I think one of the most overlooked problems with farmers trying cover crops is not paying enough attention to the herbicide program that was used in the cash crop before the cover crop was planted. We, as farmers, spend a great deal of money to control weeds in our cash crop fields. Over the last several years, many “hard” to control weeds like waterhemp and resistant marestail have pushed many of us to use herbicides that have more residual activity or we have to spray “late” rescue treatments to control late escaping weeds.
These late applications or more persistent herbicides can have a big impact on the cover crop planted that fall. Not that the herbicide will flat-out kill the cover crop, even though it could under the right conditions, but more likely the herbicide will weaken the plant or not let it grow fast enough in the fall to survive the winter.
For several years, here at Illinois Central College (ICC) we conducted trials on herbicide carryover to cover crops. We used many different corn and soybean chemistries at 1X and 2X rates and then planted eight species of cover crops into these herbicides to study their effect on the cover crop. If I had not seen these plots with my own eyes, I am sure I would not have fully understood the problems that some herbicides can have on cover crops that fall. Many times, we would see two to six inches of growth difference on the same plant species just due to which herbicide was used in that plot.
If a farmer had a 40-acre field planted to cover crops and could not see a side-by-side comparison, they would likely have no idea that the cover crop was being negatively affected by an herbicide. The cover might just appear to be growing slowly, exhibit a pale color, or some other barely noticeable growth problem. The farmer may not even notice there is a problem until the next spring when the cover crop does not survive the winter or they have a very poor stand.
My advice would be to plan ahead. If you know a field is going to be planted to a cover crop that fall, then plan the herbicide program accordingly. Try to put as many of the residue products in your preplant or pre-emergent program as possible. Make every effort to avoid late applications of persistent products and spend enough money up front in your herbicide program to not have to make a “late” rescue application. Any product you apply breaks down in the environment over time and becomes less potent, so give those residue products more time to break down before you plant the cover crop. That extra few weeks may mean the difference between a cover crop success or failure and understanding the potential problems from herbicides can make the cover crop much more productive to help achieve your goals.