Alfalfa fed to ewes at mating reduces the subsequent lambing percentage 10 to 20 percent. Prior work has shown the reduced lambing percentage is due to the presence of the phyto-estrogen coumestrol in alfalfa. The concentrations of coumestrol in alfalfa are low ordinarily, but increase rapidly in response to stress to the plant. The proposed research would establish minimum critical intakes of coumestrol which affect reproductive efficiency of animals. Reproductive effects will be studied in several species to determine possible differences in the estrogenic effect of coumestrol among animal species. Experiments will be designed to determine if the time of exposure in the estrus cycle to coumestrol is a significant factor in affecting reproduction. Additional experiments will explore the relationship exogenous coumestrol has on endogenous production of estrogen and gonadotropin. Lastly, this project will investigate the deposition of coumestrol in animal tissues and the secretion of coumestrol into bovine milk. Attempts will be made to correlate tissue deposition and milk secretion of coumestrol to dietary intake. As progress is made towards these goals, a greater appreciation for possible health risks to man will be obtained.