Illinois Field Crop Applicators Practice Test

When is rainfall most detrimental to herbicide activity?

1. Rainfall less than 30 minutes after application

2. Rainfall 2-6 hours after application

3. Rain during application

4. Rainfall after temperature drops

The correct answer is: Rainfall less than 30 minutes after application

Rainfall occurring less than 30 minutes after herbicide application can severely diminish the effectiveness of the herbicide. This is because many herbicides require sufficient time to adhere to the target plants and begin to be absorbed into their tissues. When rain falls shortly after application, it can wash the herbicide off the leaves or dilute it, preventing proper absorption and reducing its activity against the weeds it is meant to control. In contrast, rainfall that occurs a few hours after application allows for some initial absorption to take place, making it generally less detrimental to herbicide effectiveness. Rain during the actual application could also adversely affect the application process and lead to uneven distribution, but the immediate washing effect from rain shortly after application is particularly damaging. Rainfall after temperature drops may affect plant physiology but is unlikely to wash away the herbicide, thus preserving its intended effect.