It’s not just your imagination, weeds are getting harder to kill. Prior to the last postemergence herbicide application going on this summer, take a few moments to assess whether these steps will help you reduce the presence of new herbicide-resistant weeds at the end of the growing season.

• Scout, scout, scout!  Keep a close eye on fields to monitor for weeds escaping preemergence applications. This will allow you to recognize troublesome weed species and tailor postemergence applications for them. It may also save you money by allowing you to remove herbicide products that are targeting issues not present in the field. Each field should be scouted individually, as weed problems differ across landscapes and past management practices.

• Choose an effective postemergence option for the weed spectrum present. With the quick expansion of herbicide-resistant weeds, especially waterhemp, effective postemergence herbicide options are dwindling. Knowing the current resistances of weed populations is key to choosing herbicides that will kill weeds. Many farmers have noticed a rapid expansion of HG 14 (PPO) and HG 9 (glyphosate) resistance in the last several years, so some may be looking toward HG 4 (growth regulators) and HG 10 (glufosinate) products for waterhemp management in soybeans.

• Make applications in a timely manner. It’s better to err on the side of too early rather than too late. Since waterhemp is the key pest in most fields, it’s important to spray when waterhemp is within the size suggested on herbicide labels for maximum control. On most labels, waterhemp needs to be less than 4 inches tall, but when using contact herbicides, spray when waterhemp is 2 inches or shorter.

• Include a residual herbicide with your postemergence spray. Late-season emergence of waterhemp is a serious issue. While these weeds likely don’t cause yield loss, late weeds contribute significantly to the weed seedbank. Each late-emerging female waterhemp plant may be adding 20,000 or more seed to manage in future years. While including a residual product with postemergence herbicide applications may not stop all the weeds, it will extend weed control later into the season. This reduces the number of late-season emergers to contend with and reduces the likelihood of adding resistant seed to the seedbank.

• Set the sprayer up for success. Providing good coverage while limiting off-target movement is the goal. The herbicide label is a good place to start when looking for guidance in sprayer setup for a particular product application. You can also consult the manual from the sprayer manufacturer and a trusted adviser for more detailed information or clarification.

• Evaluate control after you spray. A key that many forget is to check fields seven to 10 days after a herbicide application to make sure it was successful. Look for surviving plants and try to determine why. Plants that were too large when sprayed, plants protected from spray coverage, and herbicide resistance are three common reasons why weeds survive herbicide applications.

• Clean up any stragglers. Inevitably, it seems that weeds escape herbicide applications. Any effort taken this year to remove these plants will pay dividends in future years given the seed production and longevity of seed in the soil.

Anderson is the ISU Extension field agronomist in east central Iowa. Contact her at [email protected].