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Abstract
Three cow groups similar in growth and mature size but different in amount of milk provided to their calves (low (L) = Hereford x, medium (M) = Red Poll x and high (H) = Milking Shorthorn x Angus) were studied. Two gestation and one lactation trials were conducted to estimate maintenance energy requirements. Daily maintenance requirements during gestation were 18% lower than those during lactation. The H and M cows required 12% more energy per unit metabolic weight than L cows to maintain body weight. Variation in milk production explained 23% of the variation in maintenance requirements. Repeatabilities of mesurements ranged from.44 to.64. Maintenance requirements for calves under feedlot conditions in the postweaning phase were 11% higher for the H and M groups than those for the L group.
Reproductive performance was not significantly affected by milk group under the pasture-feeding and management conditions in this study. Days to first postpartum estrus, calving date, pregnancy rate and calf-crop percentage were essentially the same. Average mature weights were 530, 476 and 503 kg for L, M and H cows, respectively. The highest weights were at weaning; the lowest were at calving (spring) for L and M cows and at the start of the breeding season for H cows.
Biological efficiencies to weaning and slaughter were estimated as the ratio of estimated calf weight weaned or carcass weight produced by a 100-cow herd, to estimated metabolizable energy required by the cows and preweaning non-milk or preweaning non-milk and postweaning by the calves. Efficiencies to weaning were 28.1, 28.2 and 28.0 g/Mcal and to slaughter were 21.8, 20.8 and 20.0 g/Mcal for L, M and H cows, respectively.
