Potential Nutritional Strategies for Intensively Managed Cattle during Thermal Stress

Thermal stress can impact negatively on performance of intensively managed livestock in the southern United States and other subtropical and tropical regions of the world. Three fundamental management practices, physical protection, genetic development and nutritional management schemes, have been proposed to alleviate partially the extent and severity of thermal stress. Focus of this paper is on 1) interrelationships of thermal stress with functions involved in acquisition of nutrients and their metabolism, 2) considerations for provision of energy and nutrients during thermal stress and 3) potential practical management strategies to enhance animal productivity and efficiency. Discussion concentrates primarily on cattle. Acquisition of potentially absorbable nutrients and energy by the thermal-stressed animal is affected negatively by reduction in dry matter intake, positively by slightly enhanced digestibility because of greater retention time in the gut and, probably, negatively by reduced blood flow to and nutrient absorption from the alimentary tract. Negative influences of reduced intake and absorption of nutrients largely offset small enhancement in digestibility, resulting in less total nutrients being available to the thermal-stressed animal. Physiological adaptations to acute and(or) chronic thermal stress alter metabolism of absorbed energy substrates and nutrients. Hormones involved in adaptation to thermal stress include prolactin, growth hormone, thyroxine, glucocorticoids, antidiuretic hormone and aldosterone. These are either involved with nutrient partitioning and homeorhesis or with need for homeostatic regulation, potentiated by the thermal stressor. Increasing the energy density of diets fed to thermal-stressed animals by increasing concentrate to forage ratios or adding protected fat is discussed. For supplemental protein, the idea is suggested that increasing nutrient content may not always result in the most biologically and economically efficient production for a thermal-stressed animal. Water and macromineral needs, influenced heavily by demands to maintain homeostasis and homeothermy, are altered for lactating dairy cows. However, this may not be true for animals with lower turnover rates of these nutrients. A number of nutritional technologies are available to aid the thermal-stressed animal; others are not yet developed. Establishment of prediction equations for requirements of nutrients fed to livestock in different productive states during various intensities of thermal stress are needed. Forms of equations should describe the absolute nutrient needs (e.g., g·kg milk−1·d−1 ) rather than requirements based on percentage of diet dry matter.