ABSTRACT Plasma samples from two experiments were processed to determine whether ergot alkaloids associated with endophyte-infected tall fescue altered peripheral thyroxine (T4), triiodothyronine (T3), or cortisol concentrations in cattle. In Exp. 1, seven Angus steers (294 kg) received i.v. bolus injections of saline (SAL), ergonovine maleate (7 mg; EM), or ergotamine tartrate (7 mg; ET) at weekly intervals, and they received all treatments during the study. Blood was sampled every 15 min for 5 h, and treatments were given after h 1. Mean ambient temperature was 34oC. Treatment x time affected plasma concentrations of T3 (P < .05) and of cortisol (P < .001) but not that of T4 (P > .2). Plasma T3 concentrations were not affected by SAL, whereas concentrations increased (P < .01) after either EM or ET treatment. Plasma cortisol concentrations were not altered by SAL or EM, but they were increased (P < .001) by ET treatment. In Exp. 2, six Holstein cows (499 kg) nursing calves received a bolus i.v. injection of SAL, EM (9.5 mg), or ET (9.5 mg) per estrous cycle, and all treatments were given over three cycles. Blood was sampled every 20 min for 5 h; treatments were given after h 1. Mean ambient temperature was 26oC. Treatment x time affected T3 (P = .08) and cortisol (P < .001) and tended to influence (P = .16) T4 concentrations. Plasma T3, T4, and cortisol concentrations were not influenced by SAL treatment. Plasma T3 was higher (P <= .01) after EM or ET treatment compared with pretreatment concentrations. Concentrations of T4 during the 4 h after EM and ET were increased (P < .001) compared with pretreatment. Plasma cortisol concentrations were not altered by EM but were increased (P < .001) by ET. Ergot alkaloids implicated as contributing agents to fescue toxicosis alter plasma concentrations of hormones important to metabolic and thermoregulatory functions in cattle.
IMPLICATIONS Acute exposure to ergot alkaloids altered plasma concentrations of thyroid hormones and cortisol in steers and cows. It should be recognized that cattle that graze on endophytic fescue experience chronic exposure to numerous alkaloids. Nevertheless, these results revealed that ergot alkaloids associated with N. coenophialum influence peripheral concentrations of hormones that modulate metabolic function. Heat stress is detrimental to economic traits important to cattle producers, such as reproductive efficiency. Increased plasma concentrations of thyroid hormones due to ergot alkaloids may contribute to the occurrence of hyperthermia in cattle consuming endophyte-infected tall fescue. Elevated plasma cortisol concentrations after ergotamine treatment may reflect the stressful nature of fescue toxicosis. This article advances the position that modified endocrine function is one mechanism by which consumption of endophyte-infected tall fescue lowers cattle performance.
Key Words: Cattle, Festuca Poisoning, Ergot Alkaloids, Thyroxine, Triiodothyronine, Cortisol
© 1998, by the American Society of Animal Science. All rights reserved.
J. Anim. Sci. 1998. 76:1644-1650 Back to Table of Contents