EFFECTS OF PROLONGED EXPOSURE OF EWES TO ESTROGENIC PASTURE .1. PERMANENT FLOCK INFERTILITY FOLLOWING LONG-TERM GRAZING OF RED-CLOVER (GRASSLANDS-PAWERA)-DOMINANT PASTURE

Romney ewes grazed red clover (P) or mixed red clover/ryegrass (M) pas tures for about 6 months/year until mating in Year 4 (P) or 5 (M) of a 6-year study. A further group grazed ryegrass/white clover (C) pastur e continuously throughout the study. At other times all animals grazed ryegrass/white clover pasture as one mob. Ewes grazing P pasture were consistently 3-6 kg heavier than ewes that grazed C pasture. Despite similar proportions of ewes being marked by the ram, and similar ovula tion rates, during the first cycle of mating each year, ewes grazing P pastures consistently showed increased returns to service (+30%), inc reased barrenness (+30%), and fewer lambs born per ewe joined (-0.5) t han ewes grazing C pasture. Differences persisted after ewes were remo ved from treatment pastures, indicating that infertility was likely to be permanent.