Abstract:

Data on live weights collected over a 25 year period (1950-1974) at Serere Agricultural and
Animal Production Research Institute. Uganda, were used to estimate phenotypic, genetic and
environmental trends, and genetic progress in growth traits of the Small East African Zebu
(SEAZ). Data were for 1421 progeny. The traits studied were: birth weight and live weights
at 3 months interval up to 30 months of age. Fixed effects fitted in the statistical models were sex, year of birth, month within year and parity. Least squares means estimates were obtained by the GLM procedure of SAS. Variance and covariance components were estimated using
AIREML procedure, fitting a sire model and individual animal models allowing for both additive and maternal effects and their covariance. The breeding values for all animals were obtained using the later package fitting a single trait animal model. Estimates of genetic trends were calculated by regression of the annual averages of breeding values for respective
traits on year of birth, while the phenotypic trends were calculated by regression of least squares means on year of birth. Environmental trends were calculated as the difference between phenotypic and genetic trends.
Mean weights were: at birth, 19 kg; 122 kg at weaning weight (9 months); 134 kg at one year, 210 kg at 2 years and 246 kg at 2!6 years. Male calves were significantly heavier than females at all ages. Of the environmental effects, sex was by far the most important source of variation in the preweaning phase, followed by year, month within year and parity. In the postweaning phase, year effects were the most important source of variation, followed by
month within year, sex and parity. Year and sex were all highly significant for all traits, while month within year was for all traits except birth weight. Parity was significant for other traits except birth weight and weights at 21 and 30 months of age.
The heritability estimates from the sire model for birth weight, weaning weight and weight at
18 months were 0.49, 0.20 and 0.42, respectively. On the other hand, the heritability
estimates from the individual animal model for direct effects (h2a) of birth weight, weaning
weight and weight at 18 months were 0.40, 0.31 and 0.45, respectively. The heritabilities of
maternal effects (h2m) were estimated as 0.09, 0.10 and 0.01 for the respective traits at the
zero. The phenotypic correlations were between 0.08 and 0.41 whereas the genetic
correlations estimates were in the range of 0.65-0.94.
The phenotypic trends for birth weight, weaning weight, and weight at 18 months were 0.32,
2.64 and 2.83 kg/year, respectively. The genetic trends were 0.07, 0.29 and 0.40 kg/year for
the same traits and corresponding estimates for the environmental trends were 0.25, 2.37 and
annual genetic changes represented, on average, a genetic progress of 0.3 % in all the traits.
Result of this study indicate that a positive genetic progress was made in growth performance
of the SEAZ under a conventional selection programme.
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2.44 kg/year. However, the trends were not linear over the entire period. The estimated
same ages. The correlations between direct and maternal genetic effects were essentially