C.S.I.R. O., Division of Animal Genetics, P.O. Box 90, Epping, New South Wales 2121, Australia
(Received 7th January 1975)
When ewes are injected with synthetic Gn-RH on the day of oestrus, the magnitude of the LH release is greater than that following injection at other stages of the oestrous cycle (Reeves, Arimura & Schally, 1971; Hooley, Baxter, Chamley, Cumming & Findlay, 1974; Thimonier, Terqui & Pelletier, 1974). It is not known, however, whether additional LH on the day of oestrus has any effect on the number of ovulations, especially in ewes with the genetic potential for high ovulation rates. This question was investigated in four flocks of Merino sheep, designated `C', `O', `T' and `B', which are, respectively, an unselected group and groups selected since 1954, for low, medium and high litter size. The genetic history of these flocks has been described by Turner (1969).
In May 1973, eighty-nine ewes