HEAT-STRESS INTERACTIONS WITH PROTEIN, SUPPLEMENTAL FAT, AND FUNGAL CULTURES

Cows that were subjected to hot environmental temperatures yielded les s milk (3.1 kg/d) on a diet high in CP (18.4%) and of medium degradabi lity (65%) than on diets high in CP of low degradability (59%) or medi um in CP (16.1%). The high CP diets were associated with decreased DMI and higher water intake, ruminal NH4, and blood urea. Negative effect s on yield from the high CP, medium degradability diet were not observ ed at moderate temperatures. Evaporative cooling of cows in hot weathe r resulted in a greater milk yield response to low versus medium rumen -undegradabale protein diets than for uncooled cows. Evaporative cooli ng of cows also affected response to protein quality. For cooled cows, high Lys diet (soybean, fish, and blood meals) increased milk yield 1 4% over that with low Lys diet (com gluten meal), but, for uncooled co ws, a high Lys diet only increased yield by 9%. Percentage of CP, degr adability, and protein quality had no effect on body temperatures or r espiration rates of lactating cows. Some, but not other, reports showe d that supplementation of 2 to 2.5% fat to diets fed under hot summer conditions resulted in less yield response than when fat was added at moderate temperatures. In several studies, fungal cultures (3 to 5 g/d ) in the diet decreased body temperatures and respiration rates in hot , but not cool, weather. Increased milk yields and cellulose digestibi lity also resulted from dietary fungal cultures in some, but not all, trials. The mechanism of action exerted by fungal cultures on body tem perature and respiration rate is unclear.