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Beef cattle today undergo much greater and more rapid muscle growth than in previous decades (USDA NASS, 2019). This is due in part to utilizing metabolic modifiers, such as ractopamine hydrochloride (RH). This β-adrenergic agonist is fed in the month prior to harvest to promote growth of lean muscle while limiting lipid accretion. Additionally, the trace mineral Zn is needed for muscle growth. The vital role of Zn in protein synthesis has been established and previous research has shown an increase in growth when Zn is supplemented above the nationally recommended concentration of 30 mg Zn/kg dry matter to RH-fed cattle, although it is not understood how this positive interaction is occurring in the muscle. Therefore, the goal of this thesis research was to utilize proteomics analyses to better understand how Zn and RH supplementation, alone and together, affect the muscle proteome and phosphoproteome in finishing beef steers. The results of this research showed greater carcass-adjusted final body weight, average daily gain and gain to feed ratio due to increased Zn supplementation, demonstrating the importance of Zn in growth. An interaction of Zn and RH was observed in dressing percentage, such that steers receiving both supranutritional Zn and RH supplementation had the greatest dressing percentage. This effect suggests that increased Zn supplementation may increase carcass accretion stimulated by RH supplementation. Proteomics analyses suggest this growth may be occurring due to Zn and RH increasing the proportion of fast-twitch muscle fibers in the Longissimus, either through selective hypertrophy or a slow-to-fast-twitch fiber transition. The differing abundances of proteins involved in energy metabolism (L-lactate dehydrogenase, myoglobin), muscle fiber structure (slow skeletal troponin 1) and protein synthesis (eukaryotic initiation factor 5A) suggested the muscle proteome more closely resembled that of fast-twitch muscle fibers in steers receiving increased Zn and RH,

Exploring the effects of supranutritional zinc and ractopamine hydrochloride on the proteome and phosphoproteome of the Longissimus dorsi in beef steers