The influence of dietary zinc concentration during periods of rapid growth induced by ractopamine hydrochloride or dietary energy and dietary fiber content on trace mineral metabolism and performance of beef steers
The trace mineral zinc (Zn) is an important constituent of many biological processes associated with growth and maintenance. While Zn is currently recommended to be supplemented to cattle at 30 mg Zn/kg dry matter (DM), this recommended concentration stems from studies conducted in cattle genetically and phenotypically very different than the feedlot cattle of today. Prior research has shown a greater growth response when cattle fed ractopamine hydrochloride were also fed Zn at increasing supplemental concentrations as a blend of inorganic ZnSO4 and Zn-amino acid complex. Therefore, the objective of this dissertation was to investigate if improvements in cattle growth due to greater supplemental Zn concentrations are specific to ractopamine hydrochloride induced growth and if Zn source differentially influences the growth response. It was hypothesized that periods of rapid growth increase the body’s physiological need for Zn above current recommendations, regardless of how this growth is achieved. The influence of dietary energy on growth is well established and dietary energy concentrations can be altered to manipulate rates of growth in beef steers. Therefore, research reported in this dissertation investigates the interaction of Zn and rapid rates of growth. Rapid growth was induced either by ractopamine hydrochloride supplementation or diets differing in energy, designed to imitate the rates of growth typically noted during ractopamine hydrochloride supplementation, in a series of metabolism and performance studies. Results of this research indicated that supplementation of ractopamine hydrochloride did not increase Zn absorption or retention, and no final performance advantages were noted with supplemental Zn at 120 mg Zn/kg DM as either ZnSO4 or a blend of ZnSO4 and Zn-amino acid complex. However, Zn supplementation from the Zn source blend was found to increase N retention similar to the increase in N retention seen from ractopamine hydrochloride administration. Ractopamine hydrochloride acts through a G-protein coupled receptor, increasing circulating cAMP concentrations. Zinc has been shown to potentiate greater cAMP concentrations by Zrt/Irt-like protein 14 (ZIP14) import of Zn into the cell, decreasing activity of the cAMP degrading enzyme phosphodiesterase. Expression of the Zn importer ZIP14 was unaffected due to Zn supplementation or growth method, but the Zn-sequestering protein metallothionein appeared to be increased due to supplemental Zn concentration and source, and further influenced by ractopamine hydrochloride supplementation. This may be indicative of a function of ractopamine hydrochloride to influence Zn sequestering and homeostasis following administration.
The metabolism portion of the dietary energy growth model became focused on differences in dietary fiber content due to greater silage quality than expected. The supplemental Zn blend led to increased dry matter and organic matter digestibility and tended to increase N digestibility over unsupplemented controls. Additionally, the high fiber diet led to greater absorption and retention of Zn as mg/d in steers receiving supplemental Zn. Growing steers receiving greater supplemental Zn concentrations had decreased rates of absorption and retention of Zn as a portion of intake. This is in contrast to late stage finishing steers where no differences in rate of apparent absorption of Zn were observed between the control and supplemental Zn treatments. These results suggest differences in capacity or ability to absorb Zn due to age or diet composition. However, increasing supplemental Zn resulted in greater retention of Zn as mg/d for both growing and finishing steers.
Performance of beef steers was increased in steers receiving the high energy diet compared to the low energy diet. However, following transition to the high energy diet, cattle receiving the blend of ZnSO4 and Zn-amino acid complex at 120 mg Zn/kg DM had improved performance relative to ZnSO4 and control, suggesting a protective action of the blend of Zn in during this time. Supplementation of differing Zn sources at similar concentrations resulted in greater mRNA expression of the Zn storage protein metallothionein in liver relative to control. Metallothionein expression, while inconsistent, differed between sources in both cattle supplemented ractopamine hydrochloride and diets differing in energy. Additionally, supplementing Zn did not change liver Zn concentrations relative to controls when supplemented at 5 times the concentrations of the controls, confirming liver Zn concentrations are not an adequate biomarker for Zn status excluding deficiency. Tissue and plasma concentrations were shown to differ due to source, but direct comparisons of sources were not completed in the metabolism portions of this dissertation and further research of effect of source on Zn tissue and plasma concentrations is warranted.
Overall, increasing supplemental Zn concentrations as either ZnSO4 or a blend of ZnSO4 and Zn-amino acid complex over unsupplemented controls (basal diets just above NASEM (2016) recommendations) influenced interim performance but did not improve overall performance of steers experiencing increased rates of growth. However, ractopamine hydrochloride influenced expression of Metallothionein, suggesting an influence on Zn sequestering and trafficking post-absorption.