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Prepare Now for Dairy Heat Stress Season

Dr. Dick Wallace
April 6, 2000

The weather in April is ideally suited for dairy cows and high milk production, but the heat stress season is just around the corner. It is not too early to begin thinking about control strategies to minimize the production losses that occur due to heat stress.

Having just returned from Brazil, I can assure you that our period of heatstress is minor in comparison. Heat stress in parts of Brazil can last up to six months. The native cattle, Gyr (bos indicus), are well adapted to the extreme periods of prolonged heat and humidity. Unfortunately, their level of milk production is well below our average Holstein.

To increase milk production, Brazilian dairy farms have been importing American and Canadian Holsteins. Milk production increased, but the deleterious effects of heat stress are running rampant through the countrysides of Brazil. Some producers have crossed the Gyr with the Holstein; the result is a more heat-tolerant F1 cross, but these cattle do not have the same temperament as Holsteins and will not milk in conventional dairy parlors (i.e., no milk let-down and they kick).

The effects of heat stress are many. Most every producer is aware of the decline in dry atter intake and the subsequent rapid drop in daily milk production. But heat stress contributes to several more subtle, insidious effects on dairy cows. There is a significant effect of heat stress on the immune system. Heat-stressed cattle are less likely to fight off normal respiratory or mastitis infections. Dry cows that have been exposed to heat stress will have smaller calves, which in turn creates a smaller placenta. The placenta is important for contributing to the hormones that induce milk production in the subsequent lactation. With a smaller placenta (as compared to dry cows without heat stress), fewer hormones are produced and milk production in the fall after severe heat stress will be lower.

Other than production losses, the biggest impact of heat stress is reduced reproductive efficiency. After the summer of 1999, with its prolonged periods of heat stress (even in November the temperature and humidity index called for heat stress), we saw reproductive inefficiency linger into December. By January, most cows that were coming up open on herd checks earlier had finally conceived. These reproductive losses will have major impacts on cow flow for the next several years. Herds will be short replacement heifers, and the average days in milk for most herds will be excessive.

Providing heat abatement with cooling strategies will help reduce the reproductive inefficiencies in the fall, but aggressive breeding in the spring can help as well. Suggestions from subtropical dairy climates such as Florida and Brazil include aggressive breeding of ALL eligible cattle within the last month before heat stress begins and cooling, but not breeding, through the period of heat. Commence breeding when the weather has cooled off in the fall.

Usually due to spring field work, the start of the heat stress season often catches our dairy producers off guard and unprepared to cope with the rapid swings in environmental temperatures. Now is a good time to make sure all sprinkler systems are flowing properly and timers and fans are working efficiently. Preparing the shade cloth and checking curtains is prudent as well. If tin has to be removed for better air flow, get it off before the field work reduces the likelihood of it getting done. The cows are better off with a little draft than an oven for a barn.

Don't forget to provide heat abatement for dry cows. Their subsequent milk production will help offset the expense. Calves are fairly tolerant of heat as long as they can modify their own environment to suit their needs. Calf hutches well spaced and with tethers are preferred. If the calf wants shelter, it can enter the hutch. If it wants fresh air, it can rest outside the hutch. If it wants shade, the calf can use the hutch as a sun block and move with the shade.

For additional information of heat stress in dairy and other livestock species see the following Web sites:

http://dairynet.outreach.uiuc.edu/ and search on HEAT STRESS

http://edis.ifas.ufl.edu/scripts/htmlgen.exe?DOCUMENT_DS072

http://www.ianr.unl.edu/pubs/Animals/g57.htm

http://www.ces.uga.edu/pubcd/b956-w.html

Dick Wallace Dairy Extension Veterinarian Continuing Education, Public Service/Extension Assistant Professor, Dairy Production Medicine Department of Veterinary Clinical Medicine Director of Instructional Programming and Distance Learning College of Veterinary Medicine University of Illinois at Urbana-Champaign 2938 VMBSB 2001 S. Lincoln Ave. Urbana, IL 61802 Voice: 217-333-2907 Fax: 217-244-2988 dairydoc@uiuc.edu

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