Developing and validating a heat stress model and evaluating nutritional management strategies to mitigate heat stress and immune-challenges in dairy cows
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Heat stress (HS) negatively impacts animal productivity and welfare. Precisely studying HS typically requires expensive climate-controlled facilities, resources often inaccessible to most scientists. Thus, it is of interest to develop and validate alternative and cost-effective models to study HS and to assess nutritional HS mitigation strategies using this model in lactating dairy cows. Many of the negative consequences of HS appears to be mediated by intestine-derived lipopolysaccharide (LPS) and thus HS biology can be modeled by infusing LPS. Administrating LPS decreases circulating calcium (Ca) and decreases markedly both feed intake and milk yield in dairy cows. The current dissertation centered on developing, evaluating, and validating an alternative model to study HS and identifying nutritional management strategies to ameliorate negative consequences of both heat-stressed and immune-challenged dairy cows.
In studies 1 and 2 (Chapters 2 and 3), we evaluated the efficacy of using an electric heat blanket (EHB) as an alternative method to study HS and we observed similar changes in body temperature indices, production and metabolism to natural and climate-controlled HS experiments. Additionally, we validated the EHB model (using a pair-feeding design) and confirmed that lowered nutritional plane explains only ~50% of the decreased milk yield.
In study 3 (Chapter 4), we evaluated a dietary electrolyte, osmolyte, and energetic compounds (EOEC) on physiological parameters in heat-stressed cows using the EHB model. Feeding EOEC appears to increase heat dissipation by increasing skin temperature. Additionally, dietary EOEC altered metabolic and the blood gas profile in heat-stressed cows and thus is a promising nutritional strategy to utilize during the warm summer months.
In study 4 (Chapter 5), we examined a dietary Saccharomyces cerevisiae fermentation product (SCFP) on body temperature indices, metabolism, and acute phase protein response (APPR) in heat-stressed dairy cows using the EHB model. Results demonstrated that HS caused an APPR and that feeding SCFP could be beneficial at reducing circulating cortisol and the APPR.
In study 5 (Chapter 6), we investigated the effects of providing an oral supplement containing Ca and live yeast on circulating Ca and production parameters in immune-challenged dairy cows. Results suggest that increased circulating Ca improves production parameters during inflammation. Overall, utilizing an oral supplement may be a valuable management strategy to improve animal welfare and productivity during and following immunoactivation.
In conclusion, employing the EHB model provides an excellent new platform for discovery research and for evaluating pragmatic HS mitigation strategies. Results demonstrated that feeding EOEC could benefit heat dissipation and metabolism. In addition, feeding SCFP may be useful at reducing the amount of “stress” and immune activation during HS. Furthermore, infusing i.v. LPS appears to be an effective technique to model hypocalcemia and to evaluate dietary strategies aimed at increasing circulating Ca in periparturient lactating dairy cows. Collectively, understanding the biology of HS is important for identifying mitigation strategies aimed at ameliorating the negative consequences of HS in dairy cow.