Supporting muscle growth of feedlot cattle administered growth enhancing technologies through strategic zinc supplementation

Abstract

Growth enhancing technologies (GET) are a pillar of the beef production system that provide improvements to growth, feed efficiency, and carcass leanness. Steroidal implants and beta-adrenergic agonists (β-AA) are widely used in the beef industry and are critical for meeting strong consumer demand for beef with fewer animals. The performance response to GET is influenced by nutritional management, making optimized GET use paramount to capture benefits. Zinc (Zn) is an essential trace mineral involved in whole-body growth, energy metabolism, and protein and DNA synthesis. Therefore, optimizing Zn concentration in diets fed to cattle administered GET represents an opportunity to make meaningful improvements to productive efficiency. The studies described in this thesis aimed to examine the complex interactions between Zn and GET for the purpose of optimizing Zn supplementation to feedlot cattle. The studies outlined were designed to 1) examine the influence distinctly different plasma Zn concentrations (1.1 mg/L vs. 1.6 mg/L) on longissimus thoracis metabolic profile in steers who did or did not receive a high-potency combination anabolic implant, and 2) determine the effects of supplemental organic Zn concentration (30, 60 or 90 mg Zn/kg DM in addition to basal diet) on performance, carcass characteristics, and circulating metabolites of steers fed, and not fed the novel β-AA lubabegron fumarate (LUB). We have previously demonstrated improved growth performance over implanted controls with Zn supplementation up to 150 mg Zn/kg DM, and chapter 3 of this thesis sought to elucidate potential mechanisms through which Zn supports this growth. We found higher plasma Zn affected several longissimus thoracis metabolites related to energy metabolism. Lactic acid, malic acid, and hydroxybutyric acid were greater in the high plasma Zn group, suggesting Zn improves metabolic flexibility of muscle. In support of our hypothesis, these metabolites indicate greater plasma Zn aids in supplying the energy demands of lean tissue accretion. Chapter 4 of this thesis was an extension of a series of experiments examining the effects of supplemental Zn to cattle administered GET but focused on the β-3AA, LUB. All concentrations of supplemental organic Zn improved plasma Zn concentration and supported LUB-induced growth. There were minimal differences due to supplemental Zn in LUB-fed steers, but increasing supplemental Zn up to 90 mg/kg DM improved feed efficiency in steers not fed LUB. Additionally, LUB improved insulin sensitivity in late finishing steers, an effect which was magnified by increasing supplemental Zn regardless of LUB treatment, which should support improved nutrient uptake. Results from chapter 3 and 4 suggest Zn influences energy metabolism, regardless of GET utilization. Greater plasma Zn in chapter 3 influenced muscle metabolites conducive to glucose sparing and improved metabolic flexibility in muscle, and increasing dietary Zn in chapter 4 decreased glucose and insulin in circulation, potentially supporting improved growth and HCW induced by LUB. These data provide insights into metabolic mechanisms supporting our previously observed enhanced growth performance induced by Zn supplementation. Supplemental Zn studied herein ranged as high as 90 mg Zn/kg DM with no evidence of cattle requiring greater concentrations, thus, opportunities exist to refine supplementation intermediate to 30 and 90 mg Zn/kg DM to cattle fed LUB.