Influence of stress induction on trace mineral homeostasis and metabolism in ruminants

Cattle are exposed to stressors throughout the production cycle, many of which occur simultaneously. These stressors may include weaning, transportation, vaccination, and arrival into the feedlot which can be associated with co-mingling, as well as adaptation to dietary changes. Furthermore, these factors can create numerous challenges in terms of supplying adequate trace mineral nutrition to cattle, as these periods are also associated with decreased feed intake, which can further exacerbate a trace mineral deficiency. However, there is limited understanding of how stress impacts trace mineral homeostasis in the ruminant. Thus, by improving our understanding of alterations to trace mineral homeostasis in the presence of stress it may be possible to develop strategic trace mineral supplementation strategies to improve animal efficiency. Stress is a broad term that can be experimentally induced by several methodologies including, but not limited to, adrenocorticotropic hormone (ACTH), lipopolysaccharide (LPS) challenge, and feed and/or water restriction. The overall goal of this research was to evaluate the response to stress induction in ruminants with either differing trace mineral statuses or receiving differing trace mineral supplementation strategies. The first experiment sought to understand how ACTH challenge would affect the absorption and retention of trace minerals, in lambs receiving a moderately Zn deplete diet or the same diet with supplemental Zn. The results suggested ACTH may negatively impact Zn apparent absorption, regardless of Zn status of the animal. Furthermore, it was shown that ACTH administration can alter plasma Zn status of the animal; collectively, these data suggest that Zn metabolism is altered in response to stress in ruminants. The second trial sought to understand if Zn supplementation with either NASEM (2016) recommended concentrations of 30 mg of Zn/kg DM or industry standard supplemental concentrations (Samuelson et al., 2016) of 100 mg Zn/kg DM to receiving steers, would affect the innate immune response or plasma TM homeostasis in response to injection of LPS at one of two doses. Lipopolysaccharide is a potent stimulator of the innate immune system. There were minimal interactions between supplemental Zn concentration and LPS injection treatment; however, a nutritional immunity response was observed in which plasma concentrations of Zn and Fe were dramatically decreased in response to LPS, although with slightly different time courses. Interestingly, it appears that increased supplemental Zn may have allowed steers to recover plasma Zn status more rapidly than those receiving only 30 mg Zn/kg DM. While the first two projects included a targeted injection treatment to create stress, the final project sought to mimic production conditions. This was accomplished by limiting feed intake over a 5 d period in steers that were either Cu deficient, receiving a Cu deficient diet or steers with marginal Cu status, but receiving a Cu adequate diet. Copper critically supports macronutrient metabolism; thus, this study sought to evaluate the effects of Cu supplementation and feed restriction on the plasma metabolome of steers. The results of this study suggest an impaired ability of Cu deficient steers to utilize mobilized fatty acids as a fuel source during feed restriction. This is of interest, because while a role for Cu supplementation in supporting mobilization of lipids has previously been established, these data suggest Cu is also important in the utilization of these mobilized lipids. However, minimal effects were observed on the plasma metabolome due to Cu supplementation. Collectively these data support the assertion that stressors can disrupt ruminant trace mineral homeostasis. However, further research is needed to establish the underlying mechanisms creating these outcomes, to develop trace mineral supplementation strategies for the stressed ruminant