Effects on reproduction of prolonged grazing of oestrogenic pastures by ewes

Abstract

Six-year-old ewes, the survivors of two experimental groups that had grazed from May to October through the 5 preceding years on mixed pastures containing an oestrogenic variety (Dwalganup) of subterranean clover ("clover" group) and oat crops ("oats" group), were mated 6 months after their last exposure to oestrogenic pasture and allocated for slaughter to observe the fertilization rate of eggs or the numbers of surviving embryos.

In late April the clover and oats groups did not differ significantly in the proportion of eggs fertilized (13/15 eggs v. 11/11) but in May there was a highly significant difference (4/12 v. 14/17). The difference between late April and May in the clover group was significant and was associated with changes in the ratio of oestrous ewes to rams.

The ratio of live embryos to eggs shed was much higher in the oats than in the clover group (56173 v. 8/39). After estimating how many of these eggs had been fertilized it was concluded that more fertilized eggs developed successfully in the oats than in the clover group (c. 56/65 v. 8/17). The data suggested that more "clover" embryos died during days 13–19 of pregnancy.

Significantly more clover group ewes had obvious reproductive tract infections. These ewes did not show oestrus regularly though they may have ovulated more or less regularly. Other clover group ewes showed oestrus as regularly as oats group ewes.

Clover group ewes had many more macroscopic endometrial cysts in their uteri and cervices, but within groups, numbers of cysts were not significantly associated with previous or current reproductive performance.