J T Richeson, J J Ball, S L Roberts, J G Powell, T E Lawrence, E B Kegley
Crossbred beef bulls (n = 180) were blocked by initial BW (337 ± 10.9 kg; 6 blocks) and assigned randomly to 1 of 3 treatments on d 0: 1) INJ; received 1 mL (100 mg Zn) of a Zn solution in each testis, 2) BAN; received blood-restrictive rubber band placed around the dorsal aspect of the scrotum, 3) BUL; bulls with testicles remaining intact in a randomized complete block design (3 treatment pens/block and 10 cattle/pen). A subset of 54 animals (n = 3/pen) were fitted with accelerometers on d 0 to quantify behavior variables continuously for 28 d. Testis width and scrotal circumference, and serum haptoglobin (d 0, 1, 3, 5, 7 and 14) and testosterone concentrations (every 28 d until slaughter) were also determined for the subset. During the slaughter process, testes from INJ and BUL were collected to assess final testes weight and for histopathological evaluation. Data were analyzed using a mixed model (α = 0.05); pen served as the experimental unit for all dependent variables. Final BW was greater (P < 0.01) for INJ and BUL compared to BAN (672, 686, and 611 kg, respectively; SEM = 4.4). Overall ADG and G:F were greater (P ≤ 0.03) in INJ and BUL than BAN; whereas, DMI was similar between treatments for the study duration (P = 0.46). Histopathological evaluation (n = 13; INJ = 7; BUL = 6) indicated that INJ testes were degenerative and reproductively non-viable whereas BUL testes were normal. Serum testosterone concentrations on d 168 were similar (P = 0.14) between INJ and BUL whereas after d 14, BAN were non-detectable; however, initial serum testosterone concentrations were similarly low across treatments. Serum haptoglobin concentration was greater (P < 0.01) in INJ than BUL and BAN on d 1, 3, 5, and 7. Scrotal circumference (P = 0.08) and testis width (P = 0.07) on d 168 tended to be greater for BUL than INJ. Motion index (P ≤ 0.02) and step count (P = 0.04) was greater in BUL and INJ compared to BAN cattle during the 28 d monitoring period. No difference in standing time (P ≥ 0.85) or lying bouts (P = 0.35) occurred. Zinc injection resulted in sterilization but did not cause complete cessation of testicular function evidenced by testosterone concentrations more similar to BUL than BAN. This resulted in overall increased BW and G:F for INJ vs. BAN, yet the acute phase response was markedly greater directly after Zn injection. Collectively, Zn injection resulted in outcomes more similar to BUL than BAN, implying minimal efficacy of INJ as a castration method in older bulls arriving to the feedlot.
