Determination of relative bioavailability from copper carbonate (Emerald-C) and tribasic copper chloride in growing beef steers.

Two experiments were conducted to determine the bioavailability of Cu carbonate (CuCO3) and tribasic Cu chloride (TBCC) relative to Cu sulfate (CuSO4). Experiment 1 utilized 84 steers (282 ± 19 kg) in a 96-d study stratified by weight into pens (six steers per pen) equipped with GrowSafe feed bunks (GrowSafe Systems Ltd, Airdire, AB, Canada). Two pens (n = 12 steers per treatment) were randomly assigned to one of seven treatments: no supplemental Cu (CON), 5 or 10 mg Cu/kg diet dry matter (DM) from CuCO3 (CO5 and CO10, respectively), 5 or 10 mg Cu/kg diet DM from CuSO4 (SUL5 and SUL10, respectively), and 5 or 10 mg Cu/kg diet DM from TBCC (TBCC5 and TBCC10, respectively). All animals were supplemented with 2 mg Mo/kg diet DM and 0.1% S. Blood and liver samples were collected on days 5 and 4 and on days 95 and 96 with treatments stratified across sampling days. Weight was measured on days -1, 0, 26, 56, 93, and 94. Liver samples were used to determine liver Cu concentration and expression of genes relating to Cu trafficking. Blood samples were used to determine plasma Cu and ceruloplasmin concentrations. Markers of Cu status were used to determine the relative bioavailability (RBV) of each test source against CuSO4. Based on liver Cu, CuCO3, and TBCC were not as bioavailable as CuSO4 (89% and 88%, respectively, P < 0.01). There were no differences in RBV between sources based on plasma Cu; however, based on ceruloplasmin, CuCO3 tended to be more bioavailable than CuSO4 (186%; P = 0.08). Experiment 2 compared rates of repletion of Cu-depleted steers fed each source at 5 mg Cu/kg DM (plus 5 mg Mo/kg DM) for 21 d. Controls had the lowest liver Cu following depletion (P ≤ 0.01), while Cu-supplemented treatments were not different (P ≥ 0.40). Plasma Cu was affected by day (P < 0.01), where days 7 and 14 of repletion were similar (P = 0.23) and greater than day 21 (P ≤ 0.01). There was a tendency for a treatment × day interaction where ceruloplasmin tended to be greater on day 14 than on day 7 (P ≤ 0.09), while CON remained consistent on days 7 and 14 (P = 0.51), and decreased from day 14 to day 21 (P = 0.03). Environmental factors and initial Cu status may have impacted the outcome of Exp. 2. In conclusion, Cu from CuCO3 tended to result in more Cu in circulation as ceruloplasmin than CuSO4 and TBCC.