Effects of drought rations offered to lactating beef heifers on liveweight changes, milk production and composition, calf carcasses and cow fertility

摘要

Thirty-two early-summer-calving Angus heifers received 4 rations in early lactation for the last 92 days of drought and then grazed pasture. The aim was to produce calves suitable for slaughter at weaning (i.e. at about 11 months of age) and to maintain the usual calving rate of about 85% at the next calving. Variations on the control ration of 83% cracked wheat and 17% long oaten hay, plus minerals and vitamins, were 44% hay or additions of urea or sodium bicarbonate. The calves received 0.33 kg and later 0.66 kg of pasture hay/head.day. The nitrogen content of the respective rations was 19.3, 17.5, 24.3 and 19.3 g/kg of dry matter. The apparent digestibilities of the organic matter were 66, 68 and 88% for the pasture hay, oaten hay and cracked wheat, respectively. There was no acidosis, but the heifers would only consume at 80% of the intended level of 6.0 kg/head.day for the control ration. Heifer condition score, weight change and milk composition, and calf growth and milk consumption, did not differ (P>0.05) between treatments. The average daily liveweight changes during drought feeding were -0.51 kg for the heifers and +0.64 kg for the calves. There was a tendency to slower eating and fewer calves at the next calving with the urea and bicarbonate rations. The calves were all of slaughter condition at weaning, and on average 84% of the heifers calved at the next calving. The control ration appeared to contain adequate roughage and nitrogen for milk production. The heifers' energy requirement in relation to milk production and liveweight performance was as predicted by the common feeding standards. Feed consumption, although lower than intended and probably constrained by the mineral content of the diet, was adequate in heifers that were initially fat enough to tolerate prolonged weight loss.