Nutritional Physiology and Biochemistry of Dairy Cattle under the Influence of Heat Stress: Consequences and Opportunities

SIMPLE SUMMARY: Modern dairy cows have elevated internal heat loads caused by high milk production, and the effects of accumulating incremental heat are exacerbated when temperature and humidity increases in the surroundings. To shed this additional heat, cows initiate a variety of adaptive mechanisms including increased respiration rate, panting, sweating, reduced milk yield, vasodilatation, and decreased reproductive performance. Hormonal changes based on reciprocal alterations to the energetic metabolism are particularly accountable for reduced efficiency of the dairy production under the heat stress. As animals experience negative energy balance; glucose, which is also a precursor of milk lactose, becomes the preferential energy fuel. In the absence of proper mitigations, heat stress possesses potential risk of economic losses to dairy sector. Besides physical measures for the timely prediction of the actual heat stress coupled with its proper amelioration, nutritional mitigation strategies should target modulating energetic metabolism and rumen environment. ABSTRACT: Higher milk yield and prolificacy of the modern dairy cattle requires high metabolism activities to support them. It causes high heat production by the body, which coupled with increasing environmental temperatures results in heat stress (HS). Production, health, and welfare of modern cattle are severely jeopardized due to their low adaptability to hot conditions. Animal activates a variety of physiological, endocrine, and behavioral mechanisms to cope with HS. Traditionally, decreased feed intake is considered as the major factor towards negative energy balance (NEBAL) leading to a decline in milk production. However, reciprocal changes related to insulin; glucose metabolism; failure of adipose mobilization; and skeletal muscle metabolism have appeared to be the major culprits behind HS specific NEBAL. There exists high insulin activity and glucose become preferential energy fuel. Physiological biochemistry of the heat stressed cows is characterized by low-fat reserves derived NEFA (non-esterified fatty acids) response, despite high energy demands. Besides these, physiological and gut-associated changes and poor feeding practices can further compromise the welfare and production of the heat-stressed cows. Better understanding of HS specific nutritional physiology and metabolic biochemistry of the dairy cattle will primarily help to devise practical interventions in this context. Proper assessment of the HS in cattle and thereby applying relevant cooling measures at dairy seems to be the basic mitigation approach. Score of the nutritional strategies be applied in the eve of HS should target supporting physiological responses of abatement and fulfilling the deficiencies possessed, such as water and minerals. Second line of abatement constitutes proper feeding, which could augment metabolic activities and synergizes energy support. The third line of supplemental supports should be directed towards modulating the metabolic (propionates, thiazolidinediones, dietary buffers, probiotics, and fermentates) and antioxidant responses (vitamins). Comprehensive understanding of the energetic metabolism dynamics under the impact of incremental heat load and complete outlook of pros and cons of the dietary ameliorating substances together with the discovery of the newer relevant supplementations constitutes the future avenues in this context.