FEEDING
FOCUS June 2018 • Issue 32
Helping cows cope with heat stress it is important to remember the negative impact that heat stress can have on cow performance and put strategies in place to mitigate the harm that it can have on your herd.
Heat stress effects All of these actions use energy and nutrients that would normally be utilised by the cow. The effects of heat stress can be seen by:
What is heat stress? For a cow to achieve optimum performance, their ambient temperature should be between 5°C and 25°C.
• Lower milk yield • Decreased butterfat • Fertility issues • Rumen health problems It is important to remember that cows already generate a high volume of heat, so it does not take a large increase in ambient temperature for cows to start suffering from heat stress. Humidity levels also play an important role in heat stress. The temperature humidity index (THI) is a good indicator of potential heat stress problems.
There will be some variation, according to breed, weight and feeding levels, but this is a useful target temperature range to keep in mind. If cows are left exposed to temperatures in excess of 25°C for prolonged periods of time, heat stress occurs and cows will start cooling themselves down by panting, drooling and increasing internal blood flow.
Heat stress identification Heat Stress Level
THI
Temperature/ Humidity Example
Duration (h/day)
Milk Loss Potential (kg/cow/day)
Stress threshold
68 – 71
22°C/50%
4
-1.1
Mild/high
72 – 79
25°C/50%
9
-2.7
High/severe
80 – 89
30°C/75%
12
-3.9
Adapted from Burgos & Collier, 2011
The use of a hygrothermometer or local temperature and humidity data will help you make heat stress decisions.
Stress threshold 22 °C, 50% humidity Temp (oC)
Humidity (%)
0
5
10
15
0
10
20
30
Mild/high stress 25 °C, 50% humidity
40
High/severe stress 30 °C, 75% humidity
22 25
30
35
40
45
50
50
60
70
80
90
100