Supplementation of sheep grazing wheat stubble with urea, molasses and minerals: quality of diet, intake of supplements and animal response

Abstract

Supplements of urea, molasses and minerals were offered for 15 weeks to mature crossbred wethers grazing wheat stubble at Condobolin in central New South Wales. The sheep were then grazed without supplements on a phalaris-subterranean clover pasture for a further five weeks. On the wheat stubble, dead crop residues amounted to more than 4000 kg DM ha-1 and constituted 95% or more of the available plant material throughout the experiment. Diets selected by the sheep contained significant amounts of grain or green plant material only during the first six weeks, while the proportion of dead plant material increased from 66% to 98% of the diet as the experiment progressed. Supplementation had no effect on diet selection. All sheep gained some weight during the first six weeks of stubble grazing, and thereafter lost weight. This loss was reduced from 0.66 kg week-1 to 0.47 kg week-1 with a mineral block supplement, and to 0.37 kg week-1 with supplements of a mineral/urea block, or molasses or molasses/urea licks. Significant differences in mean liveweight occurred between unsupplemented sheep and those offered a mineral/urea block, or a molasses lick. These differences were reduced by half after five weeks on green pasture, due to compensatory gain. Supplementation had no effect on wool production, or on the amount of dead plant material utilized for animal production, which amounted to a mean of about 10% of the quantity initially available. Mean intakes of supplement by groups were close to the desired amounts, i.e. 10 g urea or 50 g molasses sheep-1 day-1, or both, but for all supplements intake varied widely among individual sheep, with the coefficient of variation exceeding 40% in more than half of the recordings. Apart from special circumstances where reduced rates of weight loss may be important, it seems that little benefit will be gained from feeding urea, molasses or mineral supplements to sheep grazing crop stubbles.