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Abstract
Two experiments compared the effect of long-acting melatonin implants (Regulin®) on the reproductive performance of commercial sheep flocks joined in late spring or early summer. A third experiment used melatonin implants in an attempt to induce an early January mating in Romney ewes. In Experiment 1, Corriedale and Borderdale ewes in five flocks in the South Island had joining dates from 17 December to 11 January 1988. In Experiment 2, Romney, Coopworth, Perendale, and their crosses in 11 flocks in the North Island had joining dates from 12 January to 4 March 1989. Up to half of the ewes in each flock had melatonin implants inserted 20–34 days before joining. Ultrasound scanning at 50–60 days was used to determine the number of foetuses present. There were decreases in barrenness (1–49%) and increases in ewes with multiple foetuses (4–35%) in all the South Island flocks and the eight North Island flocks joined during mid January. The mean net effect was an additional 30.8 and 23.0 foetuses/100 ewes treated with melatonin compared with untreated control flocks in the South and North Island respectively. In the three North Island flocks joined after the end of January, there were no increases in the number of foetuses, since small increases in multiple pregnancies (4–8%) was offset by small increases in barrenness (2–5%). In Experiment 3, melatonin implants were inserted into Romney ewes on 1 December and the ewes were joined on 30 December 1986. No ewes were mated before 5 February, but from then until 24 March the onset of mating with teaser rams was 8.3 days earlier (P < 0.01) in implanted than control ewes. Mating with entire rams from 24 March to 11 May (i.e. 114–162 days after start of melatonin treatment) resulted in the treated ewes having lower pregnancy rates (73.4 versus 90.5%, P < 0.01) and a suggestion of fewer multiple pregnancies (26.3 versus 39.2%, P > 0.05) than control ewes. The results indicate that melatonin implants can be used to achieve a practical increase in reproductive performance of Corriedale, Romney, and Romney-type ewes. However, the timing of treatment and joining must be optimised for each region and breed so as to avoid the lower reproductive performance associated with the development of an early refractory state.