-------, Aus(, J, BioI, Sci" 1981,34, 569-75 Induction of Ovulation in Seasonally Anovular Ewes by the Introduction of Rams: Effects of Progesterone and Active Immunization against Androstenedione Graeme B, Martin,A Rex J. Scaramuzzi B and David R. LindsayA A Department of Animal Science and Production, University of Western Australia, Nedlands, W,A, 6009. B Division of Animal Production, CSIRO, Great Western Highway, Prospect, N,S,W. 2148, Abstract The effects of progesterone priming and active immunization against androstenedione on ovulation, oestrus and premature luteal regression were observed in seasonally an ovular Merino ewes which had been induced to ovulate by the introduction of rams, Ovulation was induced in 27 out of 36 ewes, and the response was not affected by either immunization or progesterone priming, The ovulation rate (mean number of ovulations per ewe ovulating) was higher in ewes immune to androstenedione, but there was no significant effect of this treatment on oestrus or luteal main- tenance, Priming with progesterone prior to the introduction of rams prevented premature regression of the corpora lutea but had no effect on the other measures of reproductive function. If active immunization were to be applied on a commercial scale, it would not interfere with the practice of mating ewes during the anoestrous season, The increase in ovulation rate in these ewes, with no increase in the proportion of ewes ovulating, indicates that the mechanism for the control of ovulation is separate from that controlling ovulation rate, The mechanism by which progesterone pretreatment prevents premature luteal regression awaits further investigation. Introduction In Western Australia over half of the commercial Merino flocks are joined with rams during the anoestrous season, that is, in late spring and early summer, even though few of the ewes are ovulating spontaneously at this time (Knight et al. 1975). This is possible because most of these ewes can be induced to ovulate and show oestrous cycles by the introduction of rams (Schinckel 1954a), The first ovulation is rarely accompanied by oestrus (Schinckel 1954a, 1954b) and in about half of the ewes luteal regression and a second ovulation are completed within 8 days (Oldham and Martin 1978; Martin 1979). The failure to show behavioural oestrus and to maintain luteal function is due to the absence of a progestational phase prior to the introduction of rams (Hunter et al. 1971; Oldham et al. 1980). The ovulation rate at the first ovulation induced by rams is higher than that in subsequent ovulations, and if progesterone pretreatment is used to prime the ewes so that they display oestrus at this time, there may be economic gains in reproductive performance (Cognie et al. 1980). Active immunization against androstenedione has also been proposed as a method for increasing the ovulation rate of commercial flocks (Scaramuzzi et al. 1977). Merino ewes which are similarly immunized exhibit normal patterns of seasonal breeding (Martin et al. 1979), so should be capable of responding to the introduction