Effects of prolonged exposure of ewes to oestrogenic pasture 1. Permanent flock infertility following long-term grazing of red clover (‘Grasslands Pawera’)-dominant pasture

Abstract

Romney ewes grazed red clover (P) or mixed red clover/ryegrass (M) pastures for about 6 months/year until mating in Year 4 (P) or 5 (M) of a 6-year study. A further group grazed ryegrass/ white clover (C) pasture continuously throughout the study. At other times all animals grazed ryegrass/white clover pasture as one mob. Ewes grazing P pasture were consistently 3-6 kg heavier than ewes that grazed C pasture. Despite similar proportions of ewes being marked by the ram, and similar ovulation rates, during the first cycle of mating each year, ewes grazing P pastures consistently showed increased returns to service (+30%), increased barrenness (+30%), and fewer lambs born per ewe joined (-0.5) than ewes grazing C pasture. Differences persisted after ewes were removed from treatment pastures, indicating that infertility was likely to be permanent.

Effects of prolonged exposure of ewes to oestrogenic pasture 2. Occurrence of abnormalities of the external genitalia and altered mating performance

“…Subsequently an extended lambing period could be expected, with perhaps a wider spread of lambing dates and increased barrenness as ewes continue to return to service after the ram has been removed from the flock. On a flock basis, prolonged exposure to red clover results in permanent infertility over the flock's productive lifetime (Shackell et al 1993). Adams (1976) reported pathological changes in the tissue of the reproductive tracts of ewes grazing subterranean clover.…”

“…This study utilised the same animals and pastures as described by Shackell et al (1993). The materials and methods are outlined briefly below.…”

“…Pasture composition, grazing periods, and formononetin levels in the pasture have been reported by Shackell et al (1993). During the 6 years of the study, ewes on the P treatment spent approximately 21 months grazing swards that contained, on average, 53% Pawera.…”

“…Following an initial grazing for 8 weeks as ewe lambs, the P treatment group had access to oestrogenic pasture under the grazing regime described, for the next 3 consecutive years and the M treatment group for 4 consecutive years (see Fig. I; Shackell et al 1993). Subsequent to the final grazing of treatment pasture, all animals were maintained on non-oestrogenic pastures.…”

Dietary red clover (Trifolium pratense) induces oviduct growth and decreases ovary and testes growth in Japanese quail chicks

“…Phytoestrogens are a subset of plant secondary compounds (SPCs, so-called because they have no function in the primary roles of growth and development in plants), and include several classes of chemicals [6]. While phytoestrogens have been shown to exert endocrine disrupting effects on many species of vertebrates, including livestock [7,8], rodents [9–11], fish [12], and birds [13,14], little is known about the evolutionary function of these chemicals in plants. It has been suggested, however, that phytoestrogens have evolved as a chemical defense against vertebrate herbivory [15–17], similar to the well-studied ecdysteroidal disruption that plants exert on certain insects [18].…”