Effect of weaning age on pregnancy rates in Braford beef heifers bred at 13 months.

The aim of this study was to evaluate the probability of pregnancy in heifers weaned at different ages and bred at 13 to 15 months old. A total of 121 Braford heifers were used, weaned as calves at 77 days (early) or 147 days (conventional) of age. To develop the statistical models of reproductive performance, factors related to the development of the heifers were analyzed. The analysis included a diagnosis of multicollinearity using the Pearson correlation matrix, adjusting the model by means of the Hosmer and Lemeshow test. The response variable, rate of pregnancy, was analysed using the LOGISTIC procedure. Beginning with a weight of 271 kg and an age of 402 days at the start of the breeding season, the pregnancy rates increased by 18.4% and 29.0%, respectively for every 15 kg increase in body weight and 10-day increase in age. However, a reduction of 15 kg in body weight and of 10 days in age reduced the pregnancy rates in the heifers by 15.5% and 22.5%. An increase or reduction of 0.100 kg in the average daily gain between early weaning and conventional weaning represented an increase of 44.6% and a reduction of 30.9% in the chances of pregnancy. Early-weaned heifers require correct nutritional management to allow satisfactory postweaning weight gains so as not to compromise their reproductive performance.