Semler-dairy-minute

While Mother Nature tries to make up her mind what season it is, we should start thinking about summer. I hear you out there, we haven’t even planted corn yet and you want us to start thinking about summer. Yes I do. Even if you start planting soon, take that time in the cab to think. About what, you say? The coming summer heat.

While your corn will enjoy the heat units, your cows will not. Your cows need the same creature comforts you would want on a hot, humid day ... only more. Cows need relief from the heat far sooner than their human caregivers.

An important tool for gauging cow comfort during hot weather is the temperature-humidity index. The weatherman refers to this as the heat index. According to the National Weather Service, the heat index is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature. As an example, if the air temperature is 96 F and the relative humidity is 65 percent, the heat index — how hot it feels — is 121 F. NWS will initiate alert procedures when the heat index is expected to exceed 105-110 F (depending on local climate) for at least two consecutive days.

By that time your cows have been stressed for days. Remember they haven’t had the option of riding in a tractor cab, eating lunch or sleeping in air conditioning. Since heat index values were devised for shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15 F. Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.

Now back to the cows and THI, just like the heat index for humans, THI was developed to monitor and reduce heat stress-related losses. Utilizing both humidity and temperature is important compared with utilizing temperature alone to evaluate heat stress.

Bovines, unlike their human counterparts, cannot perspire and thus must dissipate heat by evaporative heat loss through the skin and lungs. Heat stress is important to dairymen everywhere but the University of Arizona has re-evaluated studies that were conducted at the University of Missouri in the 1950s. The THI formula used for these studies is: THI = Tdb - [0.55 - (0.55 x RH/100)] x (Tdb - 58). This formula uses dry bulb temperature (Tdb, ºF) and the relative humidity. The RH is divided by 100 to express the percentage in decimals.

The Tdb is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. It is the temperature that is usually thought of as air temperature.

Now you can imagine there is much less humidity in Arizona than there is here in the Mid-Atlantic. We have all heard the cliché but it’s a dry heat. So let’s look at two scenarios, first it is 80 F with 20 percent humidity in Arizona and it is 80 F with 90 percent humidity here. Using the formula above, the THI for Arizona would be 70.32 and the THI for our area it would be 78.79.

Although current cooling standards utilize a THI threshold of 72 before initiation of cooling, the Arizona research indicates that physiological parameters and milk yields were affected at THI values well below 72. Between a THI of 64 and 72 there were large reductions in milk yield. Although not reported in the paper, it is expected that milk component percentages will also be lower.

Heat-stress abatement is a critical dairy-management strategy. Since it has been found that the harmful effects of heat stress begin at lower temperatures than previously thought, all dairy producers should consider employing heat-stress abatement strategies at lower temperatures, especially for groups of high-producing cows and dry cows in the pre-fresh pens.

So the bottom line advice for dairy managers about cow cooling is, do some. Here are nine tips (in order) where you should start:

• Supply adequate water. Heat-stressed cows need up to 100 percent more water than normal.

• Provide shade in both the milking parlor holding pen and the housing area for milking cows and dry cows alike.

• Reduce walking distance to the parlor.

• Reduce standing time in the holding pen.

• Improve ventilation in the holding pen and in free stalls.

• Add cooling to the holding pen and exit lanes.

• Provide cooling for closeup cows, especially those within three weeks of calving.

• Provide cooling in fresh cow and early-lactation cow housing areas.

• Provide cooling in mid- and late-lactation cow housing areas.

Cooling cows improves their performance in every important management area and is an investment that pays long-term dividends.

Due to space limitations, resources for this article available from the author upon request.

Jeff Semler is a University of Maryland Extension educator in Washington County.