Effects of Heat Stress on the Lactation Performance of Ewes Accustomed to Tropical Conditions and the Total Fluid Intake of Their Lambs

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Abstract
The impact of heat stress on the feed intake, milk production, water intake and urine output of undernourished lactating ewes and on the growth rate and water intake of their lambs was measured in a climate chamber and during exposure to natural high ambient temperature conditions. Similar liveweight increases occurred in stressed and unstressed lambs. Feed intake was depressed in heat-stressed ewes in the climate chamber, but not under natural heat-stress conditions. During the 1st and 2nd wk of lactation calculated milk yield (200-500 ml/day) and composition were unaffected by heat stress per se. Undernutrition due to the poor quality roughage offered apparently depressed milk production of all ewes. Increased water intake (27%) and plasma prolactin concentrations (220%) were recorded in heat-stressed ewes, but daily unrine output (27-36 ml/kg body wt) was unaffected. No relationship between prolactin concentrations, milk production and antidiuretic activity was obvious. At the age of 5-6 wk water intake accounted for 67% (500 ml/day) and 80% (1000 ml/day) of total fluid intake (water plus milk) of unstressed and heat-stressed lambs, respectively. The importance of making water freely available to lambs subjected to extensive grazing systems of tropical regions is indicated.