Oestrous synchronization in cattle has not received widespread acceptance because of a period of transitory infertility which occurs following progestagen withdrawal. This reduced fertility has been attributed to changes in the time required for sperm capacitation (Lauderdale & Erickson, 1970), changes in the amount and physicochemical characteristics of cervical mucus (Johnson & Ulberg, 1965; Hill, Lamond, Henricks, Dickey & Niswender, 1971), changes in body temperature (Johnson & Ulberg, 1965; Long, Nipper & Vincent, 1969), reduced fertilization rate (Wagner, Veenhuizen, Gregory & Tonkinson, 1968), and histochemical changes in the endometrium (Wordinger, Dickey & Hill, 1970). All of these physiological aberrations can be attributed to hormonal changes. This experiment was conducted to determine changes in peripheral blood plasma progesterone concentration during and subsequent to oestrous synchronization with melengestrol acetate (MGA—The Upjohn Company) and to compare
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