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Browsing by Author "Stewart, Jacob Wesley"

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    Physiological consequences of exposure to heat stress and the mycotoxin zearalenone
    Stewart, Jacob Wesley (Virginia Tech, 2022-01-07)
    Heat stress is detrimental across many, if not most aspects of animal agriculture. Heat stress can have direct effects on the animal, effects on their environment and effects on the feedstuffs they consume. The work presented here will address direct effects on the animal (using dairy cattle) and effects on their feed (using pigs). Dairy cattle were chosen to study direct effects on the animal because they exhibit unique metabolic adaptions during heat stress. One adaptation of interest is a heat-stress induced state of hyperinsulinemia and hypoglycemia while self-limiting feed intake. In regards to effects on feedstuffs, high ambient temperatures are associated with elevated concentrations of mycotoxins in cereal grains. Mycotoxins are problematic because, at sufficient concentrations, they induce specific, toxic effects. Zearalenone is one such mycotoxin that is particularly problematic for swine, as it is a potent phytoestrogen. As such, it can reduce productivity by disrupting reproductive processes. Although these topics are disparate, the overarching goal of this work is to improve agricultural animal productivity during periods of heat stress. Overall, the three objectives of this thesis were to: 1) isolate the production-related effects of hyperinsulinemia with hypoglycemia from heat stress, 2) determine whether glucose supplementation during heat stress could improve or rescue milk production during heat stress, and finally, 3) investigate reproductive tract morphology following different durations of zearalenone consumption in peri-pubertal gilts. The first experiment compared the effects of thermoneutral, thermoneutral + hyperinsulinemic-hypoglycemic clamp (HHC), environmental heat stress, and heat stress + euglycemic clamp (EC) on milk production, milk components, and circulating glucose concentrations. Milk production, as expected, did decrease under heat stress when compared to thermoneutral conditions. Milk production during the HHC was intermediate, which was likely the result of the induced hypoglycemia. Interestingly, the glucose infused during the EC did not improve milk production over heat stress. Baseline blood glucose concentrations declined during heat stress, and those concentrations measured during the thermoneutral period were correlated with the change in blood glucose (from thermoneutral to heat stress). In the second experiment, zearalenone (6 mg) was fed to peri-pubertal gilts for 0, 7 or 21 days. Gilts were monitored daily for observable signs of zearalenone toxicity, and tissues were harvested after 21 days of treatment. No differences in the lengths or weights of any portions of the reproductive tract were observed. Likewise, there were no differences in body weight, carcass weight, dressing percent or liver weight. In summary, heat stress has varied effects on livestock production and the ultimate outcomes are not always predictable. Future work will be aimed at defining the factors that contribute to this variability.