With summer approaching it’s time to keep an eye on the Temperature-Humidity Index (THI).  When the minimum THI is greater than 65, or the average is 68 for 17 hours/day it negatively affects cows giving more than 77 pounds of milk a day.  Research is also suggesting that reproduction can be impacted at THI levels as low as 55.

“Mitigating milk production losses is oftentimes the focus when we think about heat stress,” says Dr. Kevin Herrick, senior manager, dairy and livestock technical services at Purina Animal Nutrition. “However, research shows that reproduction is impacted at a THI much lower than is typically associated with heat stress.”

The relationship between THI and early embryonic death was investigated in a research paper by Garcia-Ispierto et al., 2006. The aim of their research was to establish whether THI values from days 1 to 40 of gestation could be associated with pregnancy loss in dairy cows. Data from 1,391 pregnancies were evaluated and pregnancies were determined by trans-rectal ultrasonography. Maximum THI was used to evaluate heat stress and was calculated using the daily maximum ambient temperature and the minimum relative humidity. Figure 1 illustrates the pregnancy loss rates compared with ranges of maximum THI values during days 21-30 of gestation.

“What is interesting in this data is that even at maximum THI values of between 55 and 59, there was already some pregnancy loss,” says Dr. Herrick. “It wasn’t until maximum THI values dipped below 55 that there was no observed pregnancy loss.”

Researchers indicate that the likelihood of pregnancy loss increased by a factor of 1.05 for each unit increase in mean maximum THI from days 21 to 30 of gestation. 

Based on recent reports, it is clear that high producing dairy cattle are already affected at THI values much less than the traditional threshold value of 72. It is more likely that negative effects on production are occurring when the THI is 65 and negative effects on reproduction when THIs is as low as 55.  For accurate THI values, its recommended producers measure THI values based on the conditions inside the barn, not the general weather report of the area. Dr. Kevin Herrick on THI

For more information, contact Dr. Kevin Herrick by email: kherrick@landolakes.com or go to: www.dairyfeedtechnology.com.