Spray season rainfall can cause a lot of problems for growers, both by delaying herbicide application and reducing herbicide retention by washing the product off leaf surfaces. As a result, rainfastness – the time needed between application and rainfall to avoid reduction in efficacy – quickly becomes top of mind for pulse growers when rain clouds are on the horizon.

While rainfall after application of post-emergent herbicides can reduce weed control, the extent that control is affected will depend on the amount, intensity and duration of the rainfall, the interval between spraying and rainfall, the herbicide formulation, and the adjuvants or surfactants being used.

The following tips may help reduce the impact of rainfall following herbicide application in your pulse crop:

1. Spray fields that require herbicides that have the greatest rainfastness first.
2. Choose products with the greatest rainfastness.
3. Remember that uptake is quicker in warm weather.
4. Follow the label directions for adjuvants (which include compatibility agents, drift retardants, suspension aids and spray buffers) and surfactants (which can improve the dispersing/emulsifying, absorbing, spreading, sticking and/or penetrating properties, and rainfastness and photo degradation of a spray mixture.) Surfactant classes providing rapid rainfastness include esterfied seed oils, organo-silicates, and most nitrogen-surfactant blends.
5. Choose herbicide formulations – such as ester (oil-based) formulations – that absorb more quickly than others.
6. Use positively charged (cationic) herbicides, rather than negatively charged (anionic) herbicides, when possible. Consult your input provider for this information.
7. Use the longest rainfall time interval of all the products in the tank mix.
8. Consider that a light rain or dew may increase herbicide retention by rewetting spray droplets on the leaf (but only if no spray runs off after application).
9. Consult with your agronomist or input provider to determine your best course of action.
10. Check the herbicide label and Provincial crop protection guide for rainfastness information. If the information is not available, assume the product needs an eight hour interval between application and rainfall event.

This article was adapted from information provided by Josie Van Lent, Associate Dean of Agriculture and Environmental Sciences, Lakeland College in Vermilion, AB.