EFFECT OF HEAT STRESS ON ENERGY AND WATER UTILIZATION OF LACTATING COWS

Twelve Holstein cows 45–90 days postpartum were used to determine the effect of 32.2 C constant temperature and high levels of unsaturated and saturated fat on the digestibility and utilization of energy and various other responses during three 30-day periods. The portion of the variance associated with rations was in most cases small as compared to the effects of temperature. At 32.2 C the majority of responses showed significant changes, the greatest of which usually occurred during the first week of each period. Body surface evaporation increased markedly at 32.2 C, with the water coming mostly from a 28% increase in consumption and a 33% decrease in fecal water. Changes in urine output were small. Significant (P < .01) amounts of plasma protein were lost, but there were only small declines in urine pH. High temperature had little effect on digestibility of ration. Milk energy output declined almost twice as much as digestible energy intake, resulting in a marked decrease in the efficiency of the utilization of energy. The results show that maintenance requirements are considerably higher under thermal stress and also suggest that in hot areas where protein shortages are acute there may be additional protein losses through the sweat glands. © 1969, American Dairy Science Association. All rights reserved.