288. Award Talk: Amelioration of Heat Stress Using Dietary Mineral (selenium and Chromium) Supplementation.

Abstract Climate change is expected to increase the risk of heat stress for livestock in many parts of the world, with significant consequences for animal health, productivity, welfare and agricultural economies. A major adaptive response to heat stress (HS) is systemic oxidative stress which occurs when there is an imbalance between the production of reactive oxygen species (ROS) and the capacity of antioxidant defenses. The redistribution of blood flow away from the body core to the periphery to increase the radiant heat loss that occurs during HS deprives the viscera of blood flow, causing ischemia/hypoxia, leading to increased production of ROS and OS. One of the results of this OS is decreased gastrointestinal epithelial resistance and increased permeability as well as extravasation of multiple organs. Selenium is an important component of many enzymes involved in antioxidant activity, including glutathione peroxidase (GPx). Organic protein-bound or chelated forms of dietary Se are much more bioavailable and safer at high doses than inorganic Se and so offer a means of improving the antioxidant status of animals subject to OS. It is well established that HS also decreases feed intake and increases respiration rate, rectal temperature, heart rate, and ROS in blood and exhaled breath condensate in many livestock and laboratory species. Many of these effects can be fully or partially reversed by supra-nutritional dietary supplementation with organic Se and Vitamin E. Our initial work established that feeding Se-enriched mushrooms could reverse the effects of HS on ileal conductivity and, therefore, decrease HS-induced gut permeability in rats. This reversal in conductivity was associated with increased gastrointestinal GPx activity. It has since been established that dietary organic Se and Vitamin E were able to reverse the effects of HS on ileal and jejunal epithelial resistance and permeability in pigs. These improvements were again accompanied by increased GPx activity as well as the ratio of oxidized to reduced intestinal glutathione. These effects were dose-responsive, and the effects of Se and Vitamin E were often complementary. These findings have been extended to the beef, sheep, swine, and dairy industries, and based largely on this research several feed additive companies are now offering proprietary mixes of Se and other antioxidants to combat HS. Another metabolic adaptation to HS appears to be an increase in insulin sensitivity, possibly to reduce lipolysis and reliance on mobilized fatty acids as an energy source. Heat stress consistently causes a reduction in blood pCO2, an increase in blood pO2, and a resultant decrease in base excess and an increase in blood pH in sheep. Utilization of fatty acids as an energy source when ruminants decrease feed intake during HS would further exacerbate these effects on blood pH. So, an increase in insulin sensitivity during HS will ensure glucose is the preferred substrate, and a further improvement in insulin sensitivity may reduce the impact of HS in livestock. In a recent review, we have proposed that these changes in insulin sensitivity may be mediated, at least in part, by prolactin. Chromium is a recognized insulin mimic, and recently, we showed that dietary nano-chromium picolinate (nCrPic) can improve insulin sensitivity in sheep under both thermoneutral and HS conditions. Importantly, dietary nCrPic reduced the impact of HS on respiration rate, rectal temperature, and growth performance in sheep. This work indicated that the improvement in insulin sensitivity may be due to a reduction in JNK mRNA expression in skeletal muscle and, hence, prevent cells from progressing towards the apoptosis pathway when animals are under HS. Dietary CrPic also improved insulin sensitivity and protected against HS in growing pigs. Changes in the expression of genes involved in insulin signalling in both skeletal muscle and adipose tissue of pigs showed that dietary nCrPic improved insulin sensitivity in pigs, particularly when consuming a high-fat diet. These findings have been extended to the dairy and swine industries, and several feed additive companies are now including Cr in summer pre-mixes to combat HS. This is particularly so for sow lactation diets in Australia and Oceania. Therefore, dietary Se and Cr supplementation offer complementary means to ameliorate the effects of HS in livestock. The effects of climate change will make the issues related to HS become more critical, especially as the regions in the world where livestock production is going to increase are largely where the impacts of climate change are greatest.