HERITABILITY AND FACTORS AFFECTING GROWTH TRAITS AND AGE AT FIRST CALVING OF ZEBU BEEF HEIFERS IN SOUTH-EASTERN MEXICO

Information for this study was obtained from a commercial beef enterpr ise located in the subhumid tropics of Yucatan, Mexico. Data front 664 females of different zebu breeds born from 1972 to 1983 were consider ed for analyses. Weaning and 18-month weights were adjusted to 240 (W2 40) and to 550 (W550) days of age, respectively. The statistical model included the fixed effects of years of birth, season of birth, age of dam at calving, breed of heifer and the random effect of sire within breed. Arithmetic mean +/- standard deviations were 32.1 +/- 3.6kg, 20 8.3 +/- 29.1kg, 678.1 +/- 94.6 g, 308.7 +/- 29.7 kg, 342.3 +/- 89.8 g and 1,060.2 +/- 118.7 days for birth weight (BW), W240, average prewea ning daily gain (ADG), W550 postweaning daily gain (PDC) and age at fi rst calving (AFC), respectively. Year and season of birth had signific ant (P < 0.05) effects on W240, ADG, W550, PDG and AFC. Parity number was significant (P < 0.05) for BW, W240, ADG and AFC. Breed difference s were significant (P < 0.01) for all traits, except BW. Guzerat heife rs were heavier at weaning and W550 than heifers from the other breeds . Guzerat heifer first calved at art average age of 995 days; Brahman and commercial zebu heifers calved about 50 to 60 days Inter and the I ndubrazil and Gyr heifers about 2 months Inter still. The heritability estimates were 0.24 +/- 0.13 for BW, 0.26 +/- 0.14 for W240, 0.29 +/- 0.14 for ADG, 0.43 +/- 0.17 for W550, 0.56 +/- 0.20 for PDG and 0.46 +/- 0.15 for AFC.