Relationship of Live Weight to Calving Rate of Grade Zebu Heifers and Cows on the Eastern Plains of Colombia

Live weight and calving data were available for 320 grade Zebu cows purchased as 2- to 2%-year-old heifers on the Eastern Plains (Llanos) of Colombia. Frequency distributions relating weight at time of mating to calving rate (females calving per 100 females eligible for conception) were constructed for these animals as heifers and for their subsequent production as cows. Data were also available with which to relate live weight to frequency of abortions in heifers. There was a highly significant relationship between weight and calving rate for both heifers and cows, but the probability of abortion in heifers was distributed at random with respect to weight. No heifers weighing less than 220 kg at the end of a 4-month breeding period conceived during that period. For heifers, the probability of conception and subsequent calving rose sharply from 200 kg to 260 kilograms. Above 260 kg, there was little variation in probability of conception or calving. It was concluded that grade Zebu heifers raised under the conditions of the Colombian Llanos should weigh at least 260 kg before being bred. For cows between 4 and 8 years of age, probability of conception was near zero below 220 kg, and it increased with increasing live weight through 300 kilograms. Above 380 kg, there was a decrease in calving rate. Results indicated that 280 kg was the minimum breeding weight for cows and that maximum calving rate occurred between 280 and 380 kilograms.