Abstract
In two separate studies, 60 beef heifers (379 kg BW) and 60 beef steers (348 kg BW) were randomly assigned to six treatments in 2×3 factorial arrangements. The treatments were with or without Synovex ® implants combined with either a control diet or diets supplemented with 200 ppm Zn from ZnSO 4 or zinc methionine (Zn-Met). Near the mid-point of the feeding periods, cattle were vaccinated with a modified live virus and subsequent titers and concentrations of immunoglobulin G (IgG) were measured. Liver and blood samples were obtained 1 week prior to the start of the experiments and at intervals during the experiments. In experiment 1, average daily gains of beef heifers were ( P<0.05) affected by the interaction of implant and source of dietary Zn. Compared to control and ZnSO 4 treatments, supplementation with Zn-Met increased ( P<0.05) the concentration of Zn in serum. Antibody titers and concentrations of IgG in serum were highest ( P<0.05) in heifers fed ZnSO 4 compared to heifers fed the control or Zn-Met supplemented diets. The Synovex-H ® implant reduced the concentrations of Zn and Cu in liver. In experiment 2, Synovex-S ® implants improved ( P<0.05) weight gains of steers supplemented with 200 ppm dietary Zn from ZnSO 4 compared to non-implanted steers. However, the implant had no effect when Zn-Met was the dietary Zn source. The implant increased ( P<0.05) concentrations of Zn in liver of steers supplemented with 200 ppm dietary Zn and reduced Zn in liver of steers fed the control diet. Implanted steers had higher ( P<0.05) Cu status and IgG concentrations in serum than non-implanted steers. Steers supplemented with either ZnSO 4 or Zn-Met had greater ( P<0.05) concentrations of Zn in liver and plasma than steers fed the control diet. These results indicate both the level and source of Zn supplementation in diets of feedlot cattle affect their response to growth implants.
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