CONSEQUENCES OF SELECTION FOR WEANING WEIGHT IN ZEBU, BOS-TAURUS AND ZEBU X BOS-TAURUS CATTLE IN THE TROPICS

Direct and correlated responses to a single generation of divergent selection for weaning weight per day of age (WA/A) were examined in Bos taurus, zebu and zebu crossbred cattle in a tropical environment. Selection for high WW/A resulted in increased liveweights at all ages in all genotypes, with the exception of birth weight in the Bos taurus genotype. Preweaning daily gains were higher in all genotypes selected for high WW/A, but postweaning daily gains did not differ in two zebu crossbred genotypes. Overall parasite and heat resistance did not change significantly as a result of selection. However, selection within the more resistant genotype/sex populations resulted in higher tick and worm burdens, while in the more susceptible populations tick and worm loads decreased. An examination of lifetime cow fertility showed that cows with high preweaning growth reared more calves to weaning (P < 0.05), had fewer neonatal mortalities (P < 0.10) and also calved earlier (P < 0.01) than cows with low preweaning growth, leading to the conclusion that cows with high WW/A have improved lifetime fertility. Heritability of birth weight, WW/A and 18 month weight per day of age over all genotypes was estimated by parent-offspring regression to be 0.46, 0.21 and 0.22 for each of the traits respectively. The realized heritability of WW/A was 0.17 over all genotypes.