AbstractThe use of culled cows on grazing to rear dairy heifers is viable, but rearing beef heifers or steers may be marginal. Data on 132 calves (73 males and 59 females), from birth to 18 months, were obtained from January 1973 to November 1977. The calves were the progeny of purebred Holstein sires and purebred Holstein or grade Holstein cows from the Corozal Agricultural Experiment Substation of the University of Puerto Rico. The herd consisted of 43 cows, culled because of health disorders or delayed breeding, and 19 heifers from low-producing dams. Two Holstein bulls were kept with the herd to reb reed as early as possible following parturition. The calves remained with their dams until weaning at 4 months. They were placed on separate pastures until 18 months old. The male calves were castrated at 6 months. The animals were subjected to exclusive grazing of mixed grasses: Pangola, Guinea, and Para. These were fertilized with 2240 kg of a 15-5-10 analysis per ha per year in 4 equal applications and were managed under rotation at a stocking rate of 2.5 cows or 5 young animals per ha. The F-values due to fitting year and sex effects were significant (P .05) in all age groups. Year effects were greater in the middle age groups (8 to 14 months). Sex effects were fewer at birth (5.3%) but accounted for 10 to 13% of the variation in weight after 4 months. At 4 months, males averaged 10.1 kg heavier than females, but this difference increased only by about 8.0 kg by 18 months. The correlation between weights at parturition and at weaning was high (+0.92), suggesting little or no interaction effect of calf sex, calving season or cow age on body weight changes from parturition to weaning. Overall mean calving interval was 348 ± 73.1 days; this interval is attributed to the rapid rate of rebreeding and the few cows culled for sterility. The mean calf daily gains were 1.12, 0.27, and 0.56 kg from 0 to 4, 4 to 10, and 10 to 18 months of age, respectively, suggesting that the heifers could be bred by 17 months. With mean daily gains of 0 .52 to 0.54 kg after 18 months, expected calving age would be around 28 months with a mean weight above 480 kg.
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