Effect of hormonal growth promotant implants in weaner and hogget ewes on subsequent growth and reproductive performance

Abstract

Ninety young Merino ewes, depastured on Mitchell (Astrebla spp.) grass pastures in North West Queensland, were used in a hormonal growth promotant implantation study. The ewes were given 1 implant of Compudose, Ralgro, Revalor or Synovex-H, either at lamb marking (mid-dry season) or the start of the following summer wet season, which was ~180 days post-marking. The hormonal growth promotant implanted groups had greater (P<0.05) liveweights than the control group early in the trial (days 22 and 57) and also from the end of the period of activity of the wet season implant (day 277) until the middle of the dry season (day 412). Dry season (at lamb marking) implantation did not improve average daily gain. From the end of the wet season onwards, ewes with a wet season implant were heavier (P<0.05) than those ewes implanted at lamb marking. This liveweight advantage had diminished by the start of the autumn mating. Hormonal growth promotant implantation had a favourable (P<0.05) effect on growth rate, but adversely affected reproduction in the ewes, regardless of time of implantation. Implantation with Compudose or Synovex-H significantly (P<0.001) reduced the demonstration of oestrus, while Revalor or Synovex-H reduced pregnancy rates by up to 100%. Despite Ralgro reducing these variables by up to 25%, its effect was not significant. All ewes that were diagnosed as pregnant at 140 days later produced lambs. It was concluded from this study that hormonal growth promotant implantation at lamb marking provides no later-life advantage, while wet season implantation provides a growth or liveweight advantage to young Merino ewes and this persists for a long period after implantation. Despite the lack of an effect of Ralgro on oestrus and pregnancy results, however, implantation of young breeding ewes of any age should not be carried out, due to the long term and negative effects on reproductive performance. Wet season implantation may be best used for animals intended for slaughter.