Heat stress in crops and livestock
How heat harms crops and animals: critical temperatures, terminal heat in wheat, rice sterility, THI for dairy cattle, poultry losses and how to adapt.
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In March 2022, northern India had its hottest March on record. Temperatures rose 5–8 °C above normal just as wheat was filling its grain. Grains came out shrivelled, yields fell across Punjab, Haryana and Uttar Pradesh, and the government cut its production estimate and restricted wheat exports. In summers since, heatwaves have killed poultry, cut milk yields and scorched vegetable crops across South Asia.
Heat stress occurs when temperatures rise above the range in which plants or animals can function normally, long enough or at a sensitive enough stage to cause damage. With global warming, heatwaves are becoming more frequent, longer and more intense. A large analysis found that, without adaptation, each 1 °C rise in global temperature reduces average yields of wheat by about 6%, rice by 3%, maize by 7% and soybean by 3%.
Heat stress in crops
How heat harms plants
- Photosynthesis slows. The enzyme Rubisco activase is heat-sensitive, and thylakoid membranes are damaged.
- Respiration rises, burning more of the sugars produced.
- Reproduction is the most sensitive process. Pollen becomes sterile, the stigma dries out and pollen tubes fail to grow, so grains do not set.
- Grain filling is shortened, so grains are lighter.
- Heat usually comes with drought and high vapour pressure deficit, which together are more damaging than either alone.
Critical temperatures
| Crop | Sensitive stage | Approximate damaging temperature |
|---|---|---|
| Wheat | Anthesis and grain filling ("terminal heat") | Above about 30–32 °C during grain filling; above about 35 °C at anthesis |
| Rice | Anthesis (flowering, mid-morning) | Above about 35 °C, causing spikelet sterility |
| Maize | Tasselling and silking | Above about 35 °C; pollen viability falls |
| Chickpea and pulses | Flowering and pod set | Above about 35 °C |
| Tomato | Flowering | Day temperatures above about 32 °C, or warm nights above about 22 °C, reduce fruit set |
Night temperatures matter too. Warm nights increase respiration and reduce rice and wheat yields, even when days are not extreme.
Adapting crops
- Timely or early sowing, so that sensitive stages avoid the hottest period. Sowing wheat in the first half of November in north-west India is a key heat-avoidance measure, and zero-till wheat after rice makes earlier sowing possible.
- Heat-tolerant and early-maturing varieties, bred by ICAR and CIMMYT for terminal heat, with traits such as stay-green, canopy cooling and high grain-filling rate.
- Light irrigation during heatwaves at flowering and grain filling to cool the canopy.
- Mulching and residue retention, which lower soil temperature and conserve moisture.
- Foliar sprays recommended in some advisories, such as potassium nitrate or salicylic acid. Their benefits vary.
- Shade nets and protected cultivation for high-value vegetables.
Heat stress in livestock
Animals produce heat from metabolism, especially high-producing dairy cows. They lose it by radiation, convection, and evaporation through sweating and panting. When air temperature and humidity are high, they cannot shed heat fast enough, and body temperature rises.
The temperature–humidity index (THI)
The THI combines air temperature and humidity into a single heat stress indicator:
For dairy cattle, THI below about 72 means no stress, 72–79 mild stress, 80–89 moderate stress, and 90 or more severe stress. High-yielding cows begin to lose milk at a THI of about 68–72.
A humid afternoon in a dairy shed
At 35 °C and 60% RH: 1.8 × 35 + 32 = 95; 0.55 − 0.0055 × 60 = 0.22; 1.8 × 35 − 26 = 37. THI = 95 − 0.22 × 37 = 86.9, which is moderate stress approaching severe. A crossbred cow giving 15 litres a day might lose 10–20% of its milk, eat less, show reduced heat signs and have poorer conception. Fans with intermittent sprinklers, shade and cool drinking water at that time can bring body temperature down and recover much of the loss.
Effects on animals
- Lower feed intake and milk yield, and changes in milk fat and protein.
- Reproductive failure: weak heat signs, poor conception, and early embryo loss. Heat-stressed bulls produce poorer semen.
- Health: higher mastitis, lameness and metabolic problems.
- Poultry: broilers and laying hens are highly sensitive. Panting, reduced egg production and weight gain, thin eggshells and mass mortality can occur above about 35 °C, especially in crowded sheds.
- Pigs, which have few functional sweat glands, also suffer badly.
Indigenous zebu (Bos indicus) cattle, such as Sahiwal, Gir, Tharparkar and Red Sindhi, are more heat-tolerant than Bos taurus breeds and their crosses. They have more sweat glands, a lower metabolic rate, loose skin and short, glossy coats. This tolerance is one reason for renewed interest in indigenous breeds and in crossbreds with a higher indigenous share.
Adapting livestock
- Shade: trees, sheds with high roofs, insulated or white-painted roofs, and orientation along the east–west axis.
- Cooling: fans, misters, sprinklers with fans, wallowing for buffaloes, and evaporative cooling pads in poultry houses.
- Water: plenty of cool, clean drinking water. Needs can double in heat.
- Feeding: feed in cooler hours; adjust diets with more energy density and buffers, and supplement electrolytes, vitamin C and E, and minerals.
- Management: lower stocking density in poultry sheds; avoid handling, transport and vaccination during the hottest hours.
- Breeding for heat tolerance, using indigenous genetics and genomic tools such as the "slick" hair gene in cattle.
Early warning
IMD issues heatwave warnings and agromet advisories with specific crop and livestock actions. A heatwave in the plains is declared when the maximum temperature is at least 40 °C and departs from normal by 4.5 °C or more, or reaches 45 °C or more. Many states have heat action plans. Integrating them with farm advisories, through SMS, apps and community radio, helps farmers act in time.
Key points
- Without adaptation, each 1 °C of warming cuts average yields of wheat by about 6%, rice 3%, maize 7% and soybean 3%.
- Reproductive stages are the most sensitive. Rice and maize suffer above about 35 °C at flowering; wheat suffers terminal heat above about 30–32 °C during grain filling. Warm nights also reduce yield.
- Crop adaptation uses timely sowing, heat-tolerant and early varieties, irrigation at critical stages, mulching and protected cultivation.
- THI = (1.8T + 32) − (0.55 − 0.0055RH)(1.8T − 26). For cattle, 72 or more is stressful and 90 or more severe.
- Heat cuts milk, fertility and growth, and kills poultry. Zebu breeds are more tolerant.
- Livestock adaptation uses shade, fans and sprinklers, water, feeding changes, management and breeding.
What the exams ask
- Explain the physiological effects of heat stress on crops.
- Give critical temperatures and sensitive stages for wheat and rice.
- Calculate THI and interpret it.
- Describe management measures for heat stress in dairy cattle and poultry.
- Explain why indigenous cattle are more heat-tolerant.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does humidity make heat worse for animals?
Animals cool themselves largely by evaporating water from skin and airways. In humid air, evaporation slows, so less heat is lost. A cow can cope with 35 °C in dry air better than with 32 °C in very humid air, which is why THI includes humidity.
Can irrigation protect wheat from a heatwave?
Partly. A light irrigation cools the canopy by a few degrees through transpiration and keeps the plants functioning, which reduces damage. It cannot fully offset extreme heat, and over-irrigation at grain filling can cause lodging. Early sowing and tolerant varieties remain the main defence.
Are buffaloes more heat-tolerant than cattle?
No. Buffaloes have dark skin, few sweat glands and a thick epidermis, so they are actually more sensitive to direct sun. They cope by wallowing in water or mud. Without wallows or sprinklers, buffaloes suffer more in summer heat than zebu cattle.
Sources
- Zhao, C. et al. "Temperature increase reduces global yields of major crops in four independent estimates." PNAS 114, 9326–9331 (2017).
- Hatfield, J. L. and Prueger, J. H. "Temperature extremes: effect on plant growth and development." Weather and Climate Extremes 10, 4–10 (2015).
- Polsky, L. and von Keyserlingk, M. A. G. "Invited review: effects of heat stress on dairy cattle welfare." Journal of Dairy Science 100, 8645–8657 (2017).
- National Research Council. A Guide to Environmental Research on Animals. National Academy of Sciences, Washington DC, 1971.
- India Meteorological Department. Heat Wave: Criteria and Warnings. IMD, New Delhi.
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