Water metabolism of Merino and Border Leicester sheep grazing saltbush

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Abstract
During late summer, Border Leicester wethers running with Merinos on a pure Atriplex nummularia stand turned over 10.6 to 17.5 litres (mean 13.7 litres) of water daily, and diluted the sodium and potassium ingested to less than 1000 m-osmoles/litre in the urine. Leicesters used 46% more water than Merinos as litres/24 hr, and 71% more as ml/kg0.82/24 hr. The two breeds produced similar urine concentrations. The sheep showed a wide variation in response, and their individual water turnovers while feeding on Atriplex vesicaria and A. nummularia ranged from 120 to 833 ml/kg0.82/24 hr. This suggests differences in functional types of adjustment to saltbush associations. On a Danthonia community Leicesters again turned over more water than Merinos but only about half as much water was required as on saltbush. Merinos grazing on A. vesicaria (with some grasses available) over 2 years maintained good condition with half the water intake of Merinos on A. nummularia, although the two species of saltbush had similar sodium and potassium contents. Total body water tended to increase with water turnover rate on saltbush. Plasma sodium concentration was inversely related to water turnover, extracellular volume, and total body water on saltbush pastures. The content of extracellular sodium was, however, greatest in the sheep with the least sodium in the food taken.