Heat Stress this Season
Temperature and humidity both play a role in thermal stress. Using the Temperature Humidity Index (THI) enables farmers to determine the level of summer stress on dairy cows. The Thermo Neutral Zone for dairy cows is 5-20°C, stress symptoms begin at a THI of 68; temperature of 24° and a relative humidity of 20%. With the UK average humidity over 70%, the heat stress threshold is reached at temperatures as low as 22°C.
Impact on yield
Daily exposure to a THI of 68-71 could result in 1.1kg/cow/day less milk and at THI 72-79 you
can expect 2.7kg cow/day less milk (Dussert and Piron, 2012). This is a result of the impaired
rumen balance, the fall in rumen pH and the increased risk of acidosis.
Impact on metabolic requirement
Heat stress causes the animal to increase heat dispersion by increasing subcutaneous blood flow,
panting and drooling. This increases maintenance energy needs by an estimated 20%, meaning
that part of the cow’s production energy will be diverted to thermal regulation.
Most of the heat production in dairy cows is due to rumen fermentation. The cow will reduce
her DMI by 10-30% and be selective to what she eats; less forage, as roughages increase rumen
activity and therefore heat.
Impact on fertility
Heat stress can decrease reproduction performance in three ways:
- Oestrus intensity reduces, heat detection becomes more difficult.
(Gwazdauskas et al., 1981; Wolfenson et al., 1988).
- Fertility is reduced:
• A 10-20% drop in conception rates.
• Pregnancy loss at maximum THI over 69 (Garcia-Ispoerto et al., 2006).
- Early embryo survival is compromised:
• Embryos are more likely to develop slowly and abnormally (Putney et al., 1989).
• Increased oocyte cell death further reducing opportunities for pregnancy success.
(Zeron et al., 2001; Al-Katanani et al., 2002)
• Heat stress is detrimental to the follicle that encloses the oocyte cell and can lead
to more small and medium sized follicles and reduced dominance (Hansen, 2013).