Treatment of seasonally anestrous Romney ewes with continuous infusion of low doses of GnRH: effects on estrus, ovulation and plasma progesterone concentration

The aim of this study was to assess the suitability of a GnRH infusion regimen (125 ng/h or 250 ng/h) to induce estrous behaviour, ovulation and normal corpus luteum function in progesterone-primed Romney ewes each month of seasonal anestrus (i.e. September to February inclusive) over two years. None of the progesterone-primed control ewes (i.e. no GnRH treatment; N = 120 observations) ovulated, showed normal corpus luteum function or displayed estrous behaviour at any time during anestrus. Approximately 27 and 50% of the respective 125 ng/h and 250 ng/h GnRH-treated ewes (N = 120 observations per GnRH treatment) ovulated and showed normal luteal function. Of those which ovulated 59.2% and 52.4% in the respective 125 ng/h and 250 ng/h GnRH treatment groups showed estrous behaviour. There was a significant effect of GnRH dose on the median number of ovulations (250 ng/h > 125 ng/h; P<0.01) but no overall difference (when both treatment years and GnRH doses were pooled) in the median number of ovulations per month of anestrus. The frequency of ewes with an ovulation rate >2 was low with only 4/95 treated ewes with more than 2 corpora lutea (CL). Treatment of progesterone-primed ewes with 250 ng/h GnRH increased plasma LH (P<0.01) but not FSH concentrations; a significant increase in LH pulse amplitude (P<0.05) but not LH pulse frequency was observed. The plasma gonadotropin levels in the 125 ng/h GnRH treatment groups were not studied. We suggest that in breeds such as the Romney which have a strict (i.e. 5-6 month) anovulatory interval, the GnRH-infusion technique may be of limited practical use for inducing pregnancies during the non-breeding season.