Effect of the Duration of Progesterone Treatment on Synchronising Oestrus outside the Natural Breeding Season in Merino Ewes
This study aimed to evaluate the effect of the duration of progesterone treatment on the reproductive performance of South Africa Merino ewes during the spring breeding season. The study compared the effect of long and short-term progesterone treatment and its combination with or without equine chorionic gonadotropin on oestrous response and hormonal profiles. Seventy-six (76) Merino ewes aged between 2–5 years were allocated into two groups, 40 for long-term and 36 for short-term progesterone treatments. For the long and short-term progesterone treatment groups, controlled internal drug release devices were inserted for 14 and 11 days, respectively. At the withdrawal of the controlled internal drug release device, half of each group were injected intramuscularly with 300 IU of equine chorionic gonadotropin, resulting in four treatment groups. The oestrous response was monitored using teaser rams. Blood samples were collected to measure the serum progesterone and oestradiol concentrations. One-way ANOVA of the SPSS® software (IBM SPSS Statistics, Version 23.0, 2015, Armonk, NY., USA) was used for statistical analysis. The means were considered significant at P < 0.05. The overall oestrous response obtained was 96%. A greater proportion of the ewes showed oestrous signs at 48 and 60 hours post controlled internal drug release withdrawal. There was no significant difference (P > 0.05) in oestrous response and duration between the treatment groups. Overall, the mean interval to the onset of oestrus in this study was 46.8 ± 1.4 hours. Most ewes began exhibiting oestrus at 24 and 36 h post controlled internal drug release withdrawal. There was no significant difference (P = 0.07) in the mean interval to the onset of oestrus between the treatment groups. Progesterone hormone did not differ significantly (P > 0.05) between the treatment groups at controlled internal drug release insertion and removal and 48 hours post removal. However, oestradiol was significantly higher at controlled internal drug release removal and 48 hours post removal (P < 0.05) than at insertion in all treated groups. Therefore, it was concluded that using the progesterone synchronisation protocol, regardless of the duration or conjunction with eCG treatment, can effectively synchronise oestrus during the spring breeding season in South Africa Merino ewes. For seasonal breeders who use South Africa’s spring breeding season, the progesterone oestrous synchronisation protocol is recommended to improve the reproduction activity in Merino ewes.
oestrous synchronisation, long-term progesterone, short-term progesterone
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