Effects of steroidal growth implants alone or in combination with a beta-adrenergic agonist feed additive on the gonadal development of South African Dohne Merino rams and ewes

Abstract

The purpose of this research was to investigate the effects of different combinations of steroidal growth implants alone or in combination with a beta–adrenergic agonist feed additive on the gonadal growth, relative to ADG per treatment group of feedlot rams and ewes. The trial was conducted at the University of Pretoria’s Hatfield experimental farm. A total of 240 Dohne Merino lambs of 8 months old were randomly selected into 8 different treatment groups and fed for a period of 70 days following 3 days of adaptation. Zilpaterol hydrochloride was included into the finisher ration of half of the treatments for the last 18 days prior to slaughter (with 3 days withdrawal before slaughter). On arrival, the lambs were split by weight, first removing the outliers, and then split per sex (rams and ewes). Both sex groups were then randomly allocated into six treatment groups and then implanted as per treatment group, with one group receiving no implants only Zilmax and one group receiving no growth enhancers at all. Following the 70 days feeding period the lambs were taken to the Cavalier Group abattoir where they were humanely slaughtered, and the reproductive organs kept aside for further sampling and analysis. Data collection and statistical analysis was done on 68 selected lambs (22 rams and 46 ewes). The results indicate that the effects of steroidal growth implant and Zilmax treatments differed between ewes and rams. The results for the ewes show that Revalor G, Revalor H and Ralgro all caused a significant decrease in the number of ovarian follicles in the ewes (P<0.05) when compared to the control of 12.3 ± 4.12 follicles, with Revalor G, Revalor H and Ralgro being 6.2 ± 3.86, 7.0 ± 2.87 and 8.6 ± 2.81, respectively. No changes were observed for the length or width of the ovaries, nor in the number of corpora lutea (P>0.05). The feed supplementation with Zilpaterol hydrochloride (Zilmax) to the ewes decreased the growth of the gonads in terms of length, width and number of follicles (P<0.05) but no change was seen in the number of corpora lutea (P>0.05). The results for the rams indicate that the feed supplementation with Zilmax did not cause changes in the physical size or shape of the testes (in terms of length, width, volume and weight) (P>0.05). Revalor G significantly decreased the weight, volume, length and width of the testes (P<0.05) when compared to sheep in the control group and those implanted with Ralgro. Ralgro did not result in any physical changes (P>0.05) in the growth of the testes. Results from the growth part of the trial show that the ewes responded positively to the addition of Revalor G and Revalor H. The subcutaneous fat thicknesses of the ewes decreased from 2.11 ± 0.564mm in the control to 1.68 ± 0.565mm for both the Revalor G and Revalor H treatment groups (P<0.05), which was not seen in the rams. The final mass of the ewes also did not respond consistently to the addition of the steroids (Revalor G, Revalor H) and Ralgro (Non-steroidal), with only small numerical improvements seen in the final masses (P>0.05). No increases were seen in final mass of the rams due to the supplementation of Zilmax (P>0.05), but significant decreases in carcass subcutaneous fat thicknesses were observed with the feed supplementation of Zilmax in rams decreasing the subcutaneous fat thickness from 1.67 ± 0.564mm to 1.36 ± 0.564mm (P<0.05). No significant improvement was seen in the final mass of rams due to implant supplementation. Observations in the ewes, however, showed the opposite trend; Zilmax improved the ewes’ final mass by 800g (P<0.05) but no change in carcass subcutaneous fat thickness was observed (P>0.05).

Additional studies are needed to test the effects of the age at which lambs are implanted as well as the effects of different combinations of exogenous growth promotants and supplementation of different dosages of Zilpaterol hydrochloride on sperm counts, morphogenesis, motility and other related fertility estimates in male ruminants as well as on folliculogenesis and patterns of oestrous in females.