ENVIRONMENTAL AND GENETIC INFLUENCES ON ZEBU DEVELOPMENT

Data from performance trials in pastures of 1360 Zebu calves was used to determine the degree of influence of environmental (year and bimest er of birth and the interaction) and genetic (sire) factors on growth until 18 months of age. Calves were the progeny of 33 sires born in th e period between July 1984 add December 1989. Traits studied were: wea ning weight (WW), at one year (WY), at 15 (W15) and 18 months (W18) of age; weights for age at those ages and post-weaning gains at 1 year, at 15 and at 13 months. Data were analysed through a mixed linear mode l by the least squares method and the genetic, phenotypic and environm ental trends were estimated. Differences between years accounted for 9 to 20% of the weight variations and the weights for age and of 8 to 2 4% of that of the gains while the bimester accounted for 13 to 30% of the former and of 8 to 45% of the gain variation. Animals born in Marc h-June (end of the dry season and beginning of the rainy season) were heavier at weaning and those born in May-June were heavier at 18 month s. Extreme differences between sires were of 17, 23, 32 and 36 kg for WW, WY, W15 and W18, respectively. There was only a significant geneti c trend for gain at 18 months of the order of 8.1 g/year. The influenc e of the variations among years and bimesters of birth on pre and post -weaning growth, was confirmed.