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D. F. WISHART

Recently, a small series of screening experiments have been conducted with a view to identifying synthetic steroids of high activity for control of the oestrous cycle in mature Friesian heifers. Five synthetic steroids and progesterone were tested in cyclical animals. The activity of the steroids was assessed by their ability to suppress both oestrus and ovulation following a series of daily injections. The steroids were all chosen on the basis of their biological activity profiles in conventional laboratory animal bioassays.

The synthetic steroids were:

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D. F. WISHART and B. D. HOSKIN

Summary.

Heifers were treated with intra-vaginal pessaries impregnated with 200 mg SC-9880 (17α-acetoxy-9α-fluoro 11β-hydroxy-4-pregnene-3,20 dione). One or 2 days before removal of the pessaries (21 days after insertion) each heifer received a single intra-muscular injection of 750 i.u. pmsg.

Of the eighty-one treated heifers, sixty-six retained the pessaries for the prescribed period; fifty-five of these (83·3%) showed oestrus 24 to 72 hr after removal of the pessary. Of fifty-five heifers served or inseminated, twenty-four (43·7%) conceived. Of thirty-three heifers served at the second oestrous period, twenty-one (61·9%) conceived.