The Temperature-Humidity Index (THI) is a standard measure used to evaluate thermal comfort and heat stress in farm animals. In dairy cattle, production losses typically begin above THI 68, and serious health risks appear above 72. This article reviews THI formulas, species-specific thresholds, physiological effects, and practical heat stress management strategies based on current literature.
Economic Impact
Heat stress causes an estimated 1.5 billion USD in annual losses to the U.S. dairy industry. In Türkiye, summer milk production may fall by 10-25% and reproductive performance may decline by 20-30% (St-Pierre et al., 2003).
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Calculate THI1. What Is THI and How Is It Calculated?
THI is an index that combines air temperature and relative humidity into a single numeric value. It reflects the temperature animals actually feel more accurately than a thermometer alone, because high humidity reduces evaporative heat loss (Bohmanova et al., 2007).
THI Formula (NRC, 1971)
Most commonly used formula:
THI = (1.8 × T + 32) - (0.55 - 0.0055 × RH) × (1.8 × T - 26)T: Air temperature (°C) | RH: Relative humidity (%)
1.1 Alternative THI Formulas
Several alternative formulas have been proposed by different researchers:
| Formula | Source | Use |
|---|---|---|
THI = 0.8×T + RH×(T-14.4) + 46.4 | Mader et al. (2006) | Beef cattle |
THI = T + 0.36×Td + 41.2 | Yousef (1985) | General use (Td = dew point) |
THI = (0.35×T + 0.65×Tw) × 1.8 + 32 | Bianca (1962) | Dairy cattle (Tw = wet-bulb temperature) |
2. THI Thresholds and Risk Classification
2.1 THI Classification in Dairy Cattle
| THI Range | Stress Level | Expected Effects | Action |
|---|---|---|---|
| <68 | None (thermoneutral) | Normal production and behavior | Routine management |
| 68-71 | Mild | Feed intake ↓, respiration rate ↑ | Shade, water access |
| 72-79 | Moderate | Milk yield ↓ 10-15%, reproduction ↓ | Start active cooling |
| 80-89 | Severe | Milk yield ↓ 25%+, health risk | Intensive cooling, monitoring |
| ≥90 | Emergency / fatal | Heat stroke, death risk | EMERGENCY intervention |
Important in High-Yield Cows
In cows producing 35+ kg of milk per day, metabolic heat production is higher, so THI thresholds should be considered 5-7 points lower. These cows may begin showing stress signs even at THI 65 (Berman, 2005).
2.2 THI Thresholds in Other Species
- Normal: THI <74
- Alert: THI 74-78
- Danger: THI 79-83
- Emergency: THI ≥84
Beef cattle are slightly more tolerant than dairy cows.
- Normal: THI <72
- Mild stress: THI 72-78
- Moderate stress: THI 79-85
- Severe: THI ≥86
The wool coat makes heat loss more difficult.
- Normal: THI <72
- Stress begins: THI 72-78
- Severe stress: THI ≥79
- Critical: THI ≥85
Pigs cannot sweat and are highly sensitive.
3. Physiological Effects of Heat Stress
3.1 Thermoregulation Mechanisms
Farm animals use several mechanisms to maintain body temperature. When these mechanisms are overwhelmed, heat stress develops (Collier et al., 2008).
Heat Loss Mechanisms
- Evaporation: sweating, respiration (most important)
- Convection: heat transfer by air movement
- Radiation: heat release to the environment
- Conduction: heat transfer by contact
Changes Seen During Heat Stress
- Respiration rate: normal 20-30 → stress 80-120/min
- Rectal temperature: normal 38.5°C → stress >39.5°C
- Feed intake: 10-30% decrease
- Water intake: 50-100% increase
3.2 Effects on Milk Yield
Heat stress affects milk yield directly through metabolic changes and indirectly through reduced feed intake. For each 1-unit increase in THI, milk yield falls by about 0.2-0.5 kg/day (Bouraoui et al., 2002).
| Parameter | Normal Conditions | Heat Stress (THI >72) | Change |
|---|---|---|---|
| Milk yield | 30 kg/day | 24-27 kg/day | ↓ 10-20% |
| Milk fat | 3.8% | 3.4-3.6% | ↓ 5-10% |
| Milk protein | 3.2% | 3.0-3.1% | ↓ 3-6% |
| Somatic cells | 150,000/mL | 200,000-300,000/mL | ↑ 30-100% |
3.3 Effects on Reproduction
Heat stress has a major negative effect on reproduction. Pregnancy rates may drop by 20-30% in summer (De Rensis & Scaramuzzi, 2003).
Reproductive Effects
- Estrus behavior: shorter and less obvious heats
- Oocyte quality: impaired development
- Embryo development: increased early embryonic loss
- Pregnancy rate: falls from 40-50% to 15-25%
- Abortion risk: increased under severe heat stress
4. Heat Stress Management Strategies
4.1 Environmental Modifications
- Tunnel ventilation: 2-3 m/s air speed
- Ceiling fans: 1 fan per 10-15 m²
- Natural ventilation: open side walls
- Target air speed: 1.5-2.5 m/s at cow level
- Sprinklers: 3-5 min wet / 10-15 min dry cycle
- High-pressure mist: effective if humidity <80%
- Evaporative cooling: ideal in dry climates
- Shade: reduces radiant heat by 30-50%
4.2 Nutrition Management
When heat stress reduces feed intake, ration density should be increased and the feeding strategy changed (West, 2003).
| Strategy | Application | Expected Effect |
|---|---|---|
| Increase energy density | Add fat (3-5%), bypass fat | Less metabolic heat production |
| Protein quality | Use bypass protein | Less heat from rumen fermentation |
| Feeding time | Cool hours (60-70% in the evening) | Higher feed intake |
| Buffers | Sodium bicarbonate (0.75-1%) | Stabilizes rumen pH |
| Electrolytes | Supplement K, Na, Mg | Replaces sweat losses |
4.3 Water Management
Water Requirements During Heat Stress
A normal dairy cow drinks 80-120 L/day, but under heat stress this can increase to 150-200 L/day.
- Water trough capacity: 1 trough per 15-20 cows
- Water temperature: 15-20°C is ideal (cool water preferred)
- Trough cleaning: daily inspection, weekly cleaning
- Flow rate: minimum 10-15 L/min
5. Monitoring and Early Warning
5.1 Monitoring Protocol
- Daily THI monitoring: morning, noon, and evening
- Animal observation: respiration rate, behavior, feed intake
- Milk yield tracking: daily production records
- Weather forecast: plan based on 3-5 day forecast
Early Warning Signs
The following signs indicate the onset of heat stress:
- Respiration rate >60/min
- Open-mouth breathing (panting)
- Seeking shade, crowding around water sources
- Sudden drop in feed intake
- Milk yield drop of 10% or more
- Rectal temperature >39.5°C
6. Economic Evaluation
Investments in heat stress management usually pay back. Cooling systems recover their cost quickly by preventing milk losses (Armstrong, 1994).
| Loss Type | Estimated Cost (cow/year) | Explanation |
|---|---|---|
| Milk yield loss | 500-1500 TL | 10-20% production drop over 90 days |
| Reproductive loss | 300-800 TL | Extended service period |
| Health cost | 200-500 TL | Mastitis, metabolic diseases |
| Culling | Variable | Early reform |
| TOTAL | 1000-2800 TL/cow/year | Varies by region and farm |
7. Sources
- Armstrong, D. V. (1994). Heat stress interaction with shade and cooling. Journal of Dairy Science, 77(7), 2044-2050.
- Berman, A. (2005). Estimates of heat stress relief needs for Holstein dairy cows. Journal of Animal Science, 83(6), 1377-1384.
- Bohmanova, J., et al. (2007). Temperature-humidity indices as indicators of milk production losses due to heat stress. Journal of Dairy Science, 90(4), 1947-1956.
- Bouraoui, R., et al. (2002). The relationship of temperature-humidity index with milk production of dairy cows in a Mediterranean climate. Animal Research, 51(6), 479-491.
- Collier, R. J., et al. (2008). A review of endocrine regulation of metabolism during lactation. Journal of Animal Science, 86(13_suppl), E70-E81.
- De Rensis, F., & Scaramuzzi, R. J. (2003). Heat stress and seasonal effects on reproduction in the dairy cow. Theriogenology, 60(6), 1139-1151.
- St-Pierre, N. R., et al. (2003). Economic losses from heat stress by US livestock industries. Journal of Dairy Science, 86, E52-E77.
- West, J. W. (2003). Effects of heat-stress on production in dairy cattle. Journal of Dairy Science, 86(6), 2131-2144.