GROWTH AND CALVING PERFORMANCE OF COWS FROM CROSSES BETWEEN THE BRAHMAN AND HEREFORD

The importance of various genetic and environmental effects on three traits (birth height and weight and number of calves born over the experimental period) and four parameters (mature height and weight and maturation rate of height and weight) for 177 cows is investigated. Individual heterosis for the number of calves born was, on average, 39%. These data were in accord with the expectation that the main effect on the form of growth of individual heterosis is to increase mature weight and maternal heterosis is to increase maturation rate. Pre-weaning pasture quality improvements caused higher rates of maturation but had little effect on mature height and weight of most breed crosses. However, individual heterosis estimates for mature weight were 1%, 13% and 21% for low, medium and high quality pasture respectively. Maternal heterosis estimates were most important for maturation rate of weight and declined with increased pasture quality. It is further suggested that the mode of action of individual heterosis is through increased nutrient demand whereas that of maternal heterosis is through increased nutrient supply.