Heat Stress

Heat stress is a mismatch between the heat an organism gains, or generates, and its ability to lose that heat to the environment. In plants, this shows up as physiological disruption — for example, reduced photosynthetic efficiency or damage to pollen — rather than the active cooling response seen in animals, which instead rely on behaviours and physiological mechanisms such as increased respiration rate, panting, or sweating to shed excess heat.

High temperatures during flowering can reduce pollen viability and successful fertilisation in many crops, directly limiting yield potential. Sustained heat can also depress photosynthetic efficiency and accelerate development, shortening the grain- or fruit-fill period, and can affect fruit and grain quality through effects such as sunscald or altered composition.

In livestock, heat stress commonly reduces feed intake and growth, lowers milk yield in dairy cattle and egg production in poultry, and can reduce fertility. In severe or prolonged heat events, particularly where animals cannot access shade, water, or effective cooling, heat stress can increase mortality risk.

• For crops — heat-tolerant cultivars, adjusted planting dates to shift sensitive stages away from typical peak-heat periods, shading or reflective mulches, and evaporative cooling via irrigation in some systems
• For livestock — shade structures, increased ventilation or fans, ready access to drinking water, and adjustments to feeding times and stocking density during hot periods

Heat-stress risk is commonly tracked using indices that combine temperature and humidity, such as temperature–humidity indices used in livestock management, and, for crops, by monitoring the coincidence of forecast heat events with sensitive growth stages. Specific index thresholds are typically defined per species, breed, or crop by research institutions rather than applied universally.