The effects of endophyte consumption on milk production vary among species. Eighty-eight percent of mares were agalactic when maintained on E+ fescue up to foaling (Monroe et al., 1988; Fig. 17-1). Cattle (Strahan et al., 1987; Porter and Thompson, 1992; Schmidt and Osborn, 1993; Irwin, 1997), sheep (Stidham et al., 1982), and mice (Zavos et al., 1988, Siegel and Bush, 1988) can exhibit reduced milk yields, whereas horses (Monroe et al., 1988; Fig. 17-1) and rabbits (Daniels et al., 1984) readily exhibit reduced milk yields or complete agalactia. The connection between tall fescue toxicosis and poor lactogenesis (milk synthesis) seems to be through the effects of the ergot alkaloids on lactogenic hormones. Cattle, sheep, and mice have both placental lactogen and prolactin (Forsyth, 1986). In contrast, horses and rabbits rely on prolactin to stimulate prepartum lactogenesis (Forsyth, 1986). The depressive effects of the ergot alkaloids on prolactin secretion may suppress the effect of prolactin on lactogenesis in cattle, sheep, and mice, but have little or no effect on placental lactogen, a lactogenic hormone. Consequently, the placental lactogen and the small level of pituitary prolactin, a hormone produced by the anterior pituitary, may be sufficient to initiate prepartum (before parturition) lactogenesis in these species and allow lactation to begin after parturition. In the horse, reduced prolactin secretion from the pituitary lactotrophic cells results in agalactia. The alkaloids in the tall fescue-fungal endophyte symbiosis serve as D2 dopamine receptor agonists at the pituitary level (Strickland et al., 1992). In addition, the horse, unlike ruminant herbivores, does not benefit from pregastric metabolism of alkaloids and is subject to absorption of larger amounts of the alkaloids from E+ tall fescue (Wachenheim et al., 1992).

The milk of agalactic mares often appears as a brown or straw-colored oily-looking fluid, rather than the white milk of normal mares. This fluid has little nutritional value and foals invariably die unless bottle-fed. Another frequent complication affecting foal viability is the lack of normal immunoglobulins in foals from mares that have the straw-colored fluid rather than white milk (Kouba, 1995).


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