Pre-emerge herbicides in the north region are becoming more and more necessary year after year. With the continued occurrence of resistant weeds, growers are trying to find a way to effectively control these weeds thus resulting is cleaner rows and a better chance at maximizing yields, regardless of the production system. Typically, depending on weed spectrum, the most effective use of pre-emerge herbicides are a tank mix of group 14 and 15 herbicides to burndown what has come up already and effectively control what has not emerged yet.

Using pre-emerge chemicals with multiple modes of action will decrease the chance of resistant weed problems. As we continue to see signs of glyphosate resistance to waterhemp, kochia, pigweed, ragweed, a change in the chemistry is something rather important to consider. For example, waterhemp has shown resistance to 6 different herbicide classes. This shows how fast weeds can adapt to continuous use of certain herbicide classes making it that more critical to switch up modes of action year to year. With there being no new herbicide mode of action classes in the near future, growers likely need to adjust by hitting the weeds with residual herbicides that contain multiple modes of action. Another recommendation would be crop rotation. This will give the producer more herbicide options, particularly when the rotation includes corn.

According to cropwatch@unl.edu, several studies have shown that if weeds grow to 9 inches, soybean yield can be reduced by as much as 6%; 12-inch weeds can result in up to 10% yield loss. In corn, 12-inch weeds could cause 22% yield loss when left uncontrolled. Resistant weeds will undoubtedly reach these heights if they are resistant to a certain chemistry.

Download a copy of this technical bulletin here: Effects of Pre-emerge Herbicides