356 Exploring the manganese requirement of feedlot steers

Abstract Angus-cross steers [n = 144; body weight (BW) = 359 kg ± 13.4] were used to assess the effect of dietary Mn and steroidal implants on trace mineral (TM) status, and liver and serum metabolites related to N metabolism. Steers (n = 6/pen) were stratified by BW in a 3 × 2 factorial. GrowSafe bunks recorded individual feed intake (experimental unit = steer; n = 24/treatment). Dietary treatments included (MANG; 8 pens per treatment; Mn as MnSO4): 1) no supplemental Mn (analyzed 14 mg Mn/kg DM; Mn0); 2) 20 mg supplemental Mn/kg DM (Mn20); 3) 50 mg supplemental Mn/kg DM (Mn50). Within MANG, steers received a steroidal implant treatment (IMP) on d 0: 1) no implant; NO; or 2) combination implant (Revalor-200; REV). Liver biopsies for TM analysis and qPCR, and blood for serum urea-N (SUN) analysis were collected on d 0, 20, 40, and 77. Liver TM, serum metabolite, enzyme activity, and gene expression data were analyzed as repeated measures using PROC MIXED of SAS 9.4. The Correlation Procedure of SAS 9.4 was utilized to assess correlations between liver arginase activity, SUN concentrations, and liver Mn concentrations. No MANG × IMP effects were noted (P ≥ 0.12) for growth or carcass measures. Steroidal implants increased final BW (FBW), overall average daily gain (ADG), and gain to feed (G:F; P ≤ 0.01). Dietary Mn did not influence FBW, overall ADG, or overall G:F (P ≥ 0.14). Liver Mn concentration increased with supplemental Mn (P = 0.01). An IMP × DAY effect was noted for liver Mn (P = 0.01) where NO and REV were similar on d 0 but NO cattle increased liver Mn from d 0 to 20 while REV liver Mn decreased. Relative expression of liver Mn transporter ZnT10 tended to be least (MANG; P = 0.10) in Mn20 while Mn0 and Mn100 were similar. Relative expression of liver Mn transporter ZIP8 was not influenced by MANG, IMP, or MANG × IMP (P ≥ 0.23). Liver arginase activity was not influenced by MANG, IMP, MANG × DAY, IMP × DAY, or MANG × IMP (P ≥ 0.15) but tended to decrease over time (DAY; P = 0.10). On d 0, 20, and 40, arginase activity was correlated (r ≤ 0.33; P ≤ 0.01) with SUN and liver Mn. Relative expression of MnSOD in the liver was greater in REV (P = 0.03) compared with NO and within a MANG × IMP effect (P = 0.01) REV increased liver MnSOD activity. This study indicates current NASEM Mn recommendations are adequate to meet demands of finishing beef cattle given a steroidal implant. This study also reveals how implants and supplemental Mn influence liver enzyme activity and genes related to arginine metabolism, urea synthesis, antioxidant capacity, and TM homeostasis.