Are your cows cool? Are your cooling systems working properly? Designs for dairy cow cooling systems are much improved than in the past, but many cows still become hot. One reason is that cooling systems often do not perform as well in practice as expected. Most commonly this is just because the systems are not being operated or maintained correctly.   
Ventilation systems accomplish multiple things, removing moisture and noxious gasses, and providing airflow to cool cows. Different types of ventilation systems use different methods to accomplish these goals. Negative pressure systems such as cross-ventilated or tunnel-ventilated barns create airflow by exhausting air from the barn to create a vacuum inside. Understanding this concept is critical to operating cooling systems in negative pressure barns. There should always be noticeable airflow almost everywhere inside at the cow level.
Summer airflow should be higher to provide cooling. In winter, airflow is only necessary to the degree needed to exhaust noxious gasses and moisture. If airflow is insufficient in summer, the problem may be either insufficient exhaust, insufficient inlet size, too much inlet size or incorrect location of inlets and/or exhaust. For negative pressure systems to work as designed, it is critical that no other openings are available for air to enter other than the designed inlets; otherwise, air will enter at the point of least resistance, and the barn may lose the negative pressure. In these cases, there may be little or no airflow. It is particularly important not to let air into the barn near the exhaust fans, because air will simply enter through the opening and then exhaust through the neighboring fans.
Gasses will not be removed from the barn, and airflow will be reduced, perhaps even to zero if the inlet is large enough. This means that every time someone enters the barn, they must shut the door, especially if they are opening a large garage door. This also means that if an exhaust fan is not working and it is removed so there is now a large hole in the wall, the hole needs to be closed as soon as possible. Baffles are also important in cross-ventilated barns to direct the air flow down to the cows’ level. However, baffles only keep airflow low for a few feet away from the baffle, so ideally there should be baffles at least over every section of free stalls. Tunnel-ventilated barns can work with baffles, too, but baffles can get in the way of equipment, so most tunnel-ventilated barns use low ceilings to achieve air flow at the cow level. Tunnel-ventilated barns can work with no ceiling and circulating fans, typically located above the free stalls, to provide airflow at the cow level, but it can be difficult to achieve sufficient airflow at all areas in these types of tunnel barns. Tunnel-ventilated barns sometimes may need strategically placed positive pressure inlets to draw more air into the barn, especially if the barns are very long.
Naturally-ventilated barns allow air to flow in naturally though open side and end walls. Air typically leaves through an open ridge or through a power-ventilated open ridge. As hot air rises and exits the ridge, there is very slight negative pressure which allows outside air to enter, but the effect of even a slight breeze can be much greater than this. Fans in naturally-ventilated barns are used to create air movement in cow living areas, not to move air down the barn and exhaust out the end. Fans over free stalls are meant to cool cows by providing significant air movement in the free stalls. Thus, air movement above the stalls is of little or no value. This means fans need to be angled down to the cow level and preferable to the level of a cow lying down. It also means that the old idea of placing fans about 10-feet times the fan diameter in inches is wrong, because few fans are capable of providing sufficient airflow at that distance. The best way to evaluate airflow in stalls is just to get in there and feel it. Count the number and note the location of stalls ventilated by each fan. In some barns, only a few stalls below each fan have noticeable airflow because fans are too far apart, are not angled downward sufficiently or have migrated in position so that they blow air somewhere else rather than where the cows are.  
What about water? Cows do not sweat much, so they need water applied to the skin to create evaporative cooling. This includes in the holding pen. There needs to be air movement to aid in evaporating the water from the skin. Typically, at least one-half gallon of water should be applied per nozzle each time the nozzle turns on for feed line sprinklers. Nozzles plug easily, and it is very common to find sprinkler systems with many plugged nozzles. Like fans, nozzles often get out of adjustment and shoot water somewhere other than the backs of cows. Sprinkler system settings also seem to change over time, until someone notices they are not on for a long enough period of time or do not come on frequently enough.
The best way to determine if your cooling systems are working properly is to get into the barns on a hot day and observe. Just because fans are operating, and the electric meter is spinning, does not mean that cows are being sufficiently cooled.
Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.