Genetic parameters and trends for daughters of imported and Thai Holstein sires for age at first calving and milk yield

Age at first calving (AFC) and accumulated 305 d milk yield (MY) data from first-lactation daughters of imported Holstein (IH) and Thai Holstein (TH; purebred and crossbred Holstein-Other Breeds) sires were used to estimate genetic parameters and to compare genetic changes. The dataset included information from daughters of 3137 IH and 5686 TH sires that calved from 1991 to 2014 on 1012 dairy farms. The 2-trait animal model considered herd-year-season, difference between Other Breeds and Holstein, and heterosis as fixed effects, and animal and residual as random effects. Variance components and genetic parameters were estimated using an average information restricted maximum likelihood procedure. Regressions of mean estimated breeding value (EBV) per calving year were computed to assess genetic trends. The heritability (estimate ± standard error) was 0.149 ± 0.036 for AFC, and 0.147 ± 0.035 for MY. The genetic correlation between AFC and MY was −0.083 ± 0.174. The genetic trend was −0.008 ± 0.001 mth/yr (p < .001) for AFC, and 1.529 ± 0.219 kg/yr (p < .001) for MY. The genetic trend for AFC was −0.003 ± 0.002 mth/yr (p = .084) for daughters of IH sires, and −0.014 ± 0.002 mth/yr (p < .001) for daughters of TH sires. The genetic trend for MY was 0.585 ± 0.326 kg/yr (p = .072) for daughters of IH sires, and 4.091 ± 0.294 kg/yr (p < .001) for daughters of TH sires. The near zero genetic correlation between AFC and MY indicated that selection to increase MY would not affect AFC. The genetic predictions and trends indicated that daughters of IH sires tended to have higher EBV for MY than daughters of TH sires, but daughters of both IH and TH sires tended to have similar EBV for AFC.