Abstract
Low-risk, weaned Angus-crossbred steers (n = 72; 284 ± 25 kg) were used in a 42-d receiving study. Steers were housed in pens (n = 6 steers per pen) equipped with GrowSafe bunks for determination of individual animal feed disappearance. Dietary treatments (n = 24 steers per treatment) included: 1) trace minerals (TM) from an organic source (Availa4; Zinpro Corp., Eden Prairie, MN) at 7 g·steer-1·d-1; for 42 d (ORG); 2) ORG for entire 42-d plus AvailaZn (Zn amino acid complex, Zinpro Corp., Eden Prairie, MN) to provide 1,000 mg Zn·steer-1·d-1 for first 14 d (ORG+Z); 3) inorganic TM sources to supplemented at equivalent concentration as in ORG for 42-d (ING). Cattle were weighed on day -1, 0, 14, 41, and 42. Whole blood was collected (n = 72 steers) on day 0, 14, and 42. Liver biopsies were conducted (n = 36 steers; 3 steers per pen) on day 0, 14, and 42. Flow cytometry measures were conducted using whole blood on day 1, 14, and 42 for determination of circulating frequencies of immune cell populations. There was a tendency for improved overall average daily gain (P = 0.07) where both ORG and ORG+Z were greater than ING. Final body weight did not differ (P = 0.21) and overall dry matter intake was unaffected by dietary treatment (P ≥ 0.18). However, overall gain-to-feed ratio was improved (P = 0.01) in steers supplemented organic TM (ORG and ORG+Z) compared to ING. Plasma Zn concentration did not differ at any time point during the study (P ≥ 0.20). Liver Zn concentration did not differ between treatments on day 0 or 42; however, on day 14 ING tended (P = 0.09) to be greater than ORG+Z with ORG being intermediate. Plasma Cu was unaffected by dietary treatment (P ≥ 0.34) on day 0, 14, and 42. Plasma Fe did not differ on day 0 or 42 but tended to be greater in ORG and ORG+Z compared to ING (P = 0.08) on day 14. Dietary treatment did not alter (P ≥ 0.22) liver Fe or Mn concentration at any time point. Frequency of total circulating natural killer (NK) and CD8 T cells measured on day 0, 14, and 42 did not differ (P ≥ 0.07). However, cell surface markers of activation (CD16, CD44, and CD8) on NK cells measured on day 14 did differ because of treatment (P ≤ 0.05). Results presented herein indicate TM from an organic source supplemented to steers during receiving can positively influence growth rate and feed efficiency. Regardless of source, TM supplementation affected markers of immune function but did not influence the prevalence of circulating NK and CD8 T-cell populations.