Response of anoestrous ewes pre-treated with a single dose of oestradiol-17β, or progesterone and oestradiol-17β, to the introduction of rams and ewes in oestrus

Abstract

AIMS: To determine if the frequency distribution of time to oestrus observed after the introduction of rams, and additional ewes in oestrus, is affected by the administration of oestradiol- 17β. To determine if the fertility following mating at the first induced oestrus in progesterone-primed ewes may be increased with administration of oestradiol-17β. METHODS: In Experiment 1, ewes received 40 µg oestradiol- 17β 3 (E3; n=61) or 5 (E5; n=56) days before the introduction of rams; 61 ewes were controls (C1). In Experiment 2, a controlled internal drug-releasing (CIDR) device was inserted in 95 ewes on Day -5 (Day 0 = introduction of rams). In addition, 47 ewes received oestradiol-17β on Day 0 (CE), and the remaining 48 were controls (C2). In both experiments, additional ewes, brought into oestrus between Days 0 and 2 by hormonal treatments, were introduced with the rams. Onset of oestrus was estimated by visual observation of ewes marked by rams. Pregnancy status of ewes in Experiment 2 was determined using ultrasound, 30 days after oestrus. RESULTS: The total number of ewes detected in oestrus in Experiment 1 was similar between the three groups. The frequency distribution of ewes in oestrus in the E5 group differed from that of ewes in both C1 (p=0.05) and E3 (p<0.001). A similar number of ewes were detected in oestrus between Days 16–20, but the proportion of ewes in oestrus between Days 21–26 was greater for ewes in C1 than E5 or E3. Mean interval to the onset of oestrus was shorter in C1 than E3 which was shorter than E5 ewes (p<0.001). In Experiment 2, the total number of ewes in oestrus were similar between groups. Ewes in oestrus between Days 1–4 (65% and 64% for C2 and CE, respectively) and conception rates over the same period (81% and 80% for C2 and CE, respectively) were also similar between groups. CONCLUSIONS: Administration of oestradiol-17β to anoestrous ewes altered the pattern of onset of oestrus, but it did not affect submission or conception rates, regardless of whether or not a short period of progesterone priming with a new CIDR device was included prior to the introduction of rams.