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.