Reducing heat stress? Don’t forget the younger generation

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When hot weather rolls around most dairy producers tend to think about reducing heat stress for lactating cows – with good reason. However, dairy young stock from calves to springing heifers can also benefit from improved hot weather comfort.

Heat stress can force changes in a bovine’s physiology and behavior to cope with its immediate environment to avoid physiological malfunction. Cattle perform effectively in a comfort zone known as the thermal neutral zone. The thermal neutral zone for mature dairy cows, in still air, is 40 degrees to 75 degrees F, for calves 50 degrees to 85 degrees F, and somewhere in between each end of these ranges for growing heifers. When temperatures exceed the upper level of the thermal neutral zone, efforts to dissipate heat from the body begin. This includes dilation of blood vessels, panting, sweating, and changes in posture. Feed intake is also reduced to minimize heat production in the body. For heifers this results in reduced daily weight gain. Heifers are able to dissipate heat more effectively than cows, but that doesn’t mean they aren’t susceptible to heat stress. Signs of heat stress include increased respiration rates (greater than 80 breaths per minute), panting, higher rectal temperatures (above 103 degrees F), and reduced feed intake.

The effects of heat stress on lactating dairy cows are well documented. Similar studies on heifers are very limited, but effects on mature cows suggest heat stress can affect heifers as well. Cattle conception rates are typically lower during the summer. Higher uterine temperatures may reduce embryo survival rates from estrus onset to about one day post-insemination. Studies of dry cows experiencing heat stress indicate shortened gestation length, lower calf birth weight, and increased fetal abortions. Heat stress may also have a negative impact on postpartum cow health, production and reproduction.

The steps in reducing heat stress in dairy calves and heifers are similar to steps used with cows. Adopt the acronym ‘SAAWW’.

‘S’ represents shade that reduces solar radiation on young stock. Shaded cattle indicate they feel ‘less hot’ by lower body temperatures and respiration rates compared to cattle exposed to direct sunlight in warm weather. Shade trees, portable and permanent shade structures, and well ventilated buildings all can provide effective protection from direct solar radiation.

A’ stands for the air exchange necessary to control levels of moisture, heat, gases, and pollutants in the animal space. Favorable summer exchange rates are typically 45 seconds, or better. Naturally ventilated structures can benefit from increasing sidewall, end wall, and ridge openings. Buildings with poor or questionable air exchange that cannot be naturally ventilated typically require exhaust fans and properly placed fresh air inlets.

A’ also represents air movement necessary to improve the rate of convective heat transfer from animal to the surrounding environment. It also enhances evaporative cooling. To be beneficial air speed across the animal needs to be 2.5 to 5 miles per hour (mph). Locate circulation fans in areas where animals spend their most time, such as the resting area, and spaced to provide uniform air movement throughout the animal area. Keep in mind that circulation fans do not exchange air; a good air exchange is essential before air movement can be effective.

W’ stands for drinking water. Heifers typically consume 1 to 1.5 gallons per 100 pounds of body weight per day. Dairy animals use panting, more frequent urination, and sweating to increase heat transfer from their body to the environment. To prevent dehydration, water consumption may increase by 20 percent or more during hot weather. Convenient access to a continuous supply of clean, fresh water is essential to help cool and satisfy physiological processes of dairy cattle.

W’ also represents water used for evaporative cooling. The process of converting water into vapor (evaporation) requires heat. Direct and indirect evaporative cooling systems are used with dairy cattle. Direct cooling uses heat from the animal’s body to evaporate water directly from the skin. Intermittent spray cooling at the feed area or crossovers are effective methods used on Pennsylvania dairies. High pressure misting and evaporative pad cooling systems use latent heat in the air to evaporate moisture, cooling the air surrounding the animals (indirect cooling). Studies using water for cooling heifers are hard to find, however, based on the effect of heat stress on reproductive performance of mature cows, using water for cooling breeding age and bred heifers during hot weather may reap some benefit.

Although studies related to the effect of heat stress in dairy heifers are very limited, common sense suggests growing heifers can benefit from using heat stress abatement methods used with mature dairy cattle. Keeping heifers within their thermal neutral zone during hot weather can keep them comfortable, help maintain daily weight gain, and improve conception rates at breeding age.


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