PROLONGED GESTATION, DECREASED TRIIODOTHYRONINE CONCENTRATION, AND THYROID-GLAND HISTOMORPHOLOGIC FEATURES IN NEWBORN FOALS OF MARES GRAZING ACREMONION COENOPHIALUM-INFECTED FESCUE

Newborn foals of mares grazing on Acremonium coenophialum-infected fes cue pasture throughout gestation or from gestation day 300 to parturit ion had increased gestation duration and decreased serum triiodothyron ine concentration. Pregnant mares were allotted to 4 treatments: grazi ng continuously on endophyte-free (E(-)) fescue, grazing continuously on endophyte-infected (E(+)) fescue, grazing on E(+) fescue from gesta tion day 300 to parturition, and grazing on E(+) fescue from conceptio n to gestation day 300. Morphometric studies indicated that foals born to mares exposed to endophyte late in gestation had large, distended thyroid follicles lined by flat cuboidal epithelial cells. Mean triiod othyronine concentration in foals exposed to endophyte (395.2 ng/dl) w as decreased (P < 0.01), compared with mean values in control foals (7 78.0 ng/dl). Thyroxine and reverse triiodothyronine concentrations wer e not significantly different among groups. Foal organ weight as a per centage of foal body weight was not significantly different among expe rimental groups.