Nutritional modulation of oxidative stress in beef steers during the feedlot receiving period: A focus on transit stress
Feedlot receiving (i.e. the first 42 to 56 d after feedlot arrival) is a critical time period in the life of many beef animals. The combination of stressors (e. g. weaning, vaccination, commingling, novel pathogens and feedstuffs) experienced during feedlot receiving results in poor feed intake, decreased growth and increased susceptibility to disease. Transportation is a component of receiving period stress that is unavoidable due to the segmented nature of the beef industry. Transported cattle are subject to feed and water deprivation, psychological stress and physical exertion which may further hinder animal health and production efficiency. Therefore, investigation of strategies to mitigate the negative effects of receiving period and transit stress is warranted. The first objective of this research was to gain a more thorough understanding of how cattle biologically respond to transit stress, with an emphasis on transit-induced changes in oxidative stress biomarkers. Results from the studies presented herein have shown 1) a 10% decrease in plasma ascorbate and total antioxidant concentrations immediately post-transit, 2) a 16% increase in red blood cell lysate Mn-superoxide dismutase activity 1 d post-transit and 3) increased liver and muscle superoxide dismutase activity immediately post-transit. This research has also provided evidence that transportation of cattle stimulates tissue mobilization of energy producing substrates and elicits an inflammatory response. These biological processes can contribute to the production of reactive oxygen species and, in combination with the observed decrease in antioxidant status, result in oxidative stress and upregulation of antioxidant enzymes. Because endogenous synthesis of antioxidants requires nutrients that might otherwise be used by the animal for growth and immune function, the second objective of this research was to determine if improving antioxidant status of beef steers with nutritional supplements would increase resilience or aid in recovery from receiving period or transit stress. Increasing dietary supplementation of vitamin E, a fat-soluble antioxidant, during the receiving period linearly increased vitamin E status, but decreased concentrations of the endogenous antioxidant glutathione, likely due to a sparing effect of vitamin E. Minimal effects of supplemental vitamin E on feedlot health and performance were noted, probably because initial vitamin E status of steers was adequate and vitamin E deficiency did not develop in this 28 d study. Steers supplemented a yeast fermentation product with indirect antioxidant properties tended to have greater liver glutathione concentrations prior to a long-distance transit event and exhibited greater average daily gain for the first 30 d post-transit. In a separate study, superoxide dismutase activity and metabolites involved in the pentose phosphate pathway were increased in muscle of transported steers, suggesting the physical exertion associated with transit resulted in oxidative stress in the muscle, which could be detrimental to post-transit growth. To mitigate this response, steers were administered injectable vitamin C, a potent antioxidant found in large quantities in the muscle, immediately prior to or after a long-distance transit event. Steers administered vitamin C pre-transit had greater post-transit plasma ascorbate concentrations and exhibited greater average daily gain for the 56 d post-transit than steers that did not receive vitamin C or steers that received vitamin C after the transit event. Collectively, these data indicate nutritional strategies to improve antioxidant status are more effective in improving post-transit performance when they are adopted proactively (pre-transit) rather than retroactively (post-transit). Future research should seek to better understand the long-term implications of oxidative stress on cattle health and performance. Additionally, further refinement of the nutritional strategies utilized herein is needed to optimize dose and timing of supplementation for oxidative stress modulation.