Both temperature & humidity play a significant role in how your herd will fare this summer. Using the Temperature & Humidity Index (THI), particularly on warm days, can give you a better handle on managing heat stress in your herd…
Heat stress is caused by a combination of temperature, relative humidity, solar radiation, air movement and precipitation. The majority of studies on heat stress in livestock focus on the two main environmental stressors: temperature and relative humidity (RH).
The Temperature Humidity Index (THI) was developed and has been used since the early 1990’s to account for the combined effects of temperature & RH. It is a useful way to assess the risk of heat stress compared to just measuring temperature alone as RH has a large effect on a cow’s ability to cool itself through evaporative means. To calculate the THI on your farm at various parts of the day & night you need to be able to measure both the temperature & the RH. If you don’t have a Hygrometer then simply download the Hygrometer: check the humidity app from the Appstore 👇
There are different formulas to calculate the THI, but one of the simplest and most commonly used is:
THI = 0.8*T + RH*(T-14.4) + 46.4
- T = ambient or dry-bulb temperature in °C
- RH=relative humidity expressed as a proportion i.e. 75% humidity is expressed as 0.75.
Eg. On a hot summer’s day on your farm you may measure the ambient temperature at midday to be 32°C & the RH to be 22%. In this scenario:
THI = 0.8 * 32 + 0.22 * (32-14.4) + 46.4
THI = 75.9
Multiple studies indicate a stress threshold of 67 THI, so animals are experiencing heat stress at a THI of 68 and greater. The levels of stress are separated into:
- Mild (68 to 71 THI)
- Moderate (72 to 79 THI)
- Severe (80 to 89 THI)
You can also use the THI table below to see where your farm is sitting with regards to heat stress thresholds:
Quick THI facts:
– Mild heat stress begins in dairy cows @ 68THI, although research is proving that it is more like 65.
– When THI exceeds 72, cows are likely to begin experiencing heat stress and their in-calf rates will be affected.
– When THI exceeds 78, cows milk production is seriously affected.
– When THI rises above 82, very significant losses in milk production are likely, cows show signs of severe stress and may ultimately die.
– When THI rises above 80:
- 8-12%: decrease in dry matter intake (DMI)
- 10-25%: decrease in milk production
- 35-50%: of the decline in milk yield is due to decreased DMI
Call Eagle Direct today and talk to us about your current farm setup. We’d love to make sure you and your cows stay cool this summer.