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Effect of zeranol or melengestrol acetate (MGA)on testicular development and aggression in farmed fallow bucks
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Fifteen yearling fallow bucks were randomly assigned by body weight to one of three treatment groups: control (C; n=5), melengestrol acetate (MGA; n=5), and zeranol (Z; n=5), to evaluate effects on testicular development, aggressive behavior, antler growth, sexual activity, average daily gains and body weights. Zeranol bucks received zeranol ear implants (36 mg) at 90 d intervals, and MGA bucks received MGA in the ration (100ghead⁻¹day⁻¹). Bucks grazed ryegrass/bermudagrass pasture and were supplemented with 3:1 Corn/Soybean meal at .45kghead⁻¹day⁻¹. Body weights, body condition scores (BCS), blood samples, and testis measurements were obtained at day 0 and at 14-day intervals for 229 days. Ejaculates were collected at 14-day intervals after antlers hardened (7/29/99). Harvested antlers were weighed as the bucks reached hard antler (from 7/29/99 - 8/25/99). Aggression was evaluated using 10-minute video sessions scoring body blows, avoidance, head pushes and head bunts. Neither body weight nor condition score were affected (P>.10) by treatment. Scrotal circumference (SC) and paired testis volume (PTV) were affected by day x treatment interaction (P<.01) with testes of zeranol bucks being smaller compared to those of control or MGA bucks. First sperm in the ejaculate was delayed in zeranol bucks compared to control and MGA bucks. MGA bucks had a maximum sperm concentration in the ejaculate that was three times (P<.04) that of control bucks and nine times (P<.02) that of zeranol bucks. Antler weight was the least (P<.01) for zeranol bucks and the greatest (P<.08) for MGA bucks with intermediate values recorded for the control bucks. Aggressive behavior was delayed for zeranol bucks until treatment effects were overcome. MGA bucks had decreased aggressive behavior compared to control bucks. MGA bucks had increased (P<.05) serum testosterone concentrations compared to control and zeranol bucks. hCG stimulated serum testosterone concentrations for zeranol bucks were delayed compared to control and MGA bucks until effects of treatment were overcome. Although zeranol treated bucks overcame treatment effects, they were never able to reach testicular measurements or sperm concentrations equal to the control or MGA bucks. Zeranol and MGA treatments have advantages that could be utilized when producing slaughter bucks for venison.
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Wilson, Timothy Wayne (2000). Effect of zeranol or melengestrol acetate (MGA)on testicular development and aggression in farmed fallow bucks. Master's thesis, Texas A&M University. Available electronically from
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