Date of Graduation

8-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Poultry Science (PhD)

Degree Level

Graduate

Department

Poultry Science

Advisor/Mentor

Rochell, Samuel J.

Committee Member

Dridi, Sami

Second Committee Member

Kidd, Michael T.

Keywords

Amino acids; Broilers; Heat stress; Metabolism; Nutrition

Abstract

The simultaneous increase in the global demand for animal-based protein products and the rise in temperature associated with global warming pose important challenges to the poultry industry. Heat stress (HS) occurs when an animal’s heat production (HP) surpasses its capacity to dissipate heat into the surrounding environment, ultimately leading to reduced feed intake (FI) and negative impacts on performance. A better understanding of bird responses to elevated temperatures is required to propose effective nutritional strategies that mitigate the adverse effects of HS. Therefore, this dissertation aimed to characterize the nutritional and physiological mechanisms explaining the loss in performance and carcass quality observed in growing broilers under HS conditions, and to determine how these effects can be minimized through nutritional interventions. The first experiment compared the effects on performance, carcass characteristics, and meat quality of chronic constant and cyclic HS models, along with a pair-feeding treatment in which birds were fed the same quantity of feed consumed by HS birds. Constant HS had a more detrimental effect than cyclic HS, with reduced FI accounting for approximately 80% of the reduced body weight gain. Moreover, HS led to reduced breast muscle deposition and increased abdominal fat deposition. Further investigations were conducted to characterize the effect of these chronic HS models, as well as an acute HS model on nutrient digestibility, stress, inflammation, and markers of protein and lipid metabolism. Constant HS induced higher physiological stress compared to the other tested models, and no evidence of adaptation to chronic HS exposure was found when comparing with acute HS birds. Interestingly, reduced digestibility caused by HS only partly explained the reduced performance that occurred independently of the HS-induced reduction in FI. Lipid and protein metabolism were markedly affected by constant HS, although further investigation is needed to explore lipid metabolism regulation as gene expression markers and carcass observations did not align. Regarding protein metabolism, constant HS downregulated protein synthesis due to the HS-induced reduction in FI, while protein degradation was upregulated due to HS per se. In the last experiment, the effects of increased dietary density of select or all essential amino acids (AA) on growth and carcass characteristics were evaluated under constant HS conditions, and energy and nitrogen partitioning were quantified using the comparative slaughter technique. Reduced FI and increased energy needs for maintenance functions compromised energy availability in HS birds. Additionally, HS shifted the energy storage from fat to protein due to the lower heat increment associated with fat metabolism. However, increased dietary AA density did not show beneficial effects for HS birds, suggesting that reduced protein deposition and growth are not associated with a lack of digestible AA intake. Instead, additional essential AA may be directed toward non-productive functions, such as the production of energy for maintenance needs or proteins involved in thermoregulation and inflammation processes. Overall, this dissertation provides valuable insights into the detrimental impact of HS beyond the HS-induced reduction in FI. Further research is required to develop feeding strategies that promote protein deposition under HS conditions.

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