Some aspects of the water balance of sheep at pasture when deprived of drinking water

Abstract

In an experiment carried out near Armidale, N.S.W. (lat. 30°S., altitude 1000 m), measurements of water intake and water expenditure of grazing, lactating Merino ewes from which drinking water had been withheld for 22 months were compared with those of ewes with water supplied, in order to produce a table of water balance for each group. Estimates of the intake of water from the pasture, and of the free water available on the pasture from dew and guttation were obtained. Water intakes of the ewes supplied with water were measured directly. Estimates of water expenditure were obtained from measures of faecal and urinary water loss and daily milk production. These estimates of water intake and expenditure were compared with measurements of water turnover rates of ewes both with and without drinking water ("plus-water" and "minus-water" groups). There were large differences in turnover rates between plus- and minus-water sheep. The greatest discrepancies in water balance were between water intake and water turnover, which indicates that the contribution of water from dew and guttation has been underestimated. The sheep without water were observed to lick the dew from the pasture and fences in the early morning while the sheep with water grazed normally. There was a tendency for more grazing activity during the night and pre-dawn period by the minus-mater sheep than by the plus-water sheep. There were large differences in faecal and urinary water loss and in urine osmolality between plus- and minus-water sheep. At the lower stocking rate there was no difference in daily milk production. There was a reduction in body weight in the lactating minus-water ewes which had not been previously seen in unmated or pregnant sheep. This was probably due to the marked decrease in the feed intake of the minus-water sheep which presumably reflected the gradual reduction in the water content of the pasture as the experiment progressed into the summer months.