Modelling the reproductive performance of tropical beef herds using long-term experimental grazing data on Urochloa humidicola pastures in the Llanos of Colombia

Abstract

Long-term reproductive performance of beef breeding herds grazing sown pastures in the neotropical savanna environment of Eastern Colombia (Llanos) is not well documented, while the lifelong reproductive consequences of raising replacement heifers subject to different growth regimes is also intriguing. The study aimed to assess the effects of liveweight (LW), age, and their interaction on conception probabilities of Bos indicus replacement females by analyzing two seven- and eight-years lasting experiments that used commercial Brahman heifers subject initially to different growth rates on Urochloa humidicola pastures. The experiments included two periods, namely the growth period from weaning until 270 kg LW during which heifers were subjected to three stocking rates to attain different rates of gain, and the subsequent reproduction phase subject to common grazing for all growth treatments. The conception data fitted well logistic regressions in which LW and age were the predictor variables during the first two conceptions but the relationship disappeared in subsequent conceptions. Prediction equations compared well with extensive data from northern Australia that showed similar trends. Liveweight of adult cows only increased consistently during pregnancy, but following weaning their LWs returned to the initial low LWs attained at conception. Over the length of the experiments, weaning rates as percentages of calves born were 88% for the two treatments with higher LW gains during the growth period, and 81% for the lowest gaining treatment. Calving intervals were 19-20 months across treatments. It is suggested that the data and equations provided can guide decision making in extensive breeding herds.