REPRODUCTIVE RESPONSES ASSOCIATED WITH RAM INTRODUCTION IN MATURE TROPICAL MENZ EWES AFTER A PERIOD OF ISOLATION

This study was undertaken using 48 mature Menz ewes (3-6 years of age) and averaging 31.5 (standard deviation (SD)=4.9) kg body weight, to e valuate oestrus synchronization, conception and lambing rates after ra m introduction following a period of isolation (n=24) or constant expo sure to vasectomized rams (n=24) on a low or high level of nutrition. The overall level of oestrus synchrony was low (27%), but was higher f or animals previously isolated (38 vs. 17%). The mean interval from ra m introduction to oestrus was 12.2 (SD=7.3) days, but this was signifi cantly shorter for isolated ewes (9.4+/-1.2 vs. 14.8+/-1.7 days, P<0.0 5). Animals previously better fed were heavier at mating (P<0.01), but this had no relationship to the response to ram stimulation (P>0.05). Oestrus after ram introduction included two peaks (at 6 and 13 days) which were influenced by the behaviour of previously isolated animals. Conception rates to first and second service of 86% and 14%, respecti vely, were confirmed by plasma progesterone determinations. Subsequent lambing rate was only 72%, suggesting high prenatal reproductive wast age. Lambing was compacted, except for ewes conceiving at their second oestrus. There were as many twins born to isolated ewes as to exposed ewes but substantially more to ewes that were heavier at mating. Thes e data suggest that, although ram introduction after a period of isola tion will not increase ovulation and twinning rate in isolated Menz sh eep as judged by litter size, it is a natural tool that could be usefu l to match events of the reproductive cycle of tropical sheep with fee d and other management resources.