Zinc Injection As a Novel Castration Method in Beef Bulls: Effects on Performance, Behavior and Testosterone and Haptoglobin Concentration

One hundred and eighty bulls (BW = 337 ± 10.9 kg) were blocked by BW (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 upon the dorsal aspect of the scrotum), 3) BUL (bulls with testicles remaining intact) in a randomized complete block design (3 pens/block and 10 cattle/pen). A random subset of 54 animals (n = 3/pen) were fitted with accelerometers on d 0 for 28 d. The subset also had testis width and scrotal circumference determined, and were bled via jugular venipuncture to assess serum haptoglobin and testosterone concentrations every 28 d until slaughter. At slaughter, INJ and BUL were castrated at chain speed to assess final testes weight and for histopathological evaluation. Data were analyzed using PROC MIXED with an alpha of 0.05 and pen served as the experimental unit for all dependent variables. Final BW was greater in INJ and BUL compared to BAN cattle (P < 0.01). 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 results (n = 13; INJ = 7; BUL = 6) indicated that INJ testes were degenerative and not viable while BUL testes were normal. Final serum testosterone concentrations on d 168 were similar (P = 0.14) between INJ and BUL while after d 14, BAN were non-detectable. Serum haptoglobin concentration was greater in INJ than BUL and BAN on d 1, 3, 5, and 7 (P < 0.01). Scrotal circumference (P = 0.08) and testis width (P = 0.07) on d 168 tended to be greater in BUL compared to INJ. Motion index was greater in BUL and INJ compared to BAN cattle during the d 0 to 28 monitoring period (P ≤ 0.02). There were no differences in standing or lying time (P ≥ 0.85) or lying bouts (P = 0.35); however, step count was greater (P = 0.04) in BUL and INJ. The injection of Zn appeared to cause sterilization but did not cause complete cessation of testicular function as evidenced by testosterone concentration more similar to BUL than BAN. This resulted in increased BW and G:F compared to BAN, yet the acute phase response was markedly greater directly after injection.