Climate change has highlighted the detrimental effects of heat stress on sheep in many parts of the world, including Australia, the USA and countries in the Middle East. During periods of high heat load, physiological and immunological responses occur which allow sheep to cope with this stressor. However, these responses may have adverse effects on the performance and health of sheep.
Improved knowledge of the mechanisms of acclimation to high heat load (leading to heat stress) may contribute to the development of strategies for reducing the impact of high heat load on sheep. One such strategy involves dietary supplementation with selenium (Se) and vitamin E. However, detailed information on the effects of these micronutrients on heat-stressed livestock, particularly ruminants, is scarce. Therefore, three studies were conducted to (i) investigate the physiological responses of Australian Merino sheep to a high heat load and, (ii) to evaluate the effects of selenium and vitamin E supplementation under hot conditions.
In the first study, the heat tolerance of Australian Merino sheep was investigated by measuring physiological responses of 12 Merino wethers to repeated episodes of high heat load. The wethers were divided into two groups and the sheep were individually housed in pens within an environmental chamber and were subjected to 2 days of thermoneutral conditions (TNCs; TMAX 24°C, TMIN 16°C) followed by either 7 days of TNCs or hot conditions (TMAX 38°C, TMIN 28°C), after which they were exposed to 2 days of TNCs. Rectal temperature (RT) and respiration rate (RR) were measured 7 times each day. Feed and water intakes were measured daily and the wethers were weighed on days 1 and 11. Blood samples were collected twice to measure of the serum concentrations of creatine, glucose, total protein, cholesterol and non-esterified fatty acid. There were increases in RT (from 39.10 to 39.83°C), RR (from 39.4 to 109.9 breaths/min) and water intake for the sheep exposed to 7 days of heat, and reductions in feed intake (–22%), body weight (–2.5%) and serum creatine concentration. These results indicate that Australian Merinos are able to maintain RT within the normal range (37.5 to 40.5ºC) during exposure to a prolonged heat load and that they recover from the above negative effects of heat stress within 2 days. It would appear that they have high heat tolerance.
The aim of the second study was to determine the effects of various doses of injected Se on sheep exposed to high heat load. Fifteen Merino wethers were assigned to one of three Se treatments (0, 0.5 and 5 mg of Se), which were administered as subcutaneous sodium selenate injections on days 1, 8 and 15 of exposure to high heat load. The animals were housed individually in an environmental chamber and were exposed to a temperature of 38°C from 0700 to 1800 for 21 days. RT and RR were measured 3 times daily. Feed intake was measured daily and sheep were weighed weekly. Blood samples were collected on days 1 and 21. Sheep injected with 5 mg of Se exhibited a 0.3°C reduction in RT and a 4.5% lower bodyweight loss compared with non-injected sheep. While, injection sheep with 0.5 mg of Se did not affect on RT and bodyweight.
In the third study, 42 Merino wethers were used in a 50-day trial to investigate the effects of supplementation with Se and vitamin E on physiological and immunological responses during exposure to high heat load. Sheep were assigned to one of seven treatments that differed in the concentration of dietary Se (0 or 0.8 mg/kg DM) and vitamin E (0 or 150 mg/kg DM) and the duration of Se and vitamin E supplementation (22 or 50 days). Sheep were subjected to TNCs (24°C) for 28 days, followed by 22 days of exposure to hot conditions (38°C). Feed intake was measured daily, and sheep were weighed weekly. Blood samples were collected to measure the concentrations of biochemical, enzymatic and hematological variables, total primary antibodies and antioxidant status. Exposure to hot conditions decreased feed intake (–11.9%), average daily gain (–198g), total immunoglobulin antibodies (–12.4%), phagocytic activity of neutrophils (–17.3%) and antioxidant status (–26.2%) among all treatments. The provision of Se and vitamin E in excess of their recommended dietary levels (0.1 and 15 mg/kg DM, respectively) for 50 days appeared to decrease bodyweight loss and to improve immunological responses and oxidative status.
In conclusion, the experiments described in this thesis have contributed to our understanding of the impact of heat load on physiological responses of modern Australian Merino sheep and indicate that supplementary Se and vitamin E may be used to reduce the adverse effects of thermal stress. Furthermore, these results could be used as a foundation for further studies on the addition of Se and vitamin E to the diets of sheep during exposure to heat stress because the impact of these micronutrients on sheep under hot conditions is not completely understood. Therefore, further studies are needed to determine the dynamics of Se and vitamin E supplementation on productivity and health.