Summer is here and the cows are getting hot. Our dairy cows are large fermentation vats on legs, and they generate a lot of heat. The thermoneutral zone for an adult dairy cow is typically in the range of 40 to 70ºF, and cows suffer heat stress above the upper critical temperature – before most humans become bothered by heat.
Temperature-humidity index (THI) is a function of both temperature and humidity, and we know a cow’s eating and resting behavior starts to change at around THI of 65 to 72, with production, reproduction and other physiological and health effects apparent at around THI of 68.
Bunching is a common complaint during the summer, particularly in freestall barns where cows gravitate toward the center of the barn and away from the sidewalls.
There is a simple explanation for this behavior – cows are grazing animals and are hard-wired to seek shade (or “dark”) when hot. So bunching really tells us the cows are hot and whatever heat-abatement strategies are being used are failing to cool the cows sufficiently.
While cows may cool by conduction, convection, radiation and evaporation, once ambient temperature approaches body temperature, heat loss through evaporation becomes the only significant mechanism, through sweating and thermal panting.
Cows accumulate heat rapidly while lying down (about 1ºF per hour of rest) and dissipate heat when they stand, so our cooling strategies must focus on:
1. Reducing the rate of accumulation of heat while resting
2. Increasing the rapidity of heat dissipation while standing
Provision of shade, good ventilation and water access are essential. Ventilation can be achieved naturally with good barn design, location and freedom from wind shadows. However, mechanical cross or tunnel ventilation may be required to achieve about 50 to 60 air changes per hour in the summer, when natural ventilation is compromised.
We typically recommend a minimum of two waterers per group and 3.5 inches of accessible trough perimeter per cow for adequate water access in most climates.
In addition, there are two broad approaches to provide additional cooling: cooling the cow through the use of air movement, soaking or both; or cooling the air around the cow by using misters and evaporative cooling pads. Air conditioning has proven too expensive for most cow barns.
Re-circulation fans are used to supplement air movement in naturally ventilated barns. These fans can be located in the holding area, over the resting area in the pens and over the feedbunk, which we typically prioritize in this order. Cows need speeds of at least 350 to 450 feet per minute (about 4 to 5 mph) to begin to cool.
This provides a challenge when we look at how air travels from a typical fan ( Figure 2 ). Air leaves fans in a cone and decelerates rapidly as it moves further from the fan, so for most 3-foot to 4-foot-diameter fans, the “sweet spot” is within 10 to 15 feet of the fan– far less than the typical fan spacing recommendations of 10 times the fan diameter.
For adequate air movement, we’re going to have to double up on fans and make sure they cover the whole holding area, resting and feeding area, which is going to take a lot of electricity.
Positive-pressure tube systems have been designed to improve the delivery of fast-moving air by pulling air from the outside and distributing it evenly directly upon the cow from above. This is a much more efficient way of providing fast-moving air.
A growing number of holding areas are using this technology to help keep cows cool ( Figure 1 ). A list of trained positive-pressure tube ventilation consultants for holding areas can be found on the Dairyland Initiative website under the “Professionals” tab.
Water may be used to soak the cow or cool the air moving toward the cow. Soakers have been installed in holding areas, parlor exit lanes and over feedbunks. Thoroughly wetting the cow is a great way to improve evaporative heat loss, and we have controller units to change soaking times and intervals at different ambient temperatures.
Soaking in the pens is problematic, however. The additional water in the alleys causes wet manure to be transferred to the freestall bedding, leading to increased mastitis risk. In sand barns, the extra water leads to sand settling in transfer channels and collecting pits – causing pumping problems, and when the cows aren’t at the bunk (for 19 hours per day), they don’t get soaked.
One idea to improve soaking efficiency is to develop soaker stations around the pens where cows can voluntarily enter all day long and be soaked by activating an optic sensor.
When water is used to cool the air moving toward the cow, conditions of relatively low humidity are required. Misters are added to fans to help cool the air cone leaving them, or in cross-ventilation barns evaporative cooling pads have become common to cool the air stream as it is drawn through the pad.
This type of cooling can be very efficient under conditions of low humidity, with temperature drops of more than 10ºF possible. However, with relative humidity greater than 55 percent, the temperature drop may be less than 1ºF – making this type of cooling ineffective. In more humid climates, it is now becoming common to see the evaporative cooling pads being discarded with a return to soaking cows.
So with summer fast approaching, please take a look at cooling strategies on your farm. Watch your cows for signs of heat stress. Bunching, milk production drop, conception failure and lameness in late summer are signs you may need to make improvements. PDNigel Cook is currently chair of the Department of Medical Sciences at the University of Wisconsin – Madison School of Veterinary Medicine and manages the Dairyland Initiative.
Nigel B. Cook
UW – Madison School of Veterinary Medicine