Veterinary Science - Theses
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ItemEffect of feeding slowly fermentable grains as protection against heat stress in ruminants( 2017)The combination of high environment temperatures, the heat increment resulting from feed fermentation and digestion (HI), high metabolic rates and metabolic heat production (HP) make ruminants susceptible to heat stress (HS). The reduction in the ratio forage-to-concentrate in the diet during HS is a traditional nutrition management aimed at increasing the energy content of the diet to compensate for reductions in dry matter intake (DMI) frequently observed in heat stressed ruminants. Wheat is a rapidly fermentable grain commonly used as an energy source for ruminants in Australia. However, the rapid rate of rumen starch fermentation of wheat is associated with rumen acidosis and elevated body temperature in cattle. By contrast, slowly fermentable grains, like corn, are associated with better utilization of metabolisable energy (ME) and reduced HI. Chemical treatment of wheat grains with sodium hydroxide (NaOH) or Bioprotect (BP) may reduce the rate of starch fermentation, HI and metabolic HP. The effect of feeding cereal grains differing in rumen fermentability on physiological, metabolic and productive responses of ruminants under high thermal load has not previously been evaluated. Therefore, it was hypothesised that by feeding slowly fermentable grains, the HI would be reduced, thereby reducing the effects of HS in ruminants. The research presented herein firstly investigated the differences between rapidly (barley and wheat) and slowly fermentable grains (corn, 3 % NaOH- and BP-treated wheat) on gas production kinetic parameters, pH, and starch and dry matter (DM) digestibility using in vitro experiments. Then, using in vivo experiments the effect of feeding either slowly or rapidly fermentable grain diets at different feed intake on HS related changes on physiological, metabolic and digestibility variables of wethers and the physiological, metabolic and productive responses of lactating dairy cows fed grain-based diets differing in grain fermentability during summer were investigated. In vitro experiments demonstrated the slower fermentability and higher pH during incubation of corn, BP- and 3 % NaOH-treated wheat compared with untreated wheat and barley. In vivo experiments demonstrated that HS induces changes in physiological variables, blood-acid base balance, and energy utilization in ruminants; that elevated DMI increases the thermal load of wethers under HS; that feeding wethers either a corn- or 3 % NaOH-treated wheat based diet, may increase the glucose available for intestinal absorption, can reduce the HI and ameliorates the physiological variables negatively affected by HS compared with untreated wheat; that the reduced total apparent starch digestibility of corn diets explains the improved responses observed in corn-diet fed wethers under HS condition; and that feeding 3 % NaOH-treated wheat improves apparent starch, DM digestibility, and DMI of wheat diet without increasing the metabolic HP. In the dairy cow experiment, a lower rectal temperature and higher milk yield was found in cows fed corn diets that were associated with reduced HI and metabolic HP, and higher glucose availability than in cows fed untreated wheat. It was also demonstrated that rumen temperature is a sensitive predictor of core body temperature and physiological and productive variables negatively affected by HS. Thus, this thesis demonstrated that the use of slowly fermentable grains is a viable nutrition management to improve thermo-tolerance of ruminants under high heat loads.
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ItemImpacts of heat stress on the biomarkers of oxidative stress in sheep and potential nutritional strategies for amelioration( 2015)Heat stress (HS) is a multi-billion dollar global problem as it impairs animal performance during the summer. Heat stress has been implicated in promoting oxidative stress either through excessive production of reactive oxygen species (ROS) or decreased antioxidant defence. Excessive ROS production overwhelms the antioxidant defence, and this leads to oxidative damage of biological molecules including proteins, lipids and DNA, which in turn disrupts normal metabolism and physiology. Therefore, a robust antioxidant network capable of preventing oxidative damage of biological molecules holds promise for improving the health and performance of animals during heat stress. This thesis investigated the impacts of heat stress on the oxidative stress biomarkers, elucidated potential role and optimized the dose of vitamin E and selenium required to ameliorate heat stress in sheep. The first study investigated the impact of HS and dietary antioxidant supplementation on the oxidative and physiological status of sheep. This study showed that heat stress negatively affects the oxidative status of sheep along with the physiological responses; however some of these affects can be ameliorated through dietary vitamin E (Vit E) and selenium(Se) supplementation at supranutritional concentrations. To understand the molecular mechanisms involved in heat stress ameliorative action of Vit E and Se, mRNA expression of heat shock proteins (HSP) and pro-inflammatory genes were investigated. High dietary antioxidants proved to modulate skeletal muscle expression of HSP, pro-inflammatory cytokine and NFĸB transcription factor, which may protect against HS in sheep. The potential role of high dietary Vit E or Se in amelioration of HS in sheep, and changes in acid base balance and respiratory oxidative stress biomarkers along with systemic biomarkers, were investigated in the second study. This study suggested that the hydrogen peroxide concentration in exhaled breath condensate (EBC) can be utilized as a novel biomarker to demonstrate respiratory oxidative stress induced by HS in sheep. The study also highlighted the potential of Se to reduce respiratory oxidative stress. While Vit E improved the physiological responses of heat stressed sheep; combined supplementation of Vit E and Se was required to prevent the acid base imbalance in heat stressed sheep. Finally, optimization of dietary levels of Vit E and Se to improve the performance and oxidative status of lambs finished during hot conditions was undertaken. This study suggested that supranutritional levels of dietary Vit E and Se increased average daily feed intake and the average daily gain in lambs during finishing and maintained the oxidative balance during exposure to HS. Thus it is suggested that heat stress leads to oxidative stress in sheep and supranutritional supplementation of dietary Vit E and Se can be used as a nutritional strategy to ameliorate negative effects of HS in sheep. Further research is required to elucidate the effects of dietary Vit E and Se on retail colour stability and shelf life of meat obtained from lambs finished under hot conditions.