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Abstract
Deep understanding of the impact of heat stress on cows allows the implementation of effective strategies to enhance cow productivity and welfare during summer. In chapter 3-5, we evaluated the metabolism and inflammation of cows fed two zinc sources (Zn-hydroxychloride vs. Zn-Met complex) under two environmental conditions (cooling [CL] vs. non-cooling [NC]) prior to and following an intramammary lipopolysaccharides infusion (IM-LPS) during summer. Non-cooling reduced plasma metabolite concentrations, but increased insulin concentration. Dietary Zn source had no impact on systemic and mammary metabolism. Following IM-LPS, NC induced greater leukocyte migration into the mammary gland after IM-LPS, and NC maintained lower plasma glucose concentrations but had a more rapid and prolonged increase in serum insulin concentration than CL. Further, NC influenced the systemic and mammary mineral metabolism after IM-LPS, however, dietary Zn source had no impacts. In chapter 6, we evaluate hormonal, inflammatory and immunological responses of lactating dairy cows fed an immunomodulatory supplement (OmniGen® AF, OG) and under CL vs. NC prior to and following an intravenous LPS infusion (IV-LPS) during summer. Feeding OG enhanced cortisol release under basal conditions and after IV-LPS in CL but not NC, suggesting that heat stress inhibits the OG mediated cortisol release. Further, NC enhanced inflammatory responses of circulating lymphocytes stimulated with mitogens ex vivo than CL. Yet, OG promoted the proliferation of immune cells isolated from cows under NC. In chapter 7, we examined the behavioral responses of cows under CL and NC. NC reduced lying, rumination, and eating time but increased walking time and activity level. Cows under NC sorted more for long particles from the TMR than CL. Chapter 8 evaluated associations between behaviors and body temperature of lactating dairy cows on 3 commercial dairy farms, and we demonstrated that the effectiveness of cooling facility strongly affects the associations between lying/standing behavior and body temperature, but was not associated with daily walking activity and ruminating time under chronic heat stress condition and consistent management. These studies highlight the importance of implementing effective cooling systems and nutritional programs to reduce the adverse effects of heat stress on cow performance.