Penn State University agronomists say there are a couple things to consider when determining whether to control weeds before planting a cover crop.
1. Are there sufficient weeds in the field that will compete with a young cover crop that could reduce the stand going into the winter?
Weed specialist Paul Curran says that if the field is green with summer annual weeds that are going or have already gone to seed, you may still want to burn down the residue with a product like Gramoxone to quickly stop weed competition.
"If summer annual weeds are scattered throughout the field and are at the end of their lifecycle, then an herbicide is generally not necessary," Curran says.
2. If winter annual weeds have emerged (henbit, chickweed, etc.) or more importantly, perennials (pokeweed, Canada thistle, etc.) are still active, then a timely herbicide application prior to seeding the cover crop can pay off.
"Not only can you potentially better control the perennials with the fall application, but you will also help the cover crop succeed," Curran says.
Glyphosate can be safely used prior to the emergence of any crop, while the PGR herbicides, such as 2,4-D, will persist for a week or two and dicamba up to several weeks, depending on the rate. Curran says both can cause injury to cereal grains, ryegrass, etc. (and certainly legumes) as the seedlings attempt to emerge and establish.
As an alternative, Curran says some farmers are using shallow tillage tools (i.e. vertical coulter/rotary harrow, such as the Great Plains Turbo-Till) to manage residue prior to planting.
"These types of tools can provide some limited weed control for small annual weeds," he adds. "We have been testing a similar type of tool in the spring prior to corn and soybean planting.
"Last spring, we had a fair amount of common chickweed in our soybean trial. One no-till plot received a vertical coulter/rotary harrow treatment. Some of the chickweed had recovered — about 70% control 10 days later — but we would expect better control in the fall when winter annuals are smaller and more susceptible."