Effect of copper source and level on performance and copper status of cattle consuming molasses-based supplements1,2

Two studies were conducted to evaluate the availability of dietary Cu offered to growing beef cattle consuming molasses-based supplements. In Exp. 1, 24 Braford heifers were assigned randomly to bahiagrass (Paspalum notatum) pastures (two heifers/pasture). Heifers were provided 1.5 kg of TDN and 0.3 kg of supplemental CP/heifer daily using a molasses–cottonseed meal slurry. Three treatments were randomly assigned to pastures (four pastures/treatment), providing 100 mg of supplemental Cu daily in the form of either CuSO4 (inorganic Cu) or organic-Cu. A third treatment offered no supplemental Cu (negative control). Heifer BW was collected at the start and end of the study. Jugular blood and liver samples were collected on d 0, 29, 56, and 84. In Exp. 2, 24 Brahman-crossbred steers were fed the same molasses–cottonseed meal supplement at the same rates used in Exp. 1. Steers were housed in individual pens (15 m2) with free-choice access to stargrass (Cynodon spp.) hay. Four Cu treatments were assigned to individual steers (six pens/treatment) providing 1) 10 ppm of Cu from an organic source; 2) 10 ppm Cu from Tri-basic Cu chloride (TBCC); 3) 30 ppm of Cu from TBCC; or 4) 30 ppm of Cu, a 50:50 ratio of TBCC and organic Cu. Body weights and jugular blood and liver samples were collected on d 0, 24, 48, and 72. In Exp. 1, liver Cu concentrations did not differ between heifers supplemented with inorganic and organic Cu. Each source resulted in increased (P < 0.05) liver Cu concentrations compared with the unsupplemented control. Plasma ceruloplasmin concentrations were higher (P < 0.05) for Cu-supplemented heifers, independent of Cu source. Heifer ADG tended (P = 0.11) to increase with Cu supplementation compared with the unsupplemented control. In Exp. 2, liver Cu was greater (P < 0.05) on d 24, 48, and 72 for steers consuming 30 vs. 10 ppm of Cu. Steers supplemented with organic Cu had lower DMI than steers supplemented with 10 or 30 ppm of TBCC. These data suggest that the inorganic and organic Cu sources evaluated in these studies were of similar availability when offered in molasses supplements. A dietary Cu concentration greater than 10 ppm might be necessary to ensure absorption in beef cattle fed molasses-based supplements.