Control of oestrus and fertility in gilts with accessory corpora lutea by prostaglandin analogues, ICI 79,939 and ICI 80,996

Treatment of cows or mares with prostaglandin (PG) F-2α as early as Days 4 or 5 of the oestrous cycle causes regression of corpora lutea (CL) and results in oestrus 2–4 days after treatment (Lauderdale, 1972; Rowson, Tervit & Brand, 1972; Douglas & Ginther, 1972; Allen & Rowson, 1973) However, the CL of pigs do not regress in response to PGF-2α until Days 11–12 of the cycle (Gleeson, 1974; Guthrie & Polge, 1976). Because the secretory activity of the CL in the pig declines rapidly by Days 14 or 15 of the cycle, the effective period for synchronization of oestrus by PGF-2α treatment is too short to be of practical use. Exogenous gonadotrophins induce the formation of accessory CL at any stage of the oestrous cycle of the pig (Neill & Day 1964; Day, Neill, Oxenreider, Waite & Lasley, 1965; Caldwell, Moor, Wilmut, Polge & Rowson, 1969) and the CL formed on Days 8, 10 or 16 delayed oestrus for at least 15 days (Caldwell et al., 1969). We therefore investigated the possibility of synchronizing oestrus in gilts by inducing accessory CL to delay oestrus and ovulation and then injecting PGF-2α analogues to regress the accessory CL at a predetermined time.