Probability of pregnancy in beef cows with early-weaned calves

Abstract

The aim of this study was to evaluate over two reproductive years the factors that influence the probability of pregnancy in 185 multiparous beef cows with early-weaned calves. The effect of the following parameters were evaluated: the year; the individual and maternal heterozygosity; the percentage of Zebu in the genotype; the weight at calving, weaning and at the end of the breeding season; the body condition score at calving; the age of the cow, the Julian calving date and milk production. Logistic regression was used to analyse the variables using the LOGISTIC procedure. The variables included in the regression equation were tested for multicollinearity, and the parameters evaluated using odds ratio statistics, in which changes in the chances of pregnancy were estimated as a function of the additions to each regressor variable. The average birth rate was 74.1%. For each year beyond the mean age of the herd under evaluation (6 years) the probability of pregnancy is expected to increase by 36.6%. Every seven days from 31 October onwards the probability of pregnancy reduces by 14.3%. For every ten kilograms above the mean body weight at the end of breeding (414.5 kg) the probability of pregnancy increases by 7%. Milk production has no influence on the pregnancy of beef cows with early-weaned calves. The weight at the end of the breeding season is a good indicator of the reproductive performance of the cow, which has a higher probability of pregnancy the greater the age and the earlier calving occurs during the calving season.