Heat Stress in Dairy Cows

A cow's ideal ambient environmental temperature is approximately 50c. With high humidity and a slight increase in temperature depressed yields of 3-6 litres commonly occur. 

The Temperature and Humidity Index (THI) takes into account both ambient temperature and relative humidity, highlighting the point at which heat stress will begin to affect milk production, which is typically above 68.  For example,  this can be at 0% humidity and 270c or 50% humidity and 220c.  When considering the conditions at which heat stress becomes a concern, it is important to take account of both humidity and temperature.

Farmers are also encouraged to monitor temperature and humidity using a multi function environment meter that can easily be purchased. The results will surprise you.

Signs of heat stress in cows

The most common signs of heat stress include:

• Lethargy and depressed appetite (approx 10% reduction in DMI)
• Milk yield drop (can be 3-6 litres)
• Clinical and sub clinical acidosis resulting in depressed butter fats 
• Increased water intake & seeking out salt
• Increased standing times (approx. 2.5hr/day longer) 
• Infertility issues and risk of early embryonic death and potentially abortions
• Immune system breakdown (i.e. increased mastitis or SCC levels)
• Seeking out shade and panting to keep cool (can result in respiratory alkal

Steps to reduce the risk of heat stress

• Improve natural ventilation in housing - open up sheeted gates and remove boards to increase air inlet and ensure sufficient outlet to enable the stack effect. Removal of waste air produced by cows is essential.
• Fans may be installed in cubicle sheds and collecting yards to force ventilation - install longitudinally with spacing of up to 10 times the blade diameter.  Tilted approximately 150 down from vertical; aim at the bottom of the next fan.
• Water supply - high yielding cows can drink up to 150 litres/day in hot weather. Ensure a minimum of 0.09m water trough rim space per cow with good access to water near to the parlour exit.
• Access to shade at grass
• Provision of salt - to replace salt lost via sweating and stimulate water consumption.
• Access to cow brushes - to remove dirt and increase efficiency of cooling the cow.
• Careful monitoring of rations - check rations are not being sorted or heating up.

It is also important to consider the impact of heat stress on youngstock, who perform most effectively between 100c and 290c.  Increased temperatures will result in decreased feed intake and reduced daily live weight gains. Fertility performance of bulling heifers may also be affected.