Growth Relationships Associated with First Conception and Calving of Beef Heifers on Bermuda-Fescue Pasture

Over 3 years, 332 virgin straightbred and crossbred heifers were exposed to natural service from seven breeds of bulls for 90-day breeding seasons. Two-hundred and twelve heifers calved and 120 did not, for a calving rate of 63.9%. For those heifers calving, age at conception was estimated from calving dates and related to an estimated weight at conception and to various pre- and postweaning weights and gains. Weaning, fall and spring weights of heifers calving were approximately 7.0, 10.0 and 12.0% greater, respectively, than those of heifers not calving. Average age and weight at conception were 492 days and 277 kg, respectively. Small differences among breeding groups in calving rate were attributed to failure of the pasture regimens to provide sufficient nutrients for growth and reproduction of the larger framed heifers. Year, sire breed and dam breeding influenced (P<.01) estimated weight at conception. Age at conception was not influenced by these effects but was influenced (P<.01) by the interactions of year × dam breed and sire breed × dam breed. The pre-weaning period was the most important period of growth in determining weight at conception. In the preweaning period, each .1-kg increase in daily gain increased weight at conception by 19.3 kilograms. Gain during the three periods of growth studied did not consistently influence age at conception. In the preweaning period of growth, each .1-kg increase in daily gain decreased age at conception by 10.3 days. Age at conception was negatively correlated and weight at conception positively correlated with all growth traits. The correlation between age and weight at conception was .05. This study indicates that important considerations of the producer wishing to follow a program of calving at 2 years of age in the upper mid-South on bermuda-fescue pastures are choice of breeds, mating plan and a program of supplementary feeding adequate to support growth and reproduction.