METABOLIC RATE AND PLASMA THYROID HORMONE CONCENTRATIONS OF MATURE HORSES IN RESPONSE TO CHANGES IN AMBIENT TEMPERATURE

Abstract

Metabolic rate was assessed at controlled environmental temperatures in six winter-acclimatized mature Quarter Horse geldings. Within an environmental chamber maintained at 18 °C, metabolic rate was similar (P > 0.05) at the beginning (November) and end (mid-January) of the trial. During periods of acute cold exposure (6 h), metabolic rate was found to be elevated (P < 0.05) at temperatures below −10 °C, increasing to 142% of basal metabolic rate values at −40 °C. At temperatures above 10 °C, metabolic rate tended to be elevated (P < 0.07) over basal levels recorded at 0 °C. Thyroid hormone responses to temperature were monitored by evaluating total plasma T3 and T4 in blood samples collected from the jugular vein. Basal plasma T3 and T4 concentrations were both observed to rise (P < 0.05) during the experiment while the horses were exposed to outdoor ambient temperature. Short-term cold exposure within the environmental chamber had no effect on either plasma T3 or T4 concentration (P > 0.05). These results indicate that the thermoneutral zone of the mature horse housed outdoors during winter ranges from approximately −15 °C to 10 °C. During this experiment, outdoor housing did not result in winter acclimatization, as reflected by the constancy of metabolic rate estimates, although changes in basal concentrations of T3 and T4 were observed. Key words: Horses, metabolic rate, thyroid hormones, temperature