Supplementing Saccharomyces cerevisiae fermentation products can improve growth performance potentially through improvements in anti-oxidant status and immune system. The objective of this study was to determine impacts on immune parameters, anti-oxidant capacity, and growth of finishing steers fed a Saccharomyces cerevisiae fermentation product (SCFP; NaturSafe; Diamond V, Cedar Rapids, IA) and ractopamine hydrochloride (RAC; Optaflexx; Elanco Animal Health, Greenfield, IN). Angus-crossbred steers (N = 288) from two sources were utilized in this 90-d study. Steers were blocked by source, stratified by initial body weight to pens of six steers, and pens randomly assigned to treatments (16 pens per treatment). Three treatments compared feeding no supplemental SCFP (control; CON) and supplemental SCFP for 57 d (SCFP57), and 29 d (SCFP29) before harvest. Supplementation of SCFP was 12 g per steer per d, and all steers were fed RAC at 300 mg per steer per d for 29 d before harvest. Blood samples were collected from3 steers per pen, and muscle samples were collected from 1 steer per pen at 57, 29 (start of RAC), and 13 (midRAC) days before harvest. Blood was analyzed from 2 steers per pen for ferric reducing anti-oxidant power (FRAP). Muscle gene expression of myokines, markers of anti-oxidant and growth signaling were assessed. Individual animal BW were also collected on 57, 29, 13, and 1 d before being harvested at a commercial facility (National Beef, Tama, IA). Data were analyzed using the Mixed procedure of SAS 9.4 (Cary, NC) with pen as the experimental unit. The model included fixed effects of treatment and group. Increased BW compared to CON was observed days -29, -13, and -1 in SCFP57 steers (P <= 0.05), with SCFP29 being intermediate days -13 and -1. Overall G:F was improved in SCFP29 and SCFP57 (P = 0.01). On day -29, FRAP was greater in SCFP57 than CON (P = 0.02). The percent of gamma delta T cells and natural killer cells in both SCFP29 and SCFP57 was greater than CON on day -13 (P = 0.02). There were no treatment x day effects for muscle gene expression measured (P >= 0.25). Interleukin 6 tended to decrease in SCFP29 and SCFP57 on day -13 (P = 0.10). No other treatment effects were observed for muscle gene expression. Muscle gene expression of interleukin 15 was increased (P = 0.01), and expression of interleukin 8 was decreased (P = 0.03) due to RAC feeding. Increased growth in SCFP-fed cattle may be related to changes in anti-oxidant capacity and the immune system. Lay Summary Saccharomyces cerevisiae fermentation products (SCFP) can provide additional support for improved growth performance. This study investigated the effects of supplementing a SCFP (NaturSafe; Diamond V, Cedar Rapids, IA; 12 g per steer per d) for 29 (SCFP29) or 57 (SCFP57) d before harvest when also feeding ractopamine hydrochloride (RAC; 300 mg per steer per d; Optaflexx, Elanco Animal Health, Greenfield, IN) for 29 d before harvest. Compared to steers not fed SCFP (CON), SCFP29 and SCFP57 had improved gain:feed for the entire feeding period. Steers supplemented with SCFP had increased percentages of gamma delta T cells and natural killer cells 13 d before harvest compared to CON. Gene expression of cytokine and anti-oxidant signaling in muscle were changed in all treatments during RAC compared to before RAC. Improvements in growth during RAC with SCFP supplementation may be due to the changes in anti-oxidant and cytokine signaling in muscle.
