Field Notes Talks Post-Emergence Weed Control

Jun 17, 2025

By Angie Peltier and Ryan Miller

Weed growth progressing

While recent cool and cloudy weather may have temporarily delayed both crop and weed development, folks that were able to get their crops planted in mid-May may now be seeing 4-6 inch tall giant ragweed plants. Waterhemp, the prolific and multiple herbicide-resistant bane of many Minnesota agronomic crop producers, and lambsquarters are now also reaching 4 inches in height. This information is the equivalent of a giant banner reading, “if you didn’t start post-emergence herbicide applications yesterday, start them today.”

Depending on your location and planting dates, timely rains may have come after planting and pre-emergence herbicide applications, resulting in good activation. Pre-emergence herbicides act on germinating weed seeds as the active ingredient is taken up by the seed along with the water into which it has dissolved. In the event of delayed rains after planting and pre-emergence herbicide applications some weeds likely emerged unaffected by pre-emergence herbicides requiring sooner than anticipated post-emergence applications.

Herbicides often require an actively growing plant for good efficacy and those products with some residual activity will require a timely, activating rainfall. Now that most crops are up and so are many weeds, the forecasted good growing conditions with warmer temperatures followed by rain mean that post-emergence herbicide applications are likely to work well.

The Label is the Law: Herbicide Cutoffs and Max Use Rates

While we apply them to manage weeds, ideally herbicide applications don’t also ‘manage’ our crops. Herbicide labels have various restrictions that help us to avoid injuring both this year’s and next year’s crop. This time of year, it is important to revisit label restrictions related to the current crop’s stage of growth and development. After corn has gotten 12 inches tall, making an atrazine application would be off-label (aka: illegal). A careful perusal of labels of those products you plan to use is also required for those premix products such as Resicore and Acuron that might contain atrazine or some other active ingredient with a crop growth stage-related restriction. 2,4-D herbicide formulations also have an 8-inch height restriction. The University of Minnesota Extension Weed Science team is currently finishing up an article that will be posted on MN Crop News detailing herbicide crop-development-related application restrictions.

For some corn products that can be used both pre- and post-emergence, it is easy to get awfully close to the maximum use rate in any given growing season. Using more than the maximum label rate is not economical, is illegal and can increase the chances of your 2026 crops suffering carryover injury. While folks can use products containing fomesafen or lactofen for soybeans (ex. Flexstar and Cobra, respectively) throughout most of June, careful attention to the calendar is key as fomesafen containing products have a 10-month corn rotation restriction and as we head into July we need to consider making product changes to avoid carryover injury. Dry or droughty weather can also lengthen the time period needed for active ingredients to break down in the soil through either microbial metabolism or hydrolysis, exacerbating carryover issues.

Source : umn.edu