Most often the cause for the lack of milk and mammary development is that the cow was likely consuming endophyte-infected tall fescue or hay before calving. Endophyte is a fungus that exists in most fescue. Some stands of fescue are highly infected, while others are marginally or minimally infected. The endophyte fungus produces a large number of chemical compounds, many of which have serious biological effects on an animal’s body.

One of the primary effects of consumption of endophyte-infected tall fescue is a severe reduction in the secretion of the hormone prolactin. Prolactin is secreted by the anterior pituitary, and its main function is to stimulate the development of the mammary gland during gestation and to stimulate the production and secretion of milk from the mammary tissues after parturition. Therefore, when females consume feedstuffs highly infected with endophyte, their prolactin levels decrease significantly, and the development and function of the mammary tissue are altered.