Genetic analysis of calving date in Miles City Line 1 Hereford cattle.

A model for genetic analysis of calving date was proposed and variance components, genetic trends, and fixed effects for calving date were estimated in the Miles City Line 1 Hereford cattle population. There were 951 pedigree records that predated 1935, when data collection began, and 4,692 subsequent recorded calving dates through 1989. The statistical model included fixed linear continuous effects for inbreeding of calf and inbreeding of dam, discrete fixed effects for year, sex of calf, and age and previous parity of dam, and random sources of variation for individual (calf) and maternal additive genetic effects, and environmental effects of dams, mating groups, and residuals. All effects were estimated simultaneously by derivative-free procedures for REML. Male calves were born 1.58 +/- .40 d later than female calves. Cows failing to wean a calf in the previous year calved 2.81 +/- .81 d earlier than cows that had weaned a calf. Other fixed effects were small. Variance components (d2) were 23.0 for individual additive genetic effects, 5.8 for maternal additive genetic effects, 9.2 for maternal permanent environmental effects, 11.8 for contemporary group effects, and 161.3 for residual effects. Environmental trend was +.17 d/yr and individual additive genetic trend was -.07 d/yr. No maternal additive genetic trend was detected. These results do not encourage the use of calving day as a selection criterion for improving fertility of beef cattle.