Physiological Responses of Cattle to Heat Stress

Pakistan lies in the Warm Climate Zones of the world and is subjected to extended periods of high ambient temperature and relative humidity. The objective of this article is to review the physiologic/adaptive responses of acclimated cattle to heat stress emphasizing its affects on nutrient acquisition and metabolism, acid-base chemistry and hormonal balance of the body. Heat stress invokes a decrease in dry matter intake, enhanced digestibility and reduced blood flow to the forestomach. The energy metabolism decreases while the water and electrolyte metabolism increases. A nycthermal pattern of acid-base balance is manifested by a high respiratory rate leading to respiratory alkalosis in the hot day time hours; and lower urine pH and greater urine ammonia concentrations in the cool night time hours. Hormones involved in thermal adaptations include, prolactin, growth hormone, thyroxine, glucocorticoids, mineralocorticoids, catecholamines and antidiuretic hormone. These are either involved with nutrient partitioning and homeorhesis or for homeostatic regulation, augmented by thermal stressor. All these adaptive physiologic responses are the same in Bos taurus and Bos indicus but are less pronounced in the later, making it a thermotolerant breed. Cholistani breed of cattle may be utilized as a model to study the physiologic responses augmented by heat stress both on molecular and endocrinological levels.