Interaction of endophyte-infected fescue and heat stress on ovarian function in the beef heifer

The objective of the experiment was to examine the interaction of endophyte -infected tall fescue and environmental temperature on follicular and lutea l development and function in beef heifers. Heifers were fed endophyte-free or endophyte-infecfed tall fescue seed at thermoneutral or beat stress tem peratures (n = 6/treatment) 4 wk before and 3 wk after synchronized ovulati on. All heifers were subjected to thermoneutral conditions (19 degreesC, 50 % relative humidity) from Days -7 to -2; temperature increased incrementall y from Days -7 to 0 and cycled between 25 degreesC and 31 degreesC between Days 1 and 20 for heat-stressed heifers, Serum was collected and ovaries mo nitored every other day after induced luteolysis between Days 1 and 23 or u ntil ovulation. Size and location of follicles >4 mm and corpora lutea were recorded. Serum concentrations of prolactin were reduced in heat-stressed heifers fed infected seed and both heat stress and infected seed decreased total cholesterol. Rectal temperature and respiration rate were greatest in heifers fed the infected seed when exposed to maximal temperatures, Heat s tress led to reduced diameter of the corpus luteum and serum progesterone c ompared with thermoneutral conditions. Progesterone was reduced more so in heifers fed infected seed. The combination of infected seed and heat stress was associated with reduced diameter of the preovulatory dominant follicle , and consumption of infected seed led to fewer large follicles during the estrous cycle. Both stressors led to reduced serum estradiol. Impaired foll icle function may explain reduced pregnancy rates commonly observed in heif ers grazing infected tall fescue pasture.