Cow comfort: 13) Ventilation

Ventilation of any dairy housing structure, whether it is a newborn calf shelter or a lactating cow shelter, is of paramount importance. Emphasis is placed on fresh air. Dairy cows need a constant source of fresh, clean air to achieve their production potential. High moisture levels, manure gases, pathogens and dust concentrations present in unventilated or poorly ventilated structures create an adverse environment for animals. Stale air also adversely affects milk production and milk quality.

Barn planning

The dairy’s ventilation system should prevent high humidity in winter and heat build-up in summer. Free-stall fronts and partitions should be open enough to allow air movement across the cow. Look for excessive condensation and moisture damage, especially on the roof. Cobwebs are often a sign of inadequate air flow. Other signs of poor ventilation include air that smells of ammonia, excessive coughing, nasal discharge or open-mouthed breathing by the cows. If you run your fingers through the cow’s hair coat, it should be free of moisture in a properly ventilated building.

Proper ventilation consists of exchanging barn air with fresh outside air uniformly throughout the structure. The required rate of air exchange depends on a number of variables, including the conditions of the outside air (temperature and moisture level), animal population and density. A properly designed and managed system results in shelter air that is nearly equal in quality to the outside air on a year round basis. The shelter air’s concentrations of manure gases, dust and pathogens should be low and the relative humidity should be about the same level as that of the outside air. Fans hanging over stalls or alleys do not provide air exchange and are not a substitute for a well designed and managed ventilation system.

Heat stress

The effects of heat stress on dairy cattle physiology and productivity have been well established. First signs of heat stress can already be seen at 20ºC, with cows sweating and breathing fast. Milk yield can decrease by about 10 percent. Studies (Appendix 7) have shown that heat stress during late gestation reduces calf birth weight and subsequent milk production. Dry cows provided with shade gave birth to heavier calves and produced more milk than cows not provided with shade. Biological response to other forms of stress such as crowding, poor ventilation, poor footing and poor stall design have not been well established for dairy cows.


How do you control the air exchange to limit the temperature to within five degrees of the air outside? This depends on the type of ventilation system you have. With natural ventilation, make sure the eaves are open enough. In very cold weather, the eaves should be open about five centimetres. As the outside air temperature rises, open them more. If the wind is blowing, you can close the eaves more than on calm days. You also should make sure the ridge is opened adequately. For a cold, naturally ventilated barn, the ridge should be opened five centimetres for each three metres of building width and the eave openings sized for half that on each side. For a barn 30 metres wide, the ridge should be open 50 centimetres and the eaves should be open 25 centimetres on each side. Natural ventilation works best when the roof slope rises 10 centimetres on a 30 centimetre run. With a mechanical ventilation system, a fan should run continuously to maintain air quality. Thermostats should control the other fans. Set the thermostats so they maintain the air temperature as low as possible, while still keeping animals comfortable. Adjust the air inlets in proportion to the rate of ventilation provided by the fans. The inlet system should direct air away from the animals to avoid drafts in winter.

Stages of cow heat stress in relation to temperature and relative humidity.


Source (adapted from): Dunham, D. et al, Coping with summer weather.