Optimization of steroidal implant-induced growth in beef cattle through strategic supplementation of zinc and manganese

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Smerchek, Dathan Thomas
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Hansen, Stephanie L
Genther-Shroeder, Olivia N
Lonergan, Elisabeth J
Loy, Daniel D
Thomson, Daniel U
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Animal Science
High-potency combination steroidal implants containing estradiol (E2) and trenbolone acetate (TBA) consistently improve growth rate, feed efficiency, and carcass leanness in cattle. Steroidal implants represent an integral component of the continued economic and environmental sustainability of the U.S. beef production industry. Further optimization of growth enhancing technology use, such as steroidal implants, through strategic supplementation of trace minerals (TM) represents an opportunity to strengthen the economic sustainability of the beef industry. Trace minerals are vital components of biological functions related to whole body growth and development. Zinc (Zn) is critical in an extensive array of cellular and physiological processes as an enzyme cofactor, and a component of transcription factors. How TM accumulate in the carcass over time and as a result of implant is poorly understood. Zinc is the most utilized metal in biological processes and supports whole body growth through critical roles in protein synthesis, energy metabolism, function of insulin-like growth factor-1, growth hormone, and DNA synthesis. Manganese (Mn) is an essential TM providing catalytic activity to enzymes involved in protein glycosylation, nitrogen metabolism, and antioxidant capacity. The studies completed and described in this dissertation aim to add to a body of literature focused on strategic supplementation of TM to optimize steroidal implant-induced growth performance in finishing beef steers. Findings from studies herein will help inform TM supplementation recommendations in finishing feedlot rations. These studies were designed to 1) investigate changes in carcass TM concentration as influenced by steroidal implant use and days in the feedlot, 2) determine how increasing supplemental Zn concentration affects growth, carcass characteristics, TM status, relevant circulating metabolites, and mRNA abundance of genes related to skeletal muscle growth and TM metabolism, of implanted vs. non-implanted finishing beef steers 3) determine the effects of increasing Mn supplementation on growth performance, carcass characteristics, circulating metabolites, hepatic gene expression and enzyme activity, and markers of TM metabolism in implanted vs. non-implanted finishing beef steers. Our first objective revealed that muscle Zn accumulation is influenced by steroidal implant administration, underscoring the intricate interplay between TM metabolism and physiological processes in growing skeletal muscle related to steroidal implant-induced growth. The second objective built upon considerable prior work investigating the synergistic effect of increased supplemental Zn on steroidal implant-induced growth. Here we found Zn supplementation resulted in greater steroidal implant-induced growth during the first 28 days post-implant when ADG was greatest but did not affect overall ADG across the finishing period. Additionally in this study, steroidal implants resulted in improved insulin sensitivity and caused a 13.8% decrease in liver Mn in implanted steers from d 0 to 20. Finally, our third objective revealed dietary Mn of 14 mg/kg DM did not limit growth of implanted or non-implanted cattle. This supports the current NASEM (2016) Mn recommendation of 20 mg Mn/kg DM as sufficient, especially with the presence of common Mn antagonists such as iron in cattle diets. Additionally, correlations suggest changes in liver Mn are related closely with liver arginase activity and circulating SUN in finishing beef cattle. Results from experiments detailed in this dissertation found implanted cattle have increased muscle Zn content and supplementing Zn (as ZnSO4) at concentrations greater than NASEM (2016) recommendations is valuable and can result in tangible improvements in steroidal implant-induced growth performance. Here we found 84 mg total Zn/kg DM was sufficient to allow for optimized growth, but consideration must be given to cattle type and basal diet Zn concentration. Additionally, our results suggest supplementing Mn at greater than NASEM (2016) recommendations does not affect overall growth performance in implanted or non-implanted cattle. Thus, the current NASEM (2016) recommendation for supplemental Mn (20 mg Mn/kg DM) will allow for optimal implant-induced growth. Overall, this series of experiments indicates implanted cattle have greater muscle demand for Zn and Zn appears to contribute to enhanced implant-induced growth to a greater degree than Mn. Further investigation of other TM such as cobalt (Co) may be warranted to further elucidate synergistic effects of increased TM supplementation to cattle given a steroidal implant.
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