INFLUENCE OF TRIIODOTHYRONINE INJECTIONS ON CALF IMMUNE-RESPONSE TO AN INFECTIOUS BOVINE-RHINOTRACHEITIS VIRUS CHALLENGE AND NITROGEN-BALANCE OF LAMBS

Three experiments were conducted to determine the influence of triiodo thyronine (T-3) or propylthiouracil (PTU) on the humoral immune respon se of calves challenged with infectious bovine rhinotracheitis virus ( IBRV) and on nitrogen depletion and repletion of lambs deprived of fee d and water for 3 d. In Exp. 1, 18 steer calves (BW 284 +/- 6 kg) chal lenged with IBRV were limit-fed (1.5% BW) a 60% concentrate diet and i njected (s.c.) daily with alkaline saline, .4 mg of T-3, or .8 mg of T -3 Injections of T-3 did not affect serum antibody titers to IBRV, blo od leukocyte counts, or plasma free fatty acid, ceruloplasmin, and cho lesterol concentrations but increased (P < .05) plasma glucose concent rations and decreased (P < .05) plasma urea N concentrations. In Exp. 2, 36 IBRV-challenged steers (BW 266 +/- .8 kg) were given ad libitum access to a 60% concentrate diet and injected (s.c.) daily with alkali ne saline, .2 mg of T-3, or .4 mg of T-3. In contrast to Exp. 1, injec tions of T-3 did not affect plasma glucose or urea N concentrations an d reduced (P < .05) serum antibody titers to IBRV. In Exp. 3, eight we ther lambs were limit-fed (600 g/d) a 36% concentrate pelleted diet an d assigned to one of four treatments in a replicated 4 x 4 Latin squar e designed nutrient balance experiment involving periods of nutrient d epletion and repletion. Treatments were as follows: 1) alkaline saline injection (s.c.), 2) 4 mg of PTU/kg BW in water, 3) .15 mg of T-3 s.c . daily for 15 d, and 4) .15 mg of T-3 s.c. daily for 7 d after the 3- d feed and water deprivation period. Thyroid status affected (P < .05) predeprivation N balance but did not affect N losses during the feed and water deprivation period. Retention of N during realimentation was affected (P < .05) by T-3 treatment. Results of these experiments ind icate that there is a complex interrelationship among stress, nutrient status, and thyroidal status and that the effects of T-3 injections o n immune and metabolic responses may he dependent on the nutritional s tatus of the animal.