Estimates of genetic and environmental trends of growth traits in Boran cattle

Data from a Boran cattle breeding scheme, run by the Ministry of Agriculture in Ethiopia, were analysed by separating the genetic and environmental trends in growth traits. The data used included weight records at birth (BW), weaning (WW), and as yearlings (YW), for animals born between 1959 and 1985. The mean generation interval calculated Tvas 6.75 years. In a 24-year actual selection period, 3.29 generations of selection occurred, during this time inbreeding increased, to an average oi 1.7% in animals born in 1985. The regressions oi estimated direct and maternal breeding value on year of birth were -0.002 and 0.003 kg, 0.32 and 0.02 kg, and 0.22 and -0.005 kg, for BW, WW, and YW respectively. The aggregate breeding value, which is the sum of the maternal and direct breeding values, showed an Increase of 0.34 and 0.21 kg per year for WW and YW, respectively. Whereas the maternal environmental trends were slight, the variation in the direct environmental effects was the greatest. Generally, a less marked annual genetic trend was shown, when breeding values were regressed on generation coefficients than on year of birth. Despite the good opportunity for selection, that was demonstrated by the more than 50 kg difference between the maximum and minimum breeding values for WW and YW, the maximum genetic gain in aggregate breeding value was only 0.2 % of the mean WW per year. This limited response was due to the high level of genetic ant agonism between direct and maternal effects, and also to problems in running the selection scheme, for example, selection on phenotypic performance alone ani relatively long generation intervals.