The influence of supplemental zinc and ractopamine hydrochloride on the performance and longissimus dorsi proteome of finishing beef steers

To determine how Zn and ractopamine hydrochloride (RH) supplementation affect the longissimus dorsi muscle proteome in beef cattle, 48 high percentage Angus steers (494 ± 18.2 kg) were utilized in a 2 × 2 factorial study design. Steers were blocked by body weight (BW) and genetic gain potential (GeneMax; Zoetis, Parsippany, NJ) into control (CON; analyzed 36 mg Zn/kg dry matter [DM]) or supranutritional Zn (SUPZN; CON + 60 mg Zn/kg DM from ZnSO4 + 60 mg Zn/kg DM from Zn-amino acid complex; Availa-Zn, Zinpro, Eden Prairie, MN) dietary treatments (ZNTRT). Starting 28 days prior to harvest steers were blocked by BW within ZNTRT to RH treatments (RACTRT) of 0 (NO) or 300 mg·steer-1·d-1 (RAC; Actogain45, Zoetis). After 15 days on RACTRT, longissimus dorsi biopsies were collected from all steers for proteomic analysis. Twenty-eight steers (n = 7 per treatment) were harvested at a commercial abattoir on d 90. A ZNTRT × RACTRT effect showed SUPZN-RAC steers had a greater dressing percentage than other treatments (P ≤ 0.02). Steers fed SUPZN had a greater HCW, carcass-adjusted final BW, overall average daily gain and overall gain to feed (P ≤ 0.05). Differentially abundant proteins involved in energy metabolism, muscle fiber structure and protein synthesis indicated an increased proportion of fast-twitch fibers due to Zn and RH supplementation. Both myosin light chain kinase 2 and eukaryotic translation initiation factor 5A-1 were more abundant in SUPZN-RAC steers than SUPZN-NO (P ≤ 0.05) or CON-NO (P ≤ 0.04) steers when analyzed for phosphorylation modifications. L-lactate dehydrogenase B was more abundant in SUPZN-NO steers than CON-NO (P = 0.03) or CON-RAC (P = 0.01) steers. In conclusion, increased Zn supplementation may be needed to optimize the hypertrophic effects of RH through its effects on the longissimus dorsi proteome.