LACTATION IN THE SOW DURING HEAT-STRESS

Ambient temperatures above the evaporative critical temperature (ECT) of lactating sows lead to a reduction in food intake, milk yield, reproductive performance and growth rate of piglets. The fall in food intake of the lactating sow is closely associated with a rise in deep body temperature as is also observed in growing pigs. Evidence is presented indicating that the reduction in milk yield of sows exposed to high temperatures may be greater than would be expected from an equivalent decline in food intake for sows housed under thermoneutral conditions. It is suggested that the direct effect of high temperatures on milk yield may result from a redirection of blood flow to the skin and away from other tissues, including the mammary gland. Experimental observations showing that skin temperature is maintained while deep body temperature declines after initially rising rapidly in lactating sows exposed to 28-degrees-C for four days lends indirect support to this theory. Oxygen uptake of lactating sows was observed to decrease from 523 to 411 ml/min when ambient temperature was increased from 18 to 28-degrees-C. This decline of 20% in heat production was associated with a 25% decline in milk yield and a 40% reduction in food intake. The oxygen uptake of lactating sows was found to be similar to that of non-breeding animals with an equivalent food intake. Consequently, ECT for lactating and non-lactating animals was found to be similar. Performance of lactating sows exposed to high temperatures can be improved by reducing the animal's heat production through decreasing the fibre and increasing the fat content of the diet. However, increasing heat loss from the sow, particularly through increasing the area of wet skin, has a greater positive effect on animal performance than modifying the diet.