Heat Stress and Shed Design in a Broiler Enterprise

Increasing stocking density is not appropriate under the current conditions. The heat event already demonstrated that the sheds cannot maintain a safe environment at 32 kg/m²: temperatures reached 38°C and mortality hit 4%, which is well above the typical acceptable level of under 1% for a flock at 35 days. Birds at higher density generate more metabolic heat per unit floor area and have less ability to move away from hot zones or to access drinkers and feeders, so mortality and heat stress would worsen. The evaporative cooling pads are three seasons old and the bore water is hard, so scale and mineral fouling are likely reducing pad efficiency and airflow. Before any density increase, the grower should service or replace the pads, descale the water system, and verify that tunnel air speed meets the target of about 2.5-3 m/s. Only after the sheds can hold temperatures below about 30°C at the current density should a staged increase be considered, and even then, a summer flock should be run at a lower density than a winter flock. The second feed line is a reasonable improvement for feed access, but it does not offset the heat load problem.

The first priority is to restore evaporative cooling capacity. Replace or thoroughly descale the pads and fit a water treatment or filtration system for the hard bore water, because scaled pads cannot evaporate water effectively and the resulting air is hot and humid rather than cooled. The second priority is to maximise air speed at bird level. Tunnel ventilation relies on air movement over the birds to increase convective heat loss; panting alone is not enough at 38°C and 60% humidity, because high humidity limits evaporative loss from the respiratory tract. Check fan belts, shutter openings and any obstructions, and aim for at least 2.5-3 m/s. The third priority is to adjust management: reduce stocking density in summer, move feeding to the coolest parts of the day so the heat of digestion does not coincide with peak temperature, and ensure cool drinking water is available close to the birds. The 18% drop in feed intake is a normal response to heat, so feed formulation may need higher nutrient density to compensate. Adding a second feed line helps access but is lower priority than fixing the environment.

The 35-day-old flock has a much higher metabolic heat production per bird because body mass is greater and feed intake is near its peak. Heat production scales with body mass, so a 2.4 kg bird produces far more heat than a younger, lighter bird, and its surface area relative to mass is lower, which reduces its ability to lose heat. The older flock also occupies more of the floor area at the target density, so the total heat load in the shed is higher and air movement at bird level is reduced by the mass of birds. Younger birds have a higher surface area to mass ratio and lower feed intake, so they cope better with the same air temperature. This implies that the enterprise should manage stocking density and cooling by flock age, not by a single shed-wide target. The grower should plan thinning or partial depletion of older flocks before forecast heatwaves, run lower summer densities for flocks approaching processing weight, and monitor shed temperature and bird behaviour daily as flocks age. It also reinforces that the density increase proposal is riskiest for the oldest flocks.