EVALUATION OF HEREFORD AND FIRST-CROSS COWS ON 3 PASTURE SYSTEMS .3. MILK-YIELD AND ITS INFLUENCE ON CALF PERFORMANCE

Milk yield was determined by the weigh-suckle-weigh method over 2 year s (1983 and 1984 calvings), for a total of 305 purebred Hereford (H x H) and first-cross Brahman x Hereford (B x H), Simmental x Hereford (S x H) and Friesian x Hereford (F x H) cows grazing three pasture syste ms at Grafton, New South Wales, Australia. The age of the cows ranged from 6 to 11 years. The data were used to evaluate different estimates of milk yield and to examine the effects of milk yield on growth of c alves up to weaning. Of all the cow traits studied, average lactation milk yield (average of early, mid and late-lactation milk yields) had the highest correlation coefficient with calf 210-day weight (r = 0.73 ) and preweaning average daily gain (ADGtotal, r = 0.73), and explaine d > 50 % of the variation in the calf traits. However, milk yield was also moderately correlated with other cow traits (liveweight and body condition score). Thus, to predict calf performance, milk yield data m ay not be required if detailed data on other cow traits are available. This is supported by the finding that differences in the coefficients of determination (R-2) between models for calf 210-day weight and ADG total which included average lactation milk yield and other cow traits (highest R-2 = 69 %) and models which included other cow traits but n o milk yield estimate (highest R-2 = 57%) were < 13%. Cow breed rankin gs for average lactation milk yield were similar to those for calf 210 -day weight and ADGtotal. On high quality pasture, S x H and F x H cow s produced the most milk (S x H, 7.5 kg/day; F x H, 8.3 kg/day; B x H, 5.7 kg/day; H x H, 5.5 kg/day) and weaned the heaviest calves (S x H, 255 kg; F x H, 252 kg; B x H, 215 kg; H x H, 217 kg), while on low qu ality pasture, B x H and F x H cows produced the most milk (B x H, 4.2 kg/day; F x H, 3.7 kg/day; S x H, 2.9 kg/day; H x H, 2.7 kg/day) but B x H cows weaned the heaviest calves (B x H, 180 kg; F x H, 168 kg; S x H, 159 kg; H x H, 124 kg).