ABSTRACT Heat stress results from the animal's inability to dissipate sufficient heat to maintain homeothermy. Environmental factors, including ambient temperature, 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 contribute to excessive heat load, describe how disruption of homeothermy alters physiologic systems of the cow, and discuss nutritional modifications that help to maintain homeostasis or prevent nutrient deficiencies that result from heat stress. Changes in diet are needed during hot weather to maintain nutrient intake, increase dietary nutrient density, or to reestablish homeostasis. Formulation for adequate nutrient intake is challenging because of the competition between nutrient density and other needs for the cow, including energy density and adequate dietary fiber. Lower DMI during hot weather reduces nutrients available for absorption, and absorbed nutrients are used less efficiently. An excess of degradable dietary protein is undesirable because of energy costs to metabolize and excrete excess N as urea. Optimizing ruminally undegraded protein improves milk yield in hot climates. Mineral losses via sweating (primarily K) and changes in blood acid-base chemistry resulting from hyperventilation reduce blood bicarbonate and blood buffering capacity and increase urinary excretion of electrolytes. Theoretical heat production favors feed ingredients with a lower heat increment, such as concentrates and fats, whereas forages have a greater heat increment. Improved dietary energy density and the lower heat increment associated with the inclusion of dietary fat must be coupled with limitations to fat feeding to avoid ruminal and metabolic disorders. Numerous nutritional modifications are used for hot weather feeding; however, many need further investigation to achieve specific recommendations.
Implications The modern dairy cow with high genetic potential consumes and metabolizes a large quantity of nutrients, and it is apparent that if the cow is to produce to her potential that some heat abatement procedures are necessary to dissipate the large amount of metabolic heat produced. By minimizing the increase in body temperature that occurs with hot environmental conditions, greater DMI is encouraged and the gross efficiency with which dietary nutrients are used by the cow is improved. A reduction in body temperature reduces sweating and panting. The loss of electrolytes via skin secretions is minimized and disturbance of acid-base chemistry from hyperventilation is moderated. Modification of the environment by shading and cooling enhances the quantity of nutrients consumed by the cow but can also affect nutrient requirements.
With the onset of hot weather, cows will adapt to the environmental conditions and DMI and milk yield will usually stabilize at a level below that during cool weather. Reformulation of diets is necessary to achieve sufficient nutrient density to maintain nutrient intake. However, some nutrients are required at still higher levels during heat stress (K and Na, for example), and supplementation above NRC minimums (NRC, 1989) results in improved production during hot weather. There is evidence that a ratio or proportion of mineral elements benefits the heat-stressed cow, and the concept of dietary cation-anion difference may have merit. If so, not only is dietary electrolyte content important, but so is the source of electrolytes as well; KCl or NaCl are neutral in the cation-anion difference equation, whereas Na or K carbonates or bicarbonates contribute to alkalinity of the diet. Apparently, an excess of acidogenic ions such as Cl reduces DMI in the cow, especially during hot weather; this could result from the reduced blood buffering capacity that occurs in the presence of a respiratory alkalosis. Although inadequate nutrient intake is of particular importance during hot weather, excesses of nutrients such as protein can contribute to lowered efficiency in the cow. The practice of boosting nutrients such as CP to high levels to minimize potential deficiencies can have negative consequences because nitrogen in excess of requirement must be metabolized and excreted, requiring energy. Recent research demonstrates that protein content, ruminally undegradable protein value and quality of amino acids delivered to the lower digestive tract play a complex role in the nutrition of the heat-stressed cow. Additional research is needed in this area to establish minimum and maximum protein content of the diet and to determine the effects of protein content on the efficiency of nutrient use by the cow.
Heat increment of the dietary ingredients plays a role in both the efficiency with which nutrients are utilized and heat generation potential of the diet. There is a potential for formulation of diets that are particularly favorable for use during hot weather. High fiber diets result in production of large quantities of ruminal acetate, which is used with less efficiency than propionate. The efficiency of use for ME from high fiber diets is lower than for diets with high levels of rapidly fermentable carbohydrate, providing a scientific basis for the practice of feeding low fiber diets during hot weather. Low fiber diets must be implemented cautiously because of the necessity for sufficient fiber to promote good rumen function and to maintain animal health and well-being. Use of very high quality forages for hot weather feeding may ensure both adequate content of digestible fiber and improved content of fermentable sugars to enhance production of rumen VFA. Dietary fats have a relatively low heat increment because of a high efficiency of use by the cow. Fats may be of particular value during hot weather and may be undervalued by current energy equations because of the lower heat increment. Results from research with fat feeding during hot conditions have been variable. Rumen inert fat allows inclusion of relatively high levels of fat in the diet, and experiments to elucidate the effects of feeding these diets during hot weather should be conducted. Because high levels of dietary fat reduces the amount of fermentable energy in the rumen, the quantity of protein and ruminal escape value for that protein may be particularly critical when feeding high fat diets.
Practical management considerations such as frequent feeding and ready water availability are logical. Water should be available in holding pens, travel alleys, and near feed bunks. Provision of fresh feeds through multiple feedings encourages frequent feeding bouts by cows, and making the feeding area as comfortable as possible through shading and(or) cooling should enhance feeding frequency and total intake. Animal behavior during hot weather indicates that the animal will consume more feed during cooler evening hours and feeding quantities and schedules should be adjusted to accommodate changes in behavior because of season of feeding. Research evaluating animal facilities and environmental conditions including temperature, humidity, and photoperiod for their effect on feeding behavior may be beneficial in designing feeding and management programs for the lactating cow during heat-stress conditions.
Key Words: Dairy, Heat Stress, Intake, Metabolism, Milk
© 1999, by the American Society of Animal Science and the American Dairy Science Association. All rights reserved.
AM Symp. 21-35