FACTORS AFFECTING CALVING DATE AND ITS RELATIONSHIP WITH PRODUCTION TRAITS OF HEREFORD DAMS

Calving date (CD) records of 68 896 polled Hereford dams of 120 682 ca lves were coded within herd-year-season with date 1 being the date the first calf was born. Factors affecting CD and its relationship with p roduction traits were investigated. Early calving was found to be asso ciated with high performance, conceivably because dams calving early w ere able to utilize the season-dependent range pastures to their advan tage thus providing adequate milk supply to their calves. Weight and a ge of dam, weight of calf, initial CD, index of milk production potent ial, and previous CD were found to have significant influence on curre nt CD. Extremely small cows at previous weaning had a tendency to calv e late. Cows calving in the spring produced calves with higher overall weight per day of age than did cows calving in other seasons. Initial CD was found to affect CD in subsequent parities. Repeatability of CD was estimated at 0.23. Heritability was estimated at 0.16 (s.e. 0.05) and indicated that additive genetic variance constitutes a moderate p roportion of the variation in CD. Genetic correlation of CD with birth weight (-0.30, s.e. 0-10), post-weaning rate of gain (-0.64, s.e. 0.1 9) and yearling weight (-0.60, s.e. 0.11) indicated that 'plus' additi ve genes for CD influenced these traits favourably. However, the corre sponding correlations with pre-weaning rate of gain (-0.03, s.e. 0.08) and weaning weight (-0.05, s.e. 0.03) were not significant.