Effects of oestrogenic pasture on luteinizing hormone levels and the response to GnRH in female lambs

Abstract

Two Merino flocks (A and B) were divided into two groups such that half grazed oestrogenic Dwalganup subterranean clover pasture containing the phytooestrogens formononetin, genistein and biochanin A. The other half grazed non-oestrogenic rose clover pastures. Both groups began the grazing treatments 2 weeks before parturition. The female lambs born in these groups were maintained on the two pasture types for 22 weeks. Plasma LH levels (4 × 30 min samples) were measured at 8, 12, 16 and 22 weeks in Flock A (44 lambs) and 8, 14 and 19 weeks in Flock B (21 lambs). In both flocks the LH response to 100 μg GnRH was measured at the time of the last LH measurements (i.e. 22 and 19 weeks respectively). Lambs on oestrogenic pastures had significantly higher plasma LH levels at 12, 16 and 22 weeks of age in Flock A and at 19 weeks of age in Flock B. These lambs were also more sensitive to GnRH, releasing significantly more LH than those grazing nonoestrogenic pastures in both flocks. The differences in LH patterns were not associated with differences in either ovarian follicle populations at 26 weeks of age or in ovulation rate at 16 months of age. The results do, however, demonstrate clearly at two locations that oestrogenic pastures had significant effects on pituitary gonadotrophin secretion and sensitivity to GnRH early in life.