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DUBUQUE, Iowa - As dairy farmers know, summer's heat can take a toll on their cattle and their income. Not only do milk production and quality suffer, but reproduction and calf and heifer growth are also hurt.
It's been estimated that heat stress costs the United States dairy industry some $900 million a year, said Lance Baumgard, an associate animal science professor at Iowa State University. He talked about what researchers learned in 2011 about heat stress, at the recent Four-State Dairy Nutrition and Management Conference in Dubuque, Iowa.
"The entire summer was particularly warm," said Baumgard. July of 2011 was the fourth-hottest on record, and the national average was 2.67 degrees Fahrenheit warmer than the 1901-2000 average."
July of 2011 saw 8,926 daily heat records broken. Nearly 70 percent of those were daily minimum temperatures, meaning nights were warmer.
"The mid-July heat wave that blanketed the Upper Midwest was especially hard on animal agriculture, and the dairy industry in particular," Baumgard said. "The combination of heat and humidity a few days before and after July 20 was dubbed the dome of doom by the popular press."
It wasn't the daytime high temperatures that were so tough on dairy cattle, Baumgard said. Instead, it was the higher nighttime temperatures, coupled with high humidity.
For example, on July 18, the dewpoint was 80 degrees in Green Bay, Wis., and it reached 83 degrees in Rochester, Minn. The minimum temperature-humidity index (THI) came in at 73 degrees at Ames, Iowa; 75 for St. Paul, Minn.; and 72 at Madison, Wis.
"This lack of nighttime cooling," said Baumgard, "was associated with the severe decreases in milk production that accompanied the July 20 heat wave. This agrees with recent environmentally controlled experiments indicating that the daily minimum THI is closely associated with milk yield."
Milk production dropped on almost every dairy farm in Wisconsin, Minnesota, Iowa and Indiana during the heat wave two years ago, Baumgard said. But, according to dairy farmers, Extension staff, nutritionists and consultants, the declines were extremely variable.
"Not only did milk yield vary, the recovery, or long-term effects also markedly differed among producers," Baumgard said. "Presumably, much of the variation can be explained by differences in facilities, barn design and the use of heat stress abatement strategies such as fans and sprinklers."
But Baumgard noted that management practices that have nothing to do with facilities likely affect how a herd reacts to a heat wave. These practices might include nutrition and cow comfort.
To see how facilities and management affected cows' heat related production and their subsequent recovery, data from 590 farmers who ship milk to First District Association (FDA), Litchfield, Minn., were analyzed. The numbers were from July and August of 2011.
From there, 206 herds were randomly surveyed by trained personnel with no knowledge of the herds' previous production history, Baumgard said. The survey looked at heat abatement, facilities, nutrition and other management practices. Later, the owners of 108 herds agreed to share their Minnesota Dairy Herd Improvement Association reproduction and culling numbers.
"Regardless of facility design and management practices, the average FDA patron had a 20.2 percent decrease in milk production for the period of July 20 to 23, compared to the July 1 benchmark," Baumgard said. "By Aug. 31, the average patron's milk production was still 5.2 percent less than it was on July 1."
The type of barn the cows were in did not affect a herd's acute response to heat stress. But grazing herds in open lots took longer to regain their milk production. Herds on bedded packs regained their milk production fastest, followed by those in freestall barns, with those in tiestall barns next.
However, the open-lot grazing herds did not drop in milk production as much to begin with. For them, the average decline was 12.71 percent. The average milk decrease was 13.67 percent for herds on composting bedded packs. The drop averaged 15.19 percent for freestall herds, and it averaged 17.78 percent for tiestall barn herds.
Baumgard discussed the acute - or short duration - responses to the heat wave.
One acute response was that milk production tended to drop less during the heat wave on farms that used monitoring systems to track bulk tank changes and cows' body temperatures, Baumgard said. The average milk decrease was 15.5 percent on farms with such monitoring, and 18.7 percent without the monitoring.
Stocking density appeared to influence the milk production decline. In barns that were overstocked, milk production fell an average of 17.4 percent. In barns that were not overstocked, milk production dropped an average of 12 percent.
Making sure air moved across resting and loafing areas also alleviated the milk production decline. Where there was air flowing, milk production fell an average of 15.9 percent, compared to 18.8 percent without air movement.
Creating evaporative cooling from cows by using sprinklers or soakers also helped. With such cooling, milk production fell an average of 13.4 percent. Without such cooling, milk production dropped an average of 17.4 percent.
The location of sprinklers and soakers was important, too, Baumgard said. When sprinklers and soakers were in feed lanes and holding pens, the milk decline averaged 11.9 percent. With soakers and sprinklers in other locations, milk production dropped an average of 16.5 percent.
The survey also looked at herds' short term and long-term recovery from the heat wave. Having bulk tank or cow temperature monitoring systems did not aid short-term recovery of milk production. Neither did stocking density.
Keeping sidewall curtains of freestall barns wide open did speed short-term recovery. But evaporative cooling did not seem to help milk production rebound faster.
Herds whose rations were balanced by a nutritionist saw their milk production come back quicker. On those farms, milk production rose 13.8 percent above what it was during the heat wave. On farms that did not employ the services of a nutritionist, the milk production rebound was 9.3 percent.
In the longer term, monitoring environmental conditions was associated with a shorter recovery time, Baumgard said. "But," he said, "this was likely due to the fact that producers who did not monitor THI had a more severe decrease in milk production and thus had more opportunity for an increase."
Other factors associated with a faster long-term recovery included using evaporative cooling in the holding pen, having a nutritionist balance the ration, using bovine somatotropin, and feeding a rumen modifier.
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