EFFECT OF TREATMENT WITH MELATONIN IMPLANTS IN CONJUNCTION WITH TEASER RAMS ON THE REPRODUCTIVE PERFORMANCES OF POLL DORSET X MERINO EWES JOINED IN EARLY SUMMER IN THE SOUTH-WEST OF WESTERN AUSTRALIA

Melatonin pretreatment to improve the reproductive performance of first-cross Poll Dorset x Merino ewes joined in early summer was investigated in 2 experiments conducted in consecutive years. In experiment 1, melatonin pellets were administered subcutaneously to 1 of 3 groups of ewes; 14 days later they were joined with testosterone-primed wethers ('teasers'), followed after 15 days by entire rams (melatonin-teased group). The other groups were untreated and teased only. Experiment 2 consisted only of teased and melatonin-teased groups, with the ewes from experiment 1 being reallocated, as well as maiden ewes being joined for the first time. In this experiment melatonin pellets were inserted 38 days before the start of joining. Treatment with melatonin resulted in increased reproductive performances. The responses were characterised by an earlier (P<0.05) conception pattern in both experiments and higher (P<0.001) lambing percentages than controls in experiment 1. Up to 22% more lambs were born to melatonin-teased ewes than to teased ewes, and stimulatory effects were similar for ewes of different ages. In experiment 2, ewes that had received melatonin in both experiments tended (P>0.05) to have higher lambing percentages than teased ewes (up to 17% more lambs born), while ewes that had received melatonin in only experiment 1 tended (P>0.05) to have lower lambing percentages than ewes that were not treated with melatonin in either experiment (4-11%). The higher lambing performances of the melatonin-treated ewes were independent of ewe liveweights. Reproductive activity was also stimulated by the use of teasers alone in experiment 1. Conception patterns were advanced (P<0.01) and there was a trend for more (P>0.05) lambs to be born in the teased group than in the control group. The results show that both teasers and melatonin treatment will advance the conception patterns of first-cross Poll Dorset x Merino ewes joined in early summer, but the melatonin treatment also stimulated fecundity.