Hotter than You Think

May 10, 2011 11:57 PM
Hotter than You Think

65 THI is new threshold

Today’s high-producing Holsteins are 50 years and 25 generations removed from the first heat-stress studies on dairy cattle. Yet many of today’s recommendations on when cooling is required date back to the 1960s.

When those measurements were taken, production was in the low 30s per cow per day. Since then, the heat produced by the average Holstein has increased 28,000 Btu per day. That just won’t do—and today’s heat-stressed Holsteins are the evidence.

"The heat production of cows producing 40 lb. per day and 70 lb. per day of milk is 27.3% and 48.5% higher, respectively, than nonlactating cows," says Bob Collier, a University of Arizona dairy specialist. "When milk production increases from 77 lb. to 100 lb., the threshold temperature for heat stress is reduced by 9°F."

Research at Arizona suggests the standard recommended temperature-humidity index (THI) threshold of 72 is too high. Milk losses decline in a straight line between 60 and 80 THI.

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More on heat stress

The research involved 100 Holsteins housed in tiestalls in two environmentally controlled chambers. Data was collected from eight different studies over three years.

"Average losses in milk yield per day were nearly 5 lb. between a minimum THI of 65 and 73," Collier says. "Our data indicates that dairy cows producing more than 77 lb. per day need additional cooling when minimum THI is 65 or greater or when average THI is 68 for more than 17 hours per day."

A THI of 65 occurs at 72°F and a relative humidity of just 5%. A THI of 68 occurs at 72°F and a relative humidity of 45%. Once temperatures in just about any climate top 70°F, high-producing Holsteins are in danger of heat stress. Respiration rates increase, often exceeding 60 breaths per minute. Rectal temperatures exceed 101°F and reproduction losses start to occur.

Active cooling is required. Research from Israel suggests that typical shade structures increase effective ambient air temperatures by as much as 5°F for surrounding animals. Metal roofs can add 5°F to heat loads.

To calculate the costs and returns of cooling,
Collier uses the example of 100 cows. By turning on Korral Kool coolers at a THI of 68 versus 72, research shows, milk production would increase 484 lb. per day for the group. At a milk price of $17 per cwt. and feed costs of $14 per cwt., the income over feed cost for the group would be $14.52.

The estimated variable cost of running the coolers would be $7.40 per cwt., leaving an income of about $7.10 per 100 cows, or 7.1¢ per cow. In a 500-cow herd, that equates to $35 per day, $250 per week or $1,000 per month. "This does not take into account any beneficial effects on reproductive performance," Collier says.

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