Summary. Anoestrous Romney Marsh ewes with and without progesterone treatment (+P, −P) were treated with small-dose (250 ng) multiple injections of GnRH at 2-h intervals for 48 h. Animals were slaughtered on Days 4, 5, 7 and 11 after the end of GnRH treatment and luteal function was assessed by the measurement of daily plasma progesterone concentrations.
In all animals which ovulated (29/32, 91%) peripheral progesterone concentrations rose to 0·5–1·0 ng/ml within 3 days of the end of GnRH treatment. In 7/7 (100%) +P animals and 5/22 (23%) −P animals, progesterone concentrations continued to rise and were maintained at levels > 1·5 ng/ml until slaughter. In the remaining −P animals, plasma progesterone concentrations declined to reach basal levels by Day 5. Corpora lutea recovered from these animals showed signs of premature regression on Day 5 and were fully regressed by Day 7. Progesterone priming delayed the occurrence of the LH surge which occurred 39·1 ± 3·6 h after the end of GnRH treatment in the +P animals compared to 20·2 ± 1·74 h (P < 0·001) in the −P animals in which luteal function was abnormal and 22·4 ± 4·35 h in the −P animals in which luteal function was normal.
These results show that abnormal luteal function occurs in the majority of GnRH-treated ewes in the absence of progesterone pretreatment. It is characterized by a transient rise in plasma progesterone concentration to 0·5–1·0 ng/ml 3 days after GnRH treatment, which then declines to basal values between Days 4 and 5, coincident with the rapid premature regression of this abnormal corpus luteum which is complete by Day 7.
Keywords: GnRH; abnormal luteal function; sheep; anoestrus