Recalculating: A new look at ‘THI’ and cost effective ways to manage heat stress in dairy herds

Cows and conditions have changed since the Temperature Humidity Index (THI) was created. Researchers are looking at earlier starting points for management intervention, and new cost-effective cow-cooling methods.

By Ron Goble

One of the top cow comfort experts in the world, University of Arizona animal scientist Robert Collier points out the Temperature Humidity Index (THI) was originally developed for humans, not cows. The research,  conducted at the William J. Parker Research Complex at the University of Arizona in 1958, was extended to cattle using research conducted by I.L. Berry, at the University of Missouri, in the 1960s.

The data was placed into the familiar THI chart by the University of Arizona’s Frank Wiersma and Dennis Armstrong, in the 1980s, becoming a popular tool to estimate dairy cow cooling requirements and develop heat stress management strategies on dairies.

Since that early research, things have changed, according to Collier, speaking at the 2011 Western Dairy Management Conference, in Reno, Nev.

For example, the Missouri THI research was based on data from 56 cows with an average milk yield of about 34 lbs./day. Additionally, heat stress was applied for two weeks before milk yield was recorded, and the temperature was constant. During the original studies, there was also no wind speed or infrared heat load consideration.

Collier pointed out today’s typical milk production level is 70 lbs. or more/cow/day, impacting heat stress tolerance. Research in Israeli has demonstrated a milk yield of 99 lbs./day lowers the THI threshold  by 9° Fahrenheit  (5° C).

The time interval producers expect in today’s environment is next-day information. They want to know what impact today’s environmental heat load has on their cows’ production level tomorrow – not in two weeks. And, heat loads recorded from a metal roof during the summer are known to add approximately 6° F (3° C) to a cow’s heat load.

Finally, cows themselves have changed. “Since 1950, heat production per cow has increased an average of 28,000 Btu/day in the Holstein breed,” said Collier. “So the cow is more sensitive to heat stress, since she is producing more heat internally than cows from the 1960s. This requires re-evaluating the relationship between ambient temperature, milk yield and thermal stress level in cattle.”


Underestimated thresholds

Collier contends there’s still no better index to use for heat stress management. However, the current THI threshold is underestimated for today’s high-producing dairy cows.

The original THI chart had a stress threshold starting at 72, a point many used to trigger cooling management strategies. However, analyses conducted at the University of Arizona shows milk yield losses are detectable beginning at a THI of 68. Waiting until THI hits 72 is too late: Cooling methods on commercial dairies should be implemented earlier to prevent effects.

Collier stressed management strategies must focus on minimizing heat gain, while maximizing heat loss. University research showed the summer-to-winter milk production ratio in Arizona confirmed dairy cows are giving up 8.8 lbs of milk per cow/per day during summer compared to winter months.


By the numbers

“If we use an example employing 100 dairy cows cooled by a Korral Kool cooler, we should expect a milk yield gain of 4.8 lbs. of milk per day beginning cooling at 68 vs. 72,” he said. “For 100 dairy cows, that would equate to a milk yield gain of 4.84 hundredweights. Using a milk price of $17/cwt., and a feed price of $14/cwt. milk produced, the income above feed costs would be $14.52.”

Researchers used a variable cost of 14¢/cwt. of milk produced, and assuming each cooler would cool 10 cows, the total cooler variable cost would be $6.80, producing an income of $7.09/100 cows, or 7.1¢/cow/day. In a Western herd of 3,000 lactating dairy cows, the potential income would equate to $213/day, or $1,491/week. Collier pointed out that does not take into account any beneficial effects on reproductive performance in these cows.


Economic efficiency

With increasing focus on economic efficiency,  the objective of a 2009 research project, conducted at Caballero Dairy and Red River Dairy in Arizona, was to determine if lowering the threshold for cooling systems improved milk production and reproductive performance in lactating dairy cows under dairy farm conditions.

Collier concluded additional cooling – using oscillating fans or Korral Kool Coolers – did not improve lactation or reproductive performance when cooling was initiated at THI of 68 instead of 72, and was not considered cost effective.

Conventional cooling of dairy cows during the summer months consumes large amounts of electricity during the peak hours of the power demand and large quantities of water for various cow soaking and misting systems.

Energy costs on dairies continues to increase. Numbers supplied by Agriaire Industries shows a 9,500-cow dairy in Casa Grande, Ariz., spent $700,000 to cool their cows between April through October.

Collier shared a graph showing the historic increase in California’s electric rates from the years 1970 through 2006 for residential and commercial users. Rates climbed steadily, from 2¢ per kilowatt-hour to near 14¢ per kilowatt-hour during the period.

While electrical prices have been going up on an annual basis, prices can be much higher during summer months, too, Collier said.

Electricity futures prices on the Chicago Mercantile Exchange peak in July and August each year. A review of U.S. Energy Information Administration data on monthly electricity prices shows highest prices for end users peaks in June-September (

Also, the average price of water increased last year by 3.8% worldwide, according to a survey conducted by NUS Consulting Group, and an official of the firm says the survey also provides evidence to indicate even higher water price increases in the future.


Seeking a more effective way

With rising electricity and water costs to cool cows conventionally – and if heat stress management intervention is to start at a THI of 68 vs. 72 – Collier raises the question: “Would alternative ‘passive’ forms of cooling provide a more cost-effective method to reduce milk yield losses?”

One method being researched is a thermal conductive cooling system, a heat exchanger “system” installed beneath the cows’ bedding area in dairy barns. This application utilizes flexible plastic-based heat exchangers.

In the warm southern climate, well water temperatures range from mid-60s°F to low 70s°F. There is approximately a 30-35° F differential between the cow’s internal temperature and the temperature of earth-cooled well water.

The colder temperature of the water cools the cows naturally via conduction, by transferring heat from a warm source (the cow) to a colder source (the heat exchanger with the colder water), explained Collier.

Recently, collaborative work by Agriaire Industries and the University of Arizona demonstrated heat exchangers could be buried 12 inches below the surface of a freestall bed and still provide significant conductive cooling to dairy cows.

“Geothermal cooling would represent significant cost reduction in reducing heat stress on dairy cows and offers the additional opportunity of using the same approach to warm cows during cold winter months in northern dairy locations. Field testing of this concept is currently underway,” he said.

If thermal conduction cooling is proven to effectively cool cows within “profitable physiological parameters” (101° F -103° F) and implemented on dairies, researchers said the potential exists to significantly shift/reduce peak loads of energy required to cool dairy cows, providing huge savings in summer energy costs for dairy producers.


‘Proof of concept’

Phase 1, the “proof of concept” testing, was conducted June 14-23, 2009 at the University of Arizona Agricultural Research Complex in Tucson, under the supervision of the University of Arizona Animal Science Department. This study supported continued development of the concept at a commercial scale.

A commercial scale test was then conducted at 3,600-cow Rancho Teresita Dairy, Tulare, Calif., from Sept. 1-30, 2009. For this test herd,  the 52-stall test section of the freestall barn had no fans and no soaker line system operating, only conduction cooling to cool the cows. Those cows were compared to traditional cooling methods (fans and soakers), which kicked in at 72° F.

With the water supply to the heat exchanger system averaging in the low 70s° F, the control test maintained an average bed temperature of 78.57° F. The test herd with the heat exchanger system averaged 76.31° F.

Rectal temperatures recorded during the 30-day trial showed temperatures at 101.96° F in control cows and 101.98° F in test cows. A 30-day average of skin surface temperatures were reported at 99.0° F in the control group  and 98.9° F in test cows.

Researchers reported 30-day average milk weights of 97.83 lbs/day for control cows and 93.6 lbs./day for test cows, a difference of 4.23 lbs/day.

During the week of Sept. 25 through Oct. 1, temperatures soared to peaks of 99° F, with sustained high humidity. Average loss for all 3,600 cows during this particular week was 5 lbs./cow/day. Supply water to the heat exchanger averaged in the mid-70°s F during this particular week.

Despite the milk output decline in the initial study, cost-effectiveness of the alternative cooling method convinced researchers the data generated from the “proof of concept” and field studies support further development of conductive cooling of dairy cows to reduce water and electrical use. The results supported use of refrigerated water, or implementing additional cooling above air temperatures of 90° F.

Additional studies are planned in 2011-12 at dairies in California, Texas and Arizona.

By modifying management, using conduction cooling alone to cool cows – and holding off turning on fans until temperatures reached 90° F – Collier estimated this same 3,600-cow dairy, using 180 fans at 1.2 kW/hr per fan and paying 9¢/kW/hr, would save close to $26,500 for the summer in energy costs to cool cows – a savings of more than 75% in electricity costs.