LAUSR

Twenty-four Holstein heifers (165.6 kg ± 5.1 kg initial BW) were utilized in a complete randomized design. The objective was to evaluate effects of supplemental Zn source on growth, reproductive development, and immunocompetence of replacement Holstein heifers. Heifers were randomly allotted to one of two treatments: supplemental zinc sulfate (ZnSO4) or supplemental zinc methionine (ZnMet). Dietary Zn averaged 78.5 ppm for ZnSO4 heifers and 86.3 ppm for ZnMet heifers. Weights and blood were collected once weekly. On d 85 heifers were inoculated with ovalbumin to assess immunocompetence. Follicular aspirations and reproductive tract scores (RTS) were taken on d 106. Liver biopsies and body measurements were collected on d 133 and d 259. Source of Zn did not affect (P ≥ 0.54) BW, DMI, ADG, or G:F. Body measurements were similar (P ≥ 0.28) between treatments. Treatment groups did not differ in RTS, ovarian area, or follicle size or number (P ≥ 0.14). Concentration of progesterone, estradiol, and ratio of progesterone to estradiol of follicular fluid were not affected (P ≥ 0.40) by Zn source. Heifers pubertal prior to synchronization, heifers pregnant per service, total services to pregnancy, and percent heifers pregnant at conclusion of the study (P ≥ 0.34) were not different between treatments. Liver mineral concentrations (P ≥ 0.55) were not affected by Zn source. Circulating Zn in serum did not differ on either d 1, d 141, or d 252 (P ≥ 0.36) for either treatment. Heifers supplemented with ZnMet demonstrated a greater (P = 0.02) antibody response 14 d after initial ovalbumin vaccination than ZnSO4 heifers. By d 14 after second inoculation, immune response was similar (P = 0.38) for both treatments. These results suggest that in the absence of stress, ZnMet enhances immunocompetence of replacement Holstein heifers but has no effect on growth or reproductive development.