The placentas of mares grazing E+ tall fescue are thickened, reddish colored, and heavier, with an increased rate of retention than for E- mares (Monroe et al., 1988). Frequently, the foal is presented normally but encased in a tough, thickened chorioallantois (a membrane surrounding the foal in the placenta), which it cannot break through; consequently, the foal suffocates unless an attendant is present to cut the chorioallantois immediately. Using an Ingstrom meter to measure stress and strain, these E+ placentas appeared to be more resistant to forces that would tear them, partially explaining why some foals are unable to break through the thickened placentas (Monroe et al., 1988). Taylor et al. (1985) reported heavier and thicker placentas from many mares consuming E+ seed than from mares consuming E- seed; DNA, RNA, and collagen contents were greater in the placentas of mares consuming E+ seed. Brendemuehl et al. (1994a) reported on mares that grazed E+ fescue either continuously, from 300 d of gestation to foaling, or from gestation Day 60 to 300, or had no exposure to fescue. They observed an increase in weight and width of combined chorioallantois from mares exposed to E+ fescue continuously or from Day 300 to foaling. Brendemuehl (1994a) reported increased placental thickness in E+ mares immediately before parturition. Using 12 E+ and 12 E- mares, increased placental thickness was observed in 10 of the E+ mares 6.5 h before the onset of parturition, while another E+ mare demonstrated an increase in placental thickness 32 h before parturition. An elective Cesarean section was performed in one E+ mare at 358 d of gestation and within 2 h of noting an increase in placental thickening. At surgery, the placenta was reported to be thickened in a plaque-like fashion in the ventral portion of the gravid horn, the uterine horn that contains the fetus. The thickened portion of the chorioallantois was noted to be separated from the uterus. Premature placental separations are common during the last trimester in mares grazing E+ fescue and are commonly referred to as red-bag. Frequently these mares develop udders prematurely and may leak milk.


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