The effect of steroid immunisation on the seasonal pattern of oestrus, ovulation and ovulation rate in Coopworth ewes

J.F. Smith, L.T. McGowan; K.S. Maclean and P.A. Farquhar

Ruakura Animal Research Station, Ministry of Agriculture and Fisheries, Hamilton; Ministry of Agriculture and Fisheries, Gisborne and Invermay Agricultural Research Centre, Ministry of Agriculture and Fisheries, Mosgiel

Proceedings of the New Zealand Society of Animal Production. 1987, 47: 143-146

Two hundred 5-year-old Coopworth ewes were allocated to 4 groups in December 1983. Two groups (1 of which had been given a booster treatment of Fecundin(r) on 21 December) were treated with progesterone impregnated intravaginal sponges and joined with vasectomised Southdown rams on 19 January 1984. Oestrous data were recorded daily and ovulation rate determined by laparoscopy at each cycle thereafter. The other 2 groups (1 of which received a booster on 5 May 1984) were also treated with progesterone impregnated intravaginal sponges and introduced to rams on 1 June 1984. Oestrous and ovulation data were recorded until mid-September.

Immunisation had no effect on the percentage of ewes exhibiting oestrus whereas season had a significant effect with an increase from 18% in January to a peak of 95% in April. This level of oestrous activity was maintained until the end of July after which there was a rapid decline. The proportion of ewes ovulating showed a similar pattern although peak values were attained early in February. Ovulation rate showed an increase up to March, plateauing until early July, and then declining. Values for immunised ewes were significantly higher than controls and persisted for at least 6 months in the early treatment group.

These data show that although immunisation does not adversely influence the pattern of oestrus and ovulation activity in ewes subjected to the ram effect, early treatment appears to prolong the ovulation rate response.

Keywords: NZSAPAB; Immunisation; season; oestrus; ovulation rate; Coopworth

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