LAUSR

To evaluate bioavailability of Cu from Cu glycinate (CuGly; GemStoneCu; Phibro Animal Health Corporation) vs. Cu sulfate (CuSO4) in diets supplemented with 0.3% S and 2 mg Mo/kg DM 60 Angus-cross steers (288 ±26 kg initial BW) were randomly assigned to treatments within weight blocks and housed in pens (n = 6 steers/pen) with GrowSafe bunks. Treatments included: no supplemental Cu (CON; analyzed 6 mg Cu/kg DM); 5 mg supplemental Cu/kg DM from CuSO4 (SUL5) or CuGly (GLY5); or 10 mg supplemental Cu/kg DM from CuSO4 (SUL10) or CuGly (GLY10). Liver and blood samples were collected from all steers at the start (used as covariates) and end of the trial to determine Cu status. Steers were weighed on d 0, 28, 56, and 90. Data were analyzed as a completely randomized block design using Proc Mixed of SAS with steer as experimental unit (n = 12/treatment) with treatment and block fixed. Contrast statements compared supplementation source (sulfate vs. glycinate) and concentration. Copper supplementation did not affect steer final BW, or overall ADG, DMI, and G:F (P ≥ 0.19). Initial liver Cu concentrations (374 mg/kg DM) were highly adequate, such that even with antagonist supplementation final plasma Cu concentrations were not affected by supplemental Cu concentration (P ≥ 0.34). Control steers had lesser final liver Cu concentrations vs. steers supplemented 5 or 10 mg Cu/kg DM (P ≤ 0.0004). Supplementing 10 vs. 5 mg Cu/kg DM increased liver Cu concentrations (P = 0.0001). Source did not affect final liver Cu concentrations (P = 0.40). Relative bioavailability was determined by regressing liver Cu concentrations on Cu intake; relative bioavailability of CuGly was 115.5% compared with CuSO4 (100%; P = 0.27). Based on the calculated relative bioavailability CuGly may be a good supplemental Cu source when dietary Cu antagonists are present.