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 e stimated in the Miles City Line 1 Hereford cattle population. There we re 951 pedigree records that predated 1935, when data collection began , and 4,692 subsequent recorded calving dates through 1989. The statis tical model included fixed linear continuous effects for inbreeding of calf and inbreeding of dam, discrete fixed effects for year, sex of c alf, and age and previous parity of dam, and random sources of variati on for individual (calf) and maternal additive genetic effects, and en vironmental 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 f ailing to wean a calf in the previous year calved 2.81 +/- .81 d earli er than cows that had weaned a calf. Other fixed effects were small. V ariance components (d(2)) were 23.0 for individual additive genetic ef fects, 5.8 for maternal additive genetic effects, 9.2 for maternal per manent environmental effects, 11.8 for contemporary group effects, and 161.3 for residual effects. Environmental trend was +.17 d/yr and ind ividual additive genetic trend was -.07 d/yr. No maternal additive gen etic trend was detected. These results do not encourage the use of cal ving day as a selection criterion for improving fertility of beef catt le.