One hundred and fifty ewes grazed either on a high oestrogenic red clover (Pawera), or a new, low oestrogenic selection of red clover (G27), or control pastures before mating. The treatment groups (n = 25) and grazing lengths prior to mating were (1) Pawera, 6 weeks; (2) G27, 6 weeks; (3) G27, 12 weeks; (4) G27/Ryegrass-white clover (rg-wc), 6 weeks/6weeks; (5) Rg-wc (control 1), 6 weeks, and (6) White clover (control 2), 6 weeks. The ewes were mated as one group on rg-wc pasture immediately after the grazing treatments. Formononetin concentration was significantly lower in G27 (0.26%) than in Pawera red clover (0.55%) (P<0.05). Ovulation rates in ewes after the first service were similar for all treatment groups (P<0.05). The incidence of return to service was significantly higher in Pawera ewes (72.7%) than in any of the other groups (P<0.01). The return rates for the other groups were 33.3% (G27/6weeks), 25.0% (G27/12 weeks), 4.8% (G27/rg-wc), 9.5% (rg-wc) and 14.3% (white clover). Most ewes which were mated at the next two cycles became pregnant. It is concluded that the fertility in the ewes grazing G27 red clover was better than that in ewes grazing Pawera red clover, however there may have been some impairment in contrast to animals grazing control pasture.
Proceedings of the New Zealand Society of Animal Production, Volume 54, , 231-234, 1994
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