Effect of Progesterone Prior to GnRH‐PGF 2 α Treatment on Induction of Oestrus and Pregnancy in Anoestrous Awassi Ewes

Contents An experiment was conducted to examine the effect of progesterone prior to a GnRH‐PGF 2 α 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  μ g of GnRH 24 h after sponge removal followed, 5 days later, by 20 mg PGF 2 α injection. Ewes were exposed to three fertile rams at the time of PGF 2 α 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 PGF 2 α 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‐PGF 2 α ‐treated seasonally anoestrous ewes and increases their response in oestrus and pregnancy rates.