By Amit Jhala, Extension Weed Management Specialist

Due to rain the last couple of weeks and more forecast for next week, some growers may not be able to apply burndown herbicides when they would like to for utmost effectiveness. Low temperatures and wet weather can negatively impact herbicide performance.

Rainfast Period

Herbicide labels include recommendations on how much time must elapse between herbicide application and subsequent rainfall to ensure good herbicide performance. This is known as the rainfast period.

Generally, herbicide rainfast ratings (Table 1) are based on good growing conditions. Poor conditions may require a longer interval between application and any rainfall to ensure adequate herbicide translocation within the weed before the herbicide is washed off. 

For many herbicides, any amount of rainfall soon after spraying has the potential to reduce absorption, translocation and subsequent weed control. If you apply herbicide and it rains before its rainfast, herbicide performance will be reduced.

Temperature

Wide temperature fluctuations in the last couple of weeks have led to questions about possible effects on performance of burndown herbicides. The likelihood of decreased weed control due to cool temperatures will vary, depending upon the target weed, herbicide and rate applied. For example, glyphosate usually performs well under a wide range of temperatures. Best performance usually occurs when the temperature is 60-75 F at application and remains there for a few hours afterward.

When the temperature is lower than 60 F, weed growth slows, resulting in slower herbicide uptake and translocation. This increases the required rainfast period and slows the onset of symptoms and herbicide efficacy. If the temperature is below 40 F, avoid applying glyphosate-based herbicides. 

If a severe frost is predicted immediately following an intended application, it may be advisable to avoid spraying. If weeds are damaged or under stress before herbicide has properly translocated, control may be reduced.

The ideal solution is to wait for better weather conditions. However, if weed size or other situations dictate that the field be treated now, select a herbicide with excellent efficacy on the target species. Reduced herbicide rate treatments are less likely to provide acceptable control under adverse conditions than when plants are actively growing.

For more information on burndown herbicides, see the 2016 Guide for Weed, Disease, and Insect Management in Nebraska.