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Bovine Prolactin, TSH, T₄ and T₃ Concentrations as Affected by Tall Fescue Summer Toxicosis and Temperature¹

University of Kentucky, Lexington 40546

Abstract

Serum concentrations of prolactin (PRL), thyrotropin (TSH), thyroxine (T₄) and triiodothyronine (T₃) were compared between Holstein calves fed a toxic fescue (G1-307) and Holstein calves fed a less toxic fescue (Gl-306) at one of three environmental temperatures (low, 10 to 13 C; medium, 21 to 23 C; high, 34 to 35 C). Three calves were assigned to each treatment combination, and hormone concentrations were determined before (basal) and after thyrotropin-releasing hormone (TRH) was injected at 15 µg/100 kg body weight via jugular cannula. Calves fed the toxic Gl-307 had lower basal PRL concentrations than those fed Gl-306 (1.8 ± .1 vs 6.0 ± 1.2 ng/ml). Increasing ambient temperature increased basal PRL levels in calves fed Gl-306 (i.e., low [2.3 ± .3] < medium [3.6 ± .9] < high [12.1 ± 1.3 ng/ml]) but not in calves fed Gl-307 (i.e. low [1.8 ± .1] = medium [2.0 ± .1] = high [1.6 ± .1]). Similarly, after TRH injection, PRL release was greater in calves fed the less toxic Gl-306 (35.7 ± 5.2 ng/ml) than in those fed Gl-307 (5.6 ± .6 ng/ml) and was positively affected by temperature (i.e., low [6.9 ± 1.2] < medium [18.8 ± 5.6] < high [36.3 ± 5.9 ng/ml]). In contrast, basal and TRH-induced TSH concentrations were unaffected by temperature or fescue. However, T₃ (time 0) was inversely related to temperature (1.06 ± .1, 1.40 ± .1 and 1.87 ± .2 ng/ml for high, medium and low temperature, respectively). After TRH injections, T₃ increased linearly and at the same rate (.007 ng/ml/min) at all temperatures, but T₄ concentrations increased only in low temperature groups. Neither T₄ nor T₃ concentrations were affected by forage fed. We conclude that some factor(s) in toxic fescue inhibits PRL secretion, expecially at high temperature, an environmental condition closely associated with fescue summer toxicosis in cattle. Increased body temperature and respiration rates associated with fescue summer toxicosis apparently are not mediated by TSH or thyroid hormone secretion.

¹ This manuscript (79-5-191) is published with the approval of the Director of the Kentucky Agr. Exp. Sta. The study was a joint effort carried out with the Michigan Agr. Exp. Sta. The technical assistance of Pat Harkins (hormone analysis) and Dr. Roger Neitzel (computer programming) at the Michigan Station is gratefully acknowledged.

² Dept. of Anim. Sci., Univ. of Kentucky.

³ Dept. of Dairy Sci., Michigan State Univ.

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