Distribution and turnover of water in Merino sheep selected for high wool production

Abstract

Estimates of the distribution of body water and the rate of water turnover were made on medium Peppin Merino ewes grazing on Mitchell grass association pasture in south-western Queensland during January and November. It so happened that the January measurements were made while the sheep were on lush green pasture following good rains, and that the November measurements were made on dry pasture after a number of months of low rainfall. The estimates made on each sheep included those of plasma volume, extracellular volume, total body water, plasma protein, and water turnover per 24 hr. The ewes, which were approximately 4¼ and 4¾ years of age at the times of measurement, were drawn either from a flock which had been under selection for high clean wool production per head or from a control flock in which all animals used for breeding had been chosen at random from those available. Twelve ewes from each flock were observed in January, and a different set of 10 ewes from each flock was observed in November. Body fluid compartments, as millilitres per kilogram body weight, were greater during the wet: than during the dry period. Plasma volumes were significantly greater in January than in November by 10.3% for the selected ewes and 18.2% for control ewes. The estimates of extracellular volume showed a similar significant difference between the months (15.4% for selected ewes and 17.4% for controls). The January measurements of total body water exceeded the November observations by 9.6% in the selected ewes and 11.2% in the control ewes, both increases being significant. In none of these three characteristics, however, were the selected ewes significantly different from the control ewes during either January or November. The 24 hr turnover of water in millilitres per kilogram body weight was significantly greater in January than in November by 30% for selected sheep and 51.1% for controls. The high water content of pasture during January greatly reduced the free water intake from troughs (ratio of respective water intakes of control and selected sheep was 100/123.7 in November and 1.3/1.5 in January); and the low dry matter content of the food became the major determinant of water turnover. When grazing relatively dry grasses in November, the selected sheep turned over significantly more water than did the controls (ratio control to selected was 100/113.7), but in January the turnovers were not significantly different (control/selected = 137.5/128.7).