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Two trials were conducted to examine the effect of three progestagens, chlormadinone acetate (CAP), melengestrol acetate (MGA) and megestrol acetate (MA) on the sexual activity of ranch cows.

In the first trial, injection of CAP (6 mg/day) inhibited manifestation of oestrus in ten out of ten cows, while six of these cows showed full heat 4 to 6 days after cessation of treatment. When CAP was administered by intravaginal tampon (200 mg CAP/tampon), eight out of ten cows showed full heat during the 18 days of treatment, while two out of ten cows showed full heat in the 7 days immediately after cessation of treatment.

In the second trial, CAP was injected at 6 mg/day for 16 days and at 12 mg every alternate day for 16 days. MA or MGA was administered by intramuscular injection at 4 mg/day or 0·5 mg/day respectively or by intravaginal tampon at 100 mg/tampon or 60 mg/tampon respectively. With the exception of one cow (MA), all progestagens completely inhibited manifestation of oestrus when injected. When the progestagen was administered by tampon, two out of ten cows receiving MGA and two out of ten cows receiving MA showed oestrus during treatment. During the period 5 to 10 days after cessation of treatment, heat was shown by 100%, 80%, 70% and 70% of cows which received injections of CAP daily, CAP on alternate days, MGA or MA, respectively. The high incidence of anovulatory heats (20 to 80%) indicated that the first heat after cessation of progestagen treatment would be associated with low fertility.