate h_WH² when estimating breeding values. Mean h_WH² tended to be higher with daughter–dam regression (0.35 for milk yield) than with daughter–sire regression (0.24 for milk yield). Heritability estimates varied widely across herds (0.04 to 0.67 for milk yield estimated with daughter–dam regression), and h_WH² deviated from subset means more for large herds than for small herds. Correlation with REML_ANIM h_WH² was 0.68 for daughter–dam and was 0.45 for daughter–sire h_WH² for milk yield. The correlation between daughter–sire h_WH² and REML_MGS was greater than the correlation between daughter–dam h_WH² and REML_MGS. Data adjustments had a minimal impact on breeding value bias. Within-herd heritability can be estimated rapidly using regression techniques with moderate accuracy, but adjusting lactation records for h_WH² resulted in only a small improvement in the accuracy of genetic evaluations.

Key Words: heritability • daughter–dam regression • daughter–sire regression