Effectiveness of selection for growth rate in Zebu × British crossbred cattle. I. Pre-weaning growth

Abstract

From random-bred populations of Africander cross (AX) and Brahman cross (BX), bulls and cows were selected within breeds for high and low weight per day of age (W/A) at c. 2 years of age and mated assortatively. There was a similar and significant response to selection in both breeds in birth weight and pre-weaning W/A as well as in pre-weaning gains. Although the BX animals were heavier and gained more than the AX animals, the differences were not as great or as consistent as the effect of selection. Calf gains were inversely related to cow gains from birth to weaning, but positively correlated to cow weight. During the pre-weaning period high W/A line cows gained significantly more than the low W/A line cows, while AX animals gained more than the BX group. The higher gain of the high W/A line cows and the higher weaning weight of their calves showed that these lines were more efficient than the low lines in terms of production of weaning weight. Very early calf gains from birth in November to January, and W/A in January, appeared to be poor indicators of the genetic potential of the calf and of the permanent environmental effects affecting its growth. In contrast, the period from January to March, during which the most rapid gains were experienced, reflected both the genetic potential and permanent environmental effects. The previous lactational status of the dam only affected birth weight and day of birth significantly and affected the AX calves more than the BX calves. This resulted in a significant interaction between breed and previous lactational status for these two traits.