INJECTABLE COPPER AND ZINC FOR GRAZING YEARLING STEERS
Injectable Cu and Zn were evaluated as mineral supplements using 37 purebred Herefords (Group 1) and 23 Angus-Charolais-North Devon crosses (Group 2). The 60 steers were carried through a 113-d grazing phase. This was followed by a 79-d finishing phase for all steers of Group 1 and seven steers of Group 2. Treatments were combinations of the non-dosing (0Cu, 0Zn) and dosing (+Cu, +Zn) of steers with injectable Cu or Zn at the start of the grazing phase: (1) 0Cu, 0Zn; (2) +Cu, 0Zn; (3) 0Cu, +Zn; and (4) +Cu, +Zn. Injectables were the sole source of supplementary Cu and Zn during grazing and finishing phases. Means (SE) for Cu and Zn, respectively, in forages as milligrams per kilogram dry matter (DM) during the grazing phase were: grasses, 4.4 (0.17), 15.7 (0.46); alfalfa, 6.9 (0.18), 16.5 (1.49); and birdsfoot trefoil 5.0 (0.76), 15.0 (3.41). Injectable Cu did not influence (P > 0.05) body weight during the grazing phase. Body weight responses were lower (P < 0.05) for +Cu steers in the finishing phase, when dietary Cu was low, but steers could not be classified as Cu-deficient based on liver Cu. Liver Cu increased to means greater than 140 mg kg−1 DM in all steers, but was higher (P < 0.05) in +Cu steers during the grazing phase. Serum Cu varied widely during the grazing phase, but was higher (P < 0.05) for +Cu steers only on day 84 of the grazing phase. Neither liver Cu nor serum Cu was influenced (P > 0.05) by injectable Zn. Injectable Zn decreased growth rates in the latter part of the grazing phase but did not affect (P > 0.05) the body weight response during the finishing phase. The concentrations of Zn in liver and serum were not influenced (P > 0.05) by injectable Cu or Zn. Key words: Cattle, grazing, copper, zinc, injectable copper, injectable zinc