Studies on the Relationship between Thyroid Function, Cold Acclimation and Retention Time of Digesta in Sheep 1 ,2

The effects of ambient temperature (22 to 55 C vs 2 to 5 C) and injection of t r i iodothyronine (T3; O, .125 or .25 mg T3/day) on apparent digestibility, oxygen consumption, concentration of thyroid hormones in plasma, and mean retention times of digesta in the reticu!o-rumen, hind gut and total tract of sheep given brome grass (Bromus inermis) pellets were studied in two experiments in order to determine whether T3 was a mediator causing the decreased retention time observed in cold-exposed sheep. In thyroid-intact sheep, both exposure to cold and injection of T3 resulted in decreased (P<.05) apparent dry matter digestibility, and increased (P<.05) plasma T3 concentration. Mean retention time in the rumen and in the whole tract, measured by reference to the fecal excretion of a dose of lO3Ru_phenanthroline administered into the rumen and/or abomasum, was significantly reduced (P<.05) by exposure of sheep to 2 to 5 C and by injection of .25 mg T 3/day. In thyroidectomized sheep maintained at 22 to 25 C, apparent dry mat ter digestibility and mean retention time in the whole tract were decreased (P<.05), and oxygen consumption and T3 plasma concentration were increased (P<.05) by injection of T3. When thyroidectomized sheep receiving T3 were also exposed to 2 to 5 C, there was a tendency for an additional reduction in the mean retention time of digesta in the rediculo-rumen. The data indicate that T3 is involved in the mechanism for control of retention time of