Nutritional strategies to alleviate heat stress in pigs

© 2016 Dr. Fan LiuHeat stress (HS) is a production, welfare and health issue during summer. With the impacts of climate change forecast to increase global temperatures, increasing participation in tropical agriculture and selection for more productive genotypes, HS is both a current and emerging problem. Heat stress compromises efficient pig production by diverting energy from optimal pathways for growth, lactation and reproduction. Strategies to ameliorate HS in production pigs include engineering controls such as improved ventilation, shade and misting sprays as well as adjusting the macronutrient formulation of the diet. However, there is increasing evidence that development of low cost micronutrient additives can be of great benefit to production animals experiencing HS. The objective of this thesis was to investigate micronutrient additives as a means of ameliorating the impacts of HS on pig physiology. The application of this knowledge will be to augment existing strategies, reduce seasonal variation in production, improve productivity in tropical environments and provide an adaptation strategy against climate change. The main impacts of HS include the reduced feed intake, altered physiology, impaired intestinal barrier function, and altered insulin-related metabolism, and these impacts may be partially related with increased oxidative stress. Macronutrient modification is one management strategy however there is emerging evidence for the benefits of micronutrient formulations such as selenium (Se), vitamin E (VE), chromium (Cr) and betaine (Chapter 2). The unified hypothesis tested in this thesis was that supplementations with the functional nutrients (Se, VE, Cr, betaine) above the current NRC recommendations can mitigate impacts of HS in pigs. Methodology was stated in the Chapter 3, and briefly in each experimental chapter. As oxidative stress may be associated with various impacts of HS such as hyperinsulinemia and increased adiposity, the effects of supranutritional Se (1.0 ppm) and VE (200IU/kg) supplementation on oxidative stress markers as well as physiological response to HS were separately quantified in Chapters 4 and 5. The outcomes of the experiment in Chapter 4 showed that HS increased oxidative stress and reduced lipid mobilization independently of insulin concentrations. Supranutritional Se reduced hyperthermia, as indicated by reductions in rectal temperature compared to control, and VE ameliorated respiratory alkalosis in the heat-stressed pigs, indicating improved tolerance to HS. However, neither the Se or VE supplementation reduced markers of oxidative stress. As VE and Se differ in their biological function, with VE being rapidly absorbed into tissue and distributed into cell membranes where it prevents lipid peroxidation. Alternatively, Se is incorporated into newly synthesized proteins, where it contributes to a high capacity enzymatic route of elimination for free radicals. As HS was demonstrated to increase oxidative stress in Chapters 4 and 5, it was proposed that a combination of VE and Se would be more effective than either nutrient in isolation. Therefore, in Chapter 6 the influence of Se and VE combined supplementation was investigated on intestinal epithelial barrier integrity, a known site of oxidative damage during HS. This experiment confirmed that HS induced oxidative damage which in turn induced mucosal damage, as indicated by increased permeability. Addition of supranutrtional concentration of Se and VE reduced oxidative stress and protected against intestinal damage. This result was then further confirmed using a cell culture model of the porcine jejunum. Radiant heat loss is one of the primary heat dissipation measures by the pig and occurs by redistributing blood flow from the core to the periphery. The result is that visceral organs such as the intestines have insufficient blood flow and can result in tissue hypoxia and oxidative stress, compromising mucosal barrier function. Utilizing the non-transformed porcine jejunum cells (IPEC-J2), it was demonstrated that the effects of oxidative stress (Chapter 8) and hypoxia (Chapter 7) were more damaging than the direct effects of increased temperature (Chapter 7). Furthermore, consistent with its antioxidant function, Se mitigated the damage induced by oxidative but not hypoxic stress on barrier integrity of IPEC-J2 monolayers (Chapter 8). Improving insulin sensitivity may increase skin blood flow and facilitate heat dissipation during HS conditions, however, antioxidant supplementation did not improve insulin sensitivity as observed in the HS conditions (Chapters 4), therefore the effect of supplementation with a known insulin sensitizer, Cr (400 ppb in the form of chromium picolinate), was investigated in Chapter 9. Chromium augments insulin sensitivity when incorporated into the protein chromodulin, which augments insulin receptor binding. The results indicated that Cr improved insulin sensitivity. Furthermore, Cr fed pigs were more heat tolerant, as evidenced by lowered body temperature and respiration rate. The experiments conducted up this point had been under climatic controlled settings. To demonstrate application to production settings in the final experiment a standard NRC diet was reformulated to include Se, VE, Cr and betaine at levels identified through our previous investigations in lactating sows, which are highly susceptible to HS, during a typical summer at a commercial piggery (Chapter 10). The post-partum period is marked by substantial weight loss in sows as they divert energy from lactation. The primary result of the experiment was that sows on the experimental diet experienced a lesser degree of weight loss than control sows, indicating an improved tolerance to summer production and is expected to help producers counter seasonal production losses. The unified hypothesis tested in this thesis was that supplementations with the functional nutrients (Se, VE, Cr, betaine) above the current NRC recommendations can mitigate the negative impacts of HS in pigs. Taken together, the unified hypothesis of this thesis is accepted that the nutritional supplementation such as Se, VE, Cr and betaine above the current NRC recommendations can mitigate the impacts of HS in pigs