Investigation of ram effect and eCG usage in progesterone based oestrous synchronization protocols on fertility of ewes following fixed time artificial insemination

Abstract

Improving oestrus synchronization protocols is important, particularly to ensure that protocols which guarantee reduced hormonal use and environmental safety also result in acceptable reproductive efficiency. The aim of this study was to compare the effect of long and short-term progesterone (P4) treatment and their combination with either equine chorionic gonadotropin (eCG) or the ram effect (Ram) on oestrous response and fertility of ewes. Seventy-eight South African Mutton Merino ewes were randomly allocated to four treatment groups in a 2 × 2 factorial design and primed with controlled internal drug release (CIDR) for a 9 (short) or 14 d (long) period. At CIDR withdrawal, ewes in each group received either a single intramuscular injection of equine chorionic gonadotrophin (eCG; 300 IU) or exposure to the ram effect; eCGshort (n = 19), Ramshort (n = 21), eCGlong (n = 19) and Ramlong (n = 19). Oestrous behaviour was monitored from 12−84 h post CIDR withdrawal. Ultrasound was performed at 48 h post CIDR withdrawal to examine number and diameter of follicles. Artificial insemination (AI) was performed twice at 48 and 60 h post CIDR withdrawal with fresh undiluted semen using the cervical method. Pregnancy diagnosis was performed by transrectal ultrasound at 35 d post AI and confirmed by lambing data. Oestrous behaviour was observed in 98.7% of all synchronized ewes, with no significant difference among treatment groups. Overall, conception rate (CR) and the proportion of ewes lambing to synchronized oestrus were (74.4% and 52.6%, respectively). There was no significant difference among treatment groups in oestrous response, onset of oestrus, number of follicles, diameter of the largest follicle, CR and AI to lambing interval (P > 0.05). When data were pooled, CIDR-14 d protocols showed a significantly shorter interval to onset of oestrus (24.9 ± 1.6 vs 30.8 ± 2.1, P < 0.05) than CIDR-9 d protocols but there was no difference (P > 0.05) between eCG and Ram protocols when data were pooled. CIDR-9 d protocols resulted in a significantly higher CR (85.0% vs 63.2%, P < 0.05) than CIDR-14 d protocols when data were pooled, but CR between eCG and Ram protocols was not statistically different (P > 0.05). Mean AI to lambing interval was 158.2 ± 1.2 d, ranging from 147 to 154 d and 166 to 186 d post AI. Ewes lambing to synchronized oestrus per treatment group were 52.6% (eCGshort), 42.9% (Ramshort), 63.2% (eCGlong) and 52.6% (Ramlong), respectively (P > 0.05). In conclusion, the 4 protocols investigated were effective in synchronizing oestrus with similar response to synchronization of oestrus and fertility among treatment groups. Of the 4 protocols, the Ramlong protocol offers the benefit of being less costly because of reduced hormonal use in addition to the adequate fertility obtained.