Effects on cow reproductive performance, of cow age, parous state, body weight and body-weight change over the breeding season were examined using 6490 annual records from indigenous cows in Botswana.
Calving percentage rose from 69% at 2·5 yr of age to a maximum of 82% at 6 to 7 yr and declined in cows over 8 yr of age. Heifers and previously non-parous cows had a calving percentage of 85%; 8, 18 and 30% higher respectively than parous cows that had calved at the beginning, middle and end of the previous calving season. Cows weighing less than 300 kg at the beginning of the mating season achieved a 50% calving. This rose steadily to 85% as body weight increased to 430 kg.
Cows that lost weight over the mating period achieved a 67% calving, while gains of 20 kg raised this to 76%. Increased weight gains thereafter had little effect on calving percentage. In all cases, as percentages increased, days to conception decreased in a similar manner.
Under these range conditions in Botswana, animals least likely to achieve high calving percentages were young cows; cows which calved previously at the end or middle of the calving season; cows less than 310 kg in weight; and cows gaining less than 20 kg body weight during the mating season.