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

University of Alberta³, Edmonton, T6G 2E3, Canada

Abstract

The effects of ambient temperature (22 to 55 C vs 2 to 5 C) and injection of triiodothyronine (T₃; 0, .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 reticulo-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 ¹⁰³Ru-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 T₃/day.

In thyroidectomized sheep maintained at 22 to 25 C, apparent dry matter 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 digesta but is not the only mediator influencing the reduction in mean retention time and apparent digestibility in the whole tract of cold-exposed sheep.

¹ Financial assistance provided by the National Research Council of Canada is gratefully acknowledged.

² Research supported by the National Research Council of Canada.

³ Department of Animal Science.

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