Topic:Thermoneutral Zone
The thermoneutral zone (TNZ) in horses refers to the range of ambient temperatures within which a horse can maintain its body temperature without needing to expend additional energy for either heat production or heat dissipation. Within this zone, horses can regulate their body temperature through normal physiological processes without experiencing thermal stress. The lower critical temperature marks the point below which a horse must increase metabolic heat production to stay warm, while the upper critical temperature indicates the threshold above which a horse must employ mechanisms like sweating to cool down. Understanding the thermoneutral zone is important for managing horses in varying environmental conditions. This page compiles peer-reviewed research studies and scholarly articles that explore the thermoneutral zone's parameters, its implications for equine welfare, and strategies for managing horses outside of this temperature range.
Cardiovascular responses to heat and exercise in the horse. Heat-induced increases in skin blood flow (BF) are well known to be reduced if exercise is superimposed.' However, whether exercise-induced increases in muscle BF are compromised by superimposed heat stress remains a controversial issuc. The horse has now been studied because of its human-like thermoregulatory mechanisms and good exercise capacity.
Thermoregulation in sick foals aged less than one week. Metabolic rate, rectal temperature Tr and respiratory quotient (RQ) were determined in 16 sick foals, aged 0-182 h. The foals were categorized into three groups: premature, dysmature or those suffering from neonatal maladjustment syndrome. The mean metabolic rate of the premature foals was 71 watts per unit area of body surface (W m(-2)), significantly lower than that of the other two groups. The overall mean metabolic rate for the sick foals was 82 W m(-2), about 25% below that of healthy foals of similar age. Air temperature (Ta) was 9.5-26.3 degrees C, and several foals shivered despite the...
Integrative model for predicting thermal balance in exercising horses. A theoretical integrative model was developed to determine the heat balance of horses working in a given environment. This model included the following parameters: metabolic heat gain, solar heat gain, evaporative heat loss due to sweating, respiratory tract heat loss, radiation from the body and heat gain or loss due to convection and conduction. The model developed in this study includes an unique approach for estimating heat loss via evaporation of sweat from the animal's skin surface. Previous studies modelling evaporative heat dissipation were based on the volume of sweat loss. While it i...
Redistribution of cardiac output in response to heat exposure in the pony. Radioactive microspheres were used to measure cardiac output and blood flow to most major tissues in 4 ponies at rest in thermoneutral (16 degrees C/60% RH) and mildly hot (41 degrees C/34% RH) environments. In response to heat stress there were increases in cardiac output (2-fold), respiratory frequency (5-fold), blood flow to the skin of the body (3-fold), and limbs (50%), respiratory muscles (2-fold) and the upper respiratory tract (3-fold). Ponies were able to maintain body temperature in the hot environment by increasing blood flow to the tissues involved in heat dissipation, while blood ...
Thermal and cardiorespiratory responses of horses to submaximal exercise under hot and humid conditions. The objective of this study is to determine the effects of heat, and heat and high relative humidity (RH) on the thermal and cardiorespiratory responses to exercise and recovery. Five Thoroughbred horses (age 3 to 6) completed exercise tests under each of 3 environmental conditions: cool, dry (CD, room temperature (T) = 20 degrees C, RH = 45-55%), hot, dry (HD, T = 32-34 degrees C, RH = 45-55%) and hot, humid (HH, T = 32-34 degrees C, RH = 80-85%). Horses were exercised at a workload equal to 50% of VO2max on a treadmill set at a 10% slope until attainment of a pulmonary artery blood (PA) temp...
Thermoregulation in the horse in response to exercise. Conversion of stored energy into mechanical energy during exercise is relatively inefficient with approximately 80% of the energy being given off as heat. Relative to many species the horse suffers an apparent disadvantage by possessing a high metabolic capacity yet a small surface area for dissipation of heat, particularly as evaporation of sweat is the major method of heat dissipation. Under most conditions of exercise at least two-thirds of the metabolic heat load is dissipated via this means with sweat losses of more than 10 l h-1 reported. The remaining exercise-induced heat load must be ...
Microwave thermography: a non-invasive technique for investigation of injury of the superficial digital flexor tendon in the horse. Microwave thermographs were recorded from 77 normal horses. In 51% the lowest temperature was recorded in the mid-metacarpal region, and in 41% it was in the distal metacarpal region. The mean temperature of the normal limbs ranged from 25.04 to 37.4 degrees C. Maximum temperature differences between symmetrical points in both forelimbs ranged from 0 to 5.33 degrees C and differences in mean limb temperatures between both forelimbs ranged from 0 to 2.91 degrees C. In 48 horses with acute (less than 4 weeks' duration) injury of the superficial digital flexor tendon (SDFT) (36 unilateral, 12 bil...
Respiratory muscle perfusion in ponies during prolonged submaximal exercise in thermoneutral environment. Distribution of blood flow among various respiratory muscles was examined in 8 healthy ponies during submaximal exercise lasting 30 minutes, using radionuclide labeled 15-microns diameter microspheres injected into the left ventricle. From the resting values (40 +/- 2 beats/min; 37.3 +/- 0.2 C), heart rate and pulmonary arterial blood temperature increased significantly at 5 (152 +/- 8 beats/min; 38.6 +/- 0.2 C), 15 (169 +/- 6 beats/min; 39.8 +/- 0.2 C), and 26 (186 +/- 8 beats/min; 40.8 +/- 0.2 C) minutes of exertion, and the ponies sweated profusely. Mean aortic pressure also increased progr...
Hoof and distal limb surface temperature in the normal pony under constant and changing ambient temperatures. Forelimb surface temperatures were continuously monitored in four clinically normal ponies exposed to: (i) constant ambient temperature; (ii) a biphasic change in ambient temperature; and (iii) an incremental increase in ambient temperature. Limb surface temperatures were recorded at the hoof, metacarpus and forearm, and rectal temperature was also measured. Under constant ambient temperature, limb surface temperatures remained relatively constant. A pyrexic episode occurred in one pony under constant ambient temperature conditions and was characterised by an onset phase in which rectal temper...
Comparison of the binding of Ca2+ and Mn2+ to bovine alpha-lactalbumin and equine lysozyme. The enthalpy change of the binding of Ca2+ and Mn2+ to equine lysozyme was measured at 25 degrees C and pH 7.5 by batch microcalorimetry: delta H degrees Ca2+ = -76 +/- 5 kJ mol-1, delta H degrees Mn2+ = -21 +/- 10 kJ mol-1. Binding constants, log KCa2+ = 6.5 +/- 0.2 and log KMn2+ = 4.1 +/- 0.5, were calculated from the calorimetric data. Therefore, delta S degrees Ca2+ = -131 +/- 20 JK-1 mol-1 and delta S degrees Mn2+ = 8 +/- 44 JK-1 mol-1. Removal of Ca2+ induces small but significant changes in the circular dichroism spectrum, indicating the existence of a partially unfolded apo-conformatio...
Effect of ambient temperature upon the surface temperature of the equine limb. Ten clinically healthy adult horses were examined with the portable infrared thermometer at ambient temperatures of 5, 15, and 25 C to evaluate the thermal response of limbs of the horse to variations of ambient temperature. Limb surface temperature varied in direct proportion to changes in the ambient temperature, with considerable variation occurring among individual horses, especially at the lower temperatures. Areas of proximal parts of the limbs were more resistant to temperature variation than were distal parts. Ambient temperature had a statistically significant, but clinically unimport...
Limb skin thermometry in racehorses. Skin temperature measurements were recorded of the legs of 13 racehorses between 2 and 5 years old. The normal limb temperature patterns are described. In over 90 per cent of the normal horses the difference between contralateral limbs was less than 1 degree C, but in the remainder differences of up to 8 degree C were recorded due to the excessive cooling of one limb. One horse had a constant temperature elevation of one foreleg, but it remained in training for 7 weeks without any obvious ill effects. Because of the small numbers of horses in the survey and their freedom from tendon injuries i...
Cytochrome c: a thermodynamic study of the relationship among oxidation state, ion-binding and structural parameters. Cation binding to horse-heart ferrocytochrome c. The specific binding of cations to horse heart ferrocytochrome c has been studied, using the gel
filtration method. The cations investigated were: Mg2+, Co2+, cinchonine and proflavine. The stability
constants are in the range of 5-8 mM-1, and the number of binding sites per protein molecule are
1 to 2. The temperature dependence of the stability constant for the Mg2+-ferrocytochrome system
was measured. The thermodynamic parameters were found to be: dH&s = 4-12 kcal/mol, dG;,,
(25 "C) = -5.6 kcal/mol and AS&, = +57 calxmol-lx K-I.