Heat Stress - Where Do You Start?

Dr. Normand St-Pierre, Dairy Specialist, Ohio State University 

In an average year, heat stress costs $19.0 to $33.5 million to Ohio's dairy industry, or $70 to $125/cow/year. That's a lot of money being left on the table, especially in these times of low milk prices. In an average year, our cows are stressed an average of 1,200 hours. During an average heat stress hour, the ambient temperature - humidity index (THI) exceeds the upper threshold of cow comfort by 5.6 units. The problem is becoming more acute because our cows are producing an increasingly amount of heat due to their higher levels of production. A good Holstein cow producing 100 lb/day of milk is generating approximately 6,500 BTU/hour. This is the amount of heat energy that must be dissipated to the environment, every hour of the day, if we want to maintain constant normal body temperature of 101.5 (± 1.0)°F. Otherwise, feed intake drops, the number of meals eaten per day decreases, followed by a decline in milk production, a decrease in conception rate coupled with an increase in early abortion rates, an increase in subclinical acidosis, and an increase in death losses. As Murphy would say, that's not good!

Where should you start? In Table 1, we prepared a prioritized list of actions for reducing the negative impact of heat stress. Availability of clean water is a must. Cows can easily drink an additional 8 to 10 gallons/day during periods of heat stress. Production will suffer if this happens when your well is running dry or if your watering system was not designed to accommodate this increased water demand. In freestall housing, two feet of tank perimeter is adequate for every 15 to 20 cows in the winter (2.4 to 3.2 linear inches/cow), but not in the summer when 4.0 to 5.0 linear inches/cow are needed.

Solar radiation can increase substantially the heat load, and thus, the level of heat stress experienced by cows. Inexpensive shade cloths can be very effective in this regard.

Milking parlors are often the primary areas of intense heat stress for cows during the summer. Our better cows produce 6,500 BTU/hour, regardless of their location. In the holding pen, the floor surface area is reduced from 75 to 80 square feet/cow in the freestall barn to less than 15 square feet/cow. Consequently, the intensity of heat abatement must be considerably higher in the holding pen than in the freestall barn. The standard recommendation is to install rows of fans every 30 feet across the holding pen, with 36 inch fans spaced eight feet center to center along each row. In our evaluation, this recommendation is too conservative and does not provide enough air volume and velocity. Our recommendation is to center 36 inch fans on six feet center to center and to set the rows a maximum of 24 feet apart. Our recommendation reduces the surface area from 240 square feet down to 140 square feet per fan. Additionally, low-pressure sprinklers should be installed and set on a timer to operate for one minute every 5 to 6 minutes. You will know that your holding pen cooling is sufficient when a minimum of 8 cows out of 10 have normal body temperature and respiration rate as they exit the parlor.

In case you are wondering about the economics, each dollar invested in cow cooling (equipment cost and operating cost) results in an additional $3.50 of additional income. Not a bad investment, even at $10/cwt milk!

Table 1. Priorities for reducing heat stress.

1. Adequate water
2. Provide shade
3. Reduce walking distance to parlor
4. Reduce time in holding pen
5. Improve holding pen ventilation
6. Improve freestall ventilation
7. Add holding pen and exit lane cooling
8. Cool fresh cows and early lactation cows
9. Cool pre-fresh cows
10. Cool mid and late lactation cows