Heat stress in dairy calves

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Calves attempt to maintain a constant body temperature regardless of the outside temperature, and within a certain temperature range—called the thermoneutral zone—calves can accomplish this without expending extra energy.

The boundaries of the thermoneutral zone are affected greatly by the effective ambient temperature experienced by the calf, which depends on air movement, moisture, hair coat, sunlight, bedding, and rumination. Many of these factors can be influenced by the housing and environment in which the calf is placed. Each of these factors affects temperature regulation, and the impact may differ in summer and winter.

How Hot is Too Hot?

In comparison to adult animals, calves may be better able to cope with warmer temperatures due to their large surface area relative to their body weight and also due to the much smaller amount of heat generated by calves compared to cows.

Observations of calf performance in summer months show that average daily gain declined as average nursery temperature over the calf’s first 2 months of life increased from 20 to 80°F and suggest that calves may not be able to dissipate accumulated heat from their bodies when daily low temperatures (in calf housing, not the outside temperature) exceed 77°F (Bateman et al., 2012).

It seems that calves, like cows, experience less stress when temperatures drop overnight; periods with no night cooling provide no opportunity for accumulated body heat to dissipate. Grain intake is reduced and the energy required to regulate body temperature increases (a maintenance cost), so feed efficiency decreases and weight gain may suffer during heat stress. Rumen development may be slowed by reduced grain intake, leading to a more difficult transition and a growth slump after weaning.

In addition immunity can be compromised if energy is redirected to cooling functions. Body temperature rises as calves experience heat stress, and if it reaches approximately 108°F calves are very likely to die from heat stroke.

Here are a few strategies to help calves beat the heat:

Provide Shade

Studies have shown that providing shade reduces the temperature inside hutches and lowers calf body temperature and respiration rate. Shade may be from solid roofing, 80 percent shade cloth, or by moving hutches to an area shaded by trees. Calves confined to hutches may be at greater risk of heat stress than calves that are able to choose where they lie.

Providing a pen in front of the hutch or using a tether allows calves more freedom to select a comfortable spot. Calves housed in barns with solid roofs have built-in shade, but depending on the layout, some pens may experience more direct sunlight than others. If calves do not have the ability to move out of direct sunlight, shade curtains may provide some relief. In greenhouse-style barns, clear plastic covered with shade cloth or white plastic have been found to be equally effective in blocking solar radiation.

Move More Air

Calf housing should be positioned to utilize prevailing winds and should incorporate as many openings as possible to take advantage of natural air movement. Typically, open-faced buildings should face southeast. Hutches may be turned to face east in summer to maximize air movement and minimize solar heating.

Placing hutches 4 feet apart with 10 feet between rows allows air to circulate freely. Air movement can be enhanced by opening vents on hutches and placing a block under the back wall (be sure to maintain this opening as bedding builds up inside the hutch). In calf barns, fans can help keep calves cool and improve weight gains.

Depending on the facility, continual adjustments may be needed to keep ventilation adequate as weather changes occur, and automated controls can be very helpful. Greenhouse-type barns with transparent or translucent coverings will require more frequent adjustment than buildings with wooden or opaque roofs. Once temperatures reach 75°F, curtain sidewalls on calf barns should be completely open.

Offer Plenty of Water

As calves attempt to maintain their body temperature water is lost through increased respiration and evaporative cooling (sweating). In an Iowa study when the temperature exceeded 77°F calves increased water consumption independent of their grain intake (Quigley, 2011).

Whether calves are eating grain or not, they need to have access to water in hot weather. In addition, it may be critical to introduce each calf to water to insure they understand water is available. For scouring calves, early and aggressive use of fluid therapy is especially critical during hot weather; feed electrolytes at the first sign of scouring to help calves avoid dehydration.

Water buckets also may need to be filled more frequently (or switched to a larger size) in the summer, particularly for calves nearing weaning and those who have recently been weaned.

Keep Grain Fresh

Calves will naturally tend to eat less grain during periods of heat stress. This means efforts to encourage starter intake take on added importance. Offer only small handfuls at each feeding until calves begin to eat starter. Remove uneaten starter and clean out wet or moldy feed daily to maintain freshness. A divider between the grain bucket and water bucket can help keep starter fresh longer by limiting the amount of transfer between the two buckets.

Consider Inorganic Bedding

Inorganic bedding is preferred by some calf raisers as it helps keep calves cooler by absorbing body heat and dissipating it, rather than retaining it. Regardless of the material used for bedding, the priority should be to provide a clean, dry area for calves to rest.

Work Calves in the Morning

As with other classes of cattle, it is wise to handle calves in the morning so that stressful activities, such as dehorning, vaccinations, pen moves, or transportation, can be completed when both calf body temperatures and environmental temperatures are at their lowest point for the day.

Consider Feeding More Milk Replacer

There is a considerable body of evidence that most Holstein calves should be fed more than the conventional 1 pound of powder per day, simply based on the maintenance requirements of calves with typical body weights.

Cold stress gets more attention than heat stress, but as temperatures drop calves often increase their starter intake to help meet their increased energy needs (this is especially true for calves older than 3 weeks of age). Thus, in a period of cold weather, calves that have started eating grain can often meet their increased energy needs by eating more grain.

During heat stress, however, starter intake often stalls or goes down, leaving calves with less energy available to support their increased maintenance requirements. Generally speaking, healthy calves are unlikely to refuse to drink milk, so there is opportunity to increase the amount of energy provided to calves by increasing the amount of liquid feed offered.

References

Bateman, G., II, and M. Hill. 2012. How heat stress impacts the growth of calves. Progressive Dairyman. 26:55-57.

Quigley, J. 2011. Calf Note #158 – Water for calves – cold or warm? Accessed May 24, 2013. http://www.calfnotes.com/pdffiles/CN158.pdf.


This article is an excerpt from a new Penn State factsheet. Read the full version here.



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