Introduction:

Chronic stress in dairy cattle is one of the biggest problems for milk production as it reduces production, increases disease in cattle and causes them to reproduce less. Its main cause is the weather. Heat stress has been studied for many years, but the most used parameter is the Temperature and Humidity Index (THI), this comes from meteorological measurements and not from the changes that occur in the cow's body.

Method:

To find the relationship between THI and the body's response to stress, we sampled hair from 50 Holstein cows every two months from an automated intensive dairy farm, for one year, extracted the accumulated cortisol and measured it using the ELISA technique. We also obtained the climatological measurements from the local meteorological reports to calculate the THI and the averages for each period. By means of a multiple regression analysis, the correlation of the bimonthly cortisol concentrations with the averages of temperature, humidity, wind, and THI in the same periods was calculated.

Results:

The changes in cortisol levels in each period were significantly different from the previous one. The multiple regression analysis showed THI as the main regressor (p < 0.001).

Discussion or

Conclusion: The results showed the THI significantly related to the levels of cortisol in hair, this shows that the THI is a reliable parameter, if not to calculate the physiological stress, but to indirectly know the stress that weather can cause and make decisions in the farms of dairy cattle.