Nutritional strategies for managing the heat-stressed dairy cow

Heat stress results from the animal's inability to dissipate sufficient hea t to maintain homeothermy. Environmental factors, including ambient tempera ture, radiant energy, relative humidity, and metabolic heat associated with maintenance and productive processes, contribute to heat stress. The focus of this article is to identify environmental and metabolic factors that co ntribute to excessive heat load, describe how disruption of homeothermy alt ers physiologic systems of the cow, and discuss nutritional modifications t hat help to maintain homeostasis or prevent nutrient deficiencies that resu lt from heat stress. Changes in diet are needed during hot weather to maint ain nutrient intake, increase dietary nutrient density, or to reestablish h omeostasis. Formulation for adequate nutrient intake is challenging because of the competition between nutrient density and other needs for the cow, i ncluding energy density and adequate dietary fiber. Lower DMI during hot we ather reduces nutrients available for absorption, and absorbed nutrients ar e used less efficiently. An excess of degradable dietary protein is undesir able because of energy costs to metabolize and excrete excess N as urea. Op timizing ruminally undegraded protein improves milk yield in hot climates. Mineral losses via sweating (primarily K) and changes in blood acid-base ch emistry resulting from hyperventilation reduce blood bicarbonate and blood buffering capacity and increase urinary excretion of electrolytes. Theoreti cal heat production favors feed ingredients with a lower heat increment, su ch as concentrates and fats, whereas forages have a greater heat increment. Improved dietary energy density and the lower heat increment associated wi th the inclusion of dietary fat must be coupled with limitations to fat fee ding to avoid ruminal and metabolic disorders. Numerous nutritional modific ations are used for hot weather feeding; however, many need further investi gation to achieve specific recommendations.