Chapter 9 - Introduction

Heat stress in poultry is a serious problem for the industry. Mortality during extremely hot weather can be significant, especially when combined with high humidity. However, probably even more costly is the routine loss of weight and feed conversion efficiency during less severe periods of heat stress. Under normal conditions, chickens do a good job of cooling themselves with physiological and behavioral mechanisms but these mechanisms fail at high temperatures. One of the keys to minimizing production losses during hot weather is proper ventilation system design.

Air temperatures that cause heat stress and mortality are considerably below broiler body temperature. Broiler surface temperatures typically range from 95 – 100°F, with skin temperatures warmer than feathers. Air temperatures in this range can virtually stop heat loss from the broiler and accelerate heat prostration. For this reason, an important goal for hot weather ventilation systems is to keep air temperatures below 95°F.

During most of the year when cold or warm conditions prevail, a conventional natural or mechanical ventilation system is employed to ventilate the poultry house. For hot weather a specialized ventilation system is often required in order to achieve high air velocities and evaporative cooling. Hot weather systems are designed to assist broilers in dissipating heat during extreme hot weather.

Hot weather ventilation systems should be designed to:

1. Keep heat buildup within the house to a minimum through proper air exchange.
2. Provide air movement over the broilers to increase convective cooling or windchill effect.
3. Keep house temperature below 95°F through the use of evaporative cooling, if necessary.

Without evaporative cooling the air inside the broiler house will always be warmer than the air outside. The temperature difference between the inside and outside of the broiler house is determined by the solar and broiler heat added and the rate at which air in the house is exchanged by the ventilation system. The greater the amount of heat added to the air, the higher the temperature of the air within the house. The faster the air in the house is exchanged, the closer the house temperature will be to the outside temperature. Evaporative cooling is used to maintain temperatures within the comfort zonewhen outside temperatures approach or exceed the upper limit of the comfort zone (see Chapter 7 for more information on the comfort zone of chickens). Evaporative cooling in mechanically-ventilated houses is provided by, misting, or cooling pads while in naturally-ventilated houses it can only be provided by fogging or misting.

To design a hot weather ventilation system, calculation of the heat gain through the building and heat production by the broilers is needed. Heat flows into the house through the walls and roof and, to a lesser extent, through the floor, due to conduction and to heating by the sun. Heat also enters the house through the ventilation air and is added from the broilers themselves. Heat flows out of the house through the exiting ventilation air.

There is potential for cool night air to increase economic returns. By setting thermostats 10°F below the design rate, fans remain on longer into the cooler night. This strategy was shown to improve production efficiency during hot weather. The strategy did not provide any savings for late summer to early fall flocks because the increased electricity use by fans was about equal to the increased returns due to animal performance. Also, excessive windchill from cooler, high-speed air can result in additional feed energy consumption. It is extremely important that a strategy of integrated daily temperature be carefully planned and controlled so as to provide beneficial cooling without chilling the birds.