21/04/2024
Heat stress
Usually, cattle generate heat by digesting feed, producing milk and absorbing solar heat when out in the sun. Situation where the generated and absorbed heat is greater than the heat cattle can get rid of by different means is called heat stress.
Why heat stress is a serious issue?
• Decreased milk production, pregnancy rates, and dry matter intake are observed in heat-stressed lactation cows.
• Higher rates of lameness, disease incidence, and mortality are also associated with heat stress.
• Shorter gestation periods, lower birth weight calves, decreased milk production, and compromised immune systems are all characteristics of heat-stressed late gestation cows.
When does heat stress start to affect cows?
Compared to humans, cows experience heat stress at substantially lower temperatures. Mild heat stress usually starts around 72°F with 50% humidity. Cows with high yields consume more and produce more heat. Even at a temperature of 65°F, they can start suffering from heat stress in well-ventilated barns.
How do you know if a cow is heat-stressed?
Respiration
Respiration rates and body temperature are both good ways to assess how well cows are managing their heat production and losses in warm weather.
Individual cow responses will vary and conditions change throughout the day. You can observe and record respiration rates or measure body temperatures at different times of the day to assess how well your cows are coping with the weather conditions you have and your cooling practices.
You also can monitor daily milk production, but declines in milk production are commonly delayed two or more days after cows begin to experience heat stress.
Physical appearance
Cows experiencing mild heat stress may not show clinical signs at all.
Cows experiencing severe heat stress usually breathe with their mouths open and pant with their necks extended. They are lethargic and may appear unsteady.
Body temperature
Body temperature gives a direct measure of an individual cow’s heat stress level.
• Normal body temperature for an adult cow is 101.5 to 102.5 degrees F.
• If more than 5-10% of cows have a body temperature above 105 degrees F, the situation is considered an emergency and immediate action should be taken.
Milk production
Milk production decreases as the level of heat stress increases.
• Mild heat stress results in a production decrease of about 2.5 lbs. per head per day.
• Mild to moderate heat stress results in a production decrease of about 6 lbs./head/day.
• Moderate to severe heat stress results in a production decrease of about 9 lbs./head/day.
• Severe heat stress is a life-threatening condition and production decreases exceed 10 lbs./head/day.
Preventing heat stress
Many dairy farmers use multiple practices to help their cows manage their body temperature in hot, humid and sunny weather.
Shade
Shade can help reduce the solar heat load and remains one of the first recommendations to help lactating cows and dry cows manage their heat load in hot weather.
Well-ventilated barns and pole sheds are good options for providing shade. There are numerous shade structure options available from building suppliers.
When providing shade, it is important to provide plenty of shade for all of the animals at the same time without crowding. Around 40 square feet per animal is recommended.
Ventilation
Air exchange
Barn ventilation cools cows by providing air exchange between inside and outside. Ventilation can be by either natural or mechanical means.
In hot weather, provide as much air exchange as you can. Usually, this is between 60 and 90 air exchanges per hour.
Tunnel ventilation brings air in at one end of a barn and exhausts it out the other.
• Size fans and inlets correctly and make sure the fans are well maintained and inlets are open to get the desired air velocity.
Cross ventilation brings air in on the side of the barn and exhausts it out the other side.
• Cross-vent barns often use a series of baffles to try to keep the air at cow level and increase the air velocity at cow level.
• Size fans and inlets correctly. Make sure the fans are well maintained and inlets are open to get the desired air velocity.
Natural ventilation depends mainly on wind in hot weather.
• Open ridges allow some hot air to escape out the ridge.
• Fresh air enters through the open sides of the barn.
Air velocity
Increasing the air velocity flowing past a cow can help her get rid of heat. Tunnel ventilation, cross-ventilation with baffles and high-speed mixing fans are ways to increase the air velocity past a cow.
Mixing fans can be mounted above free stalls and feed alleys where cows stand to eat.
• Many fans generate air velocities over ten mph past the cows.
• Mixing fans need to be mounted high enough to not be a hazard to people, cows or equipment moving underneath the fans.
• Point high-speed fans slightly downward (20 degrees) so the air blows past the cows.
Cooling with water
Sprinklers and misters help cows get rid of body heat.
• Sprinklers wet the cow’s skin, and her body heat is used to evaporate the liquid water on the skin.
• Misting systems cool the air by evaporating water droplets using heat in the air.
Both methods increase the air’s relative humidity, but if the barn is well ventilated, the humidity levels should not become excessive.
• Sprinklers and misters are more effective in drier weather with lower dew-point temperatures.
• Sprinklers need to cycle on and off to allow time for water evaporation.
• Avoid excessive sprinkling that causes water to run down and wet the udder because this can lead to an increased risk of mastitis.
• With high-pressure misters, the droplets need to evaporate before they hit the stalls or bedding.
o Place misters near inlets.
o Misters are not as effective when ventilation blows the mist out of the barn before the air cools.
Drinking water
Hydration is incredibly important for a cow to regulate body temperature, both during times of heat stress and cold stress. Adequate drinking water should be supplied at all times.
• In hot weather (90-95 degrees F for a high daytime temp), a lactating cow producing more than 9 liters of milk drinks 25-35 gallons of water per day.
• The recommended amount of space is two linear inches of water space per cow. Consider increasing water space during hot weather.
• Water troughs should be at least three inches deep to allow cows to submerge their muzzle when drinking.
• Cows consume up to 50% of their daily water intake following milking.
• Make sure water flow to the trough and water capacity is adequate to account for large quantities needed at all at once.
