A 2 × 2 factorial experimental design was used to compare growth in entire and castrated fallow bucks, implanted with zeranol at 6 and 9 months of age, or left as controls (no. = 10 per group). Growth pattern at pasture was assessed using 10-monthly growth periods from weaning (6 months) to 17 months. There were no overall castration or zeranol effects on growth rates from weaning to 17 months (P > 0·05), although there were significant interactions of zeranol and castration with time (P < 0·05). Zeranol-treated animals grew faster than the controls from 6 to 14 months, compared with the last 3 months (P < 0·05), indicating that the zeranol implants given at 6 and 9 months of age were no longer effective over the last 3 months of the experiment. The interaction between castration and time was significant (P < 0·05), whereby entire males grew relatively faster than the castrated males in both spring and the average of the winter and spring periods, whilst there was little difference in growth rates between entire and castrated males during the autumn period.
At 17 months of age all animals were slaughtered following a 16-hfast. Entire bucks were 46g/kg live weight and 56 g/kg carcass weight heavier than the castrated bucks at slaughter (P < 0·05), although there was no effect of zeranol treatment on either live or carcass weights (P > 0·05). The mean dressing proportion for deer in all groups was 0·61.
A subset of each treatment (no. = 5) were slaughtered and half carcasses dissected into muscle, bone and fat. When compared at the same carcass weight, both castration and treatment with zeranol significantly reduced the muscle weight and increased carcass fat weight (P < 0·05). There was no effect of either castration or zeranol treatment on the distribution of primal cuts. There were significant castration × zeranol interactions (P < 0·05) for muscle: bone and muscle: fat ratios, the entire control group having the highest muscle: bone and muscle: fat ratios.
It was concluded that there is little commercial basis for the use of the growth promotant zeranol in fallow bucks being grown for venison. Although castration resulted in lower carcass weights and muscle proportion, these disadvantages should be considered against the management advantages of easier handling and lower bruising of castrates.