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 T 3 and T 4 in blood samples collected from the jugular vein. Basal plasma T 3 and T 4 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 T 3 or T 4 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 T 3 and T 4 were observed. Key words: Horses, metabolic rate, thyroid hormones, temperature