462 July 2013

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Body temperature regulation in dairy cows is constantly challenged by a combination of environmen-tal heat and that produced during rumen fermentation and nutrient metabolism. Heat stress occurs when cows cannot dissipate enough heat to maintain a core tempera-ture below 101.3°F.

Internal heat production acceler-ates at higher dry matter intakes, therefore, your high producers are more sensitive to heat stress. Body temperatures elevated just 2.7°F have been shown to result in intake reductions of almost 13 pounds.

Midlactation drop severeduring lactation, cows start to drop

intakes as temperatures exceed 75°F, with marked reductions above 86°F. Air velocity and relative humidity also need to be factored in. A thermo-neutral zone can be assumed at 68°F and 50 percent relative humidity. The thermoneutral zone is the tem-perature and moisture combination where the cow is comfortable and her organism does not need to resort to mechanisms to cool or warm up the body. When temperatures exceed 75°F, however, intake drops consid-erably even at 50 percent relative humidity. Intake is reduced more sig-nificantly in higher producing cows that consume more feed.

Intake reductions in groups of heat-stressed cows are most severe in midlactation while it is least severe early on in lactation. Early-lactation cows have the lowest intakes. How-ever, they usually are the ones that produce more milk. Their advantage is that their metabolism is in “tissue mobilization mode” and can readily utilize body reserves to compensate the effects of environmental heat.

Second-lactation or greater cows that are pregnant have greater intake reductions than pregnant first-lacta-tion cows. When cows are subjected to heat stress, a significant portion of their body heat is eliminated through the skin. As ambient temperatures rise, however, skin losses are reduced while panting hikes to augment heat emission through evaporation.

Intake and production are more closely associated with the tempera-ture of the two previous days than those of the present one. Whenever necessary, it is important to have strategies that reduce temperature at night. Cows can tolerate higher temperatures during the day when ambient temperatures at night drop.

Need evaporation timeAmong the most effective strate-

gies to improve intake and produc-tion are sprinklers over the feed-bunk. Sprinklers soaking the coat and skin should work intermittently to allow time for water evaporation before the next soaking cycle.

Researchers from the University of Missouri-Columbia placed two lines of sprinklers in the freestall; one on top of the feedbunk and the other in the alley between the stalls and side-wall. Sprinklers were set on 30-min-ute cycles (20 on, 10 off). Although they reduced rectal temperature by 0.5°F, they only improved yield by 1.5 pounds. Production rose by 4.4 pounds per day when the experiment was replicated with forced air over the feedbunk and stalls.

In an Alabama experiment, sprin-klers over the feedbunks improved intake and milk yield by 2.2 pounds per day. In this experiment, sprin-klers were on 15-minute cycles (3 on, 12 off) with fans over feedbunks and stalls. It was determined that fans alone did not improve cow comfort. Both strategies need to be combined to treat cows under severe heat stress.

Kansas State University (KSU) research evaluated three combina-tions of sprinklers and fans dur-ing the summer in barns with four rows of stalls. All treatments had sprinklers over the feedbunk pro-grammed in 15-minute cycles. one treatment had two rows of fans over the stalls, another had one row of fans over the stalls and one over the feedbunk, and the last treatment had two rows of fans over the stalls and one row over the feedbunk.

Cows cooled with fans over the stalls and feedbunk produced more milk than those with fans only over the feedbunk (98.8 versus 93.9 pounds per day). There was no pro-duction response when the number of fans over the stalls was doubled.

Combinations work bestA subsequent experiment evalu-

ated two cooling systems, both with sprinklers and fans over the feed-bunk, but only one of them had a line of fans over the stalls. It was confirmed that, when fans over the feedbunk were complemented with those over the stalls, milk yield rose by 5.7 pounds per day.

The effectiveness of the cooling system depends on the number of rows of stalls:

Four rows: sprinklers over the feed-bunk and two rows of fans, one over the stalls, one over the feed bunk.

Two rows: one row of sprinklers

over the feedbunk and one of fans over the stalls.

Researchers at KSU compared the capacity of soaking frequencies to reduce body temperature in cycles of 5, 10 and 15 minutes. Each cycle supplied approximately 0.35 gallon of water per headlock per minute. Airflow was continuous at 700 cubic feet per minute (CFM). The drop in body temperature rose with soak-ing frequency. The most effective system was continuous forced air and 5 minutes (1 on, 4 off) soaking frequency. With this soaking fre-quency, there were no advantages of airflow above 750 CFM. It’s impor-tant that cows are dry before the next cycle starts.

According to KSU research, mis-ters over the feedbunk coupled with forced air can have a similar effi-ciency to 5-minute cycle sprinklers only when they soak the cow’s skin completely. To achieve this they must work with a minimum water flow of 3.4 gallons per hour. With high-pressure misting, there’s a combined effect of cooling both the surrounding air and the animal through water evaporation.

Misters with water f lows of 1.7 gallons per hour were able to drop body temperatures in cows, but were not as efficient as those with 3.4 gallons per hour or sprinklers with 5-minute cycles. In addition, continuous high-pressure misting systems at 3.4 gallons per hour use 18 percent less water than sprinklers in 5-minute cycles.

A cooling program could raise soaking frequency at the feedbunk as ambient temperatures elevated.

Extended periods of high ambient temperature coupled with high rela-tive humidity compromise the ability of the cows to dissipate excess body heat. Supplemental fan cooling and low pressure feedbunk sprinklers can reduce the effects of heat stress on milk production and intake.

Cool cows keep intakes up

Feedingby Fernando Díaz-Royón and Álvaro García, D.V.M.

The authors are in the Dairy Science Depart-ment, South Dakota State University, Brookings.

How often should you soak?

Ambient temperature

Soaking cyclefrequency

Between 77°Fand 86°F

12 minutes (1 on - 11 off)

Above 86°F 8 minutes (1 on - 7 off)

INTAKES BEGIN TO DECLINE as temperatures exceed 75°F. This drop will be most severe for your midlactation cows along with those in their second lactation. Sprinklers over the bunk are one of the best methods to foster higher intakes.

Used by permission from the July 2013 issue of Hoard’s dairyman.Copyright 2013 by W.d. Hoard & Sons Company, Fort Atkinson, Wisconsin.