An experiment was conducted to examine the effect of progesterone prior to a GnRH-PGF2alpha treatment on oestrus and pregnancy in seasonally anoestrous Awassi ewes. Twenty-four ewes were randomly assigned to three groups to be pre-treated with 60 mg medroxyprogesterone acetate sponges (group A), 600 mg progesterone sponges (group B) or blank sponges (group C) for 4 days. All ewes were injected with 100 microg of GnRH 24 h after sponge removal followed, 5 days later, by 20 mg PGF2alpha injection. Ewes were exposed to three fertile rams at the time of PGF2alpha injection (day 0, 0 h) and were checked for breeding marks at 6-h intervals for 5 days. Blood samples were collected from all ewes 1 day (day -10) prior to sponge insertion, at the time of sponge removal (day -6), 1 day following sponge removal (day -5, at the time of GnRH injection) and at the time of PGF2alpha injection (day 0) for analysis of progesterone. Progesterone concentrations on days -10 and -5 were basal and averaged 0.2 +/- 0.04 and 0.2 +/- 0.2 ng/ml, respectively. Progesterone concentrations on day -6 were elevated only in group B ewes and were higher (p < 0.0001) than those of groups A and C. Progesterone concentrations on day 0 were higher (p = 0.002) in groups A and B than group C. Oestrous responses occurred only in ewes of groups A and B (p > 0.05). Induced oestrus conception rate was greater (p < 0.01) in group A than groups B and C. Ewes returned to oestrus 17-20 days following day 0 were two of eight, six of eight and three of eight of groups A, B and C, respectively, all of which eventually lambed. The overall lambing rate was 82% in progesterone-primed ewes compared with only 38% non-progesterone-primed ewes (p < 0.05). Progesterone priming apparently sensitizes GnRH-PGF2alpha-treated seasonally anoestrous ewes and increases their response in oestrus and pregnancy rates.

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