As we start to transition into fall, now is a good time to evaluate how well your weed control system worked this year to be vigilant about herbicide resistance issues. However, it should be noted that this year weed control was challenging because drought-stressed weeds can be more difficult to control with herbicides. As you are making rounds across fields this fall, start taking notes of which weeds are your most problematic and in which fields. The first step to effective weed management is knowing what weeds are problematic and in which fields.
Weeds to pay attention to for resistance issues are kochia, common ragweed, horseweed and waterhemp. All of these weeds have confirmed resistance issues in North Dakota. A list of confirmed resistant weeds in North Dakota can be found on pages 96 and 97 of the ND Weed Guide (NDSU publication W253-21). If you suspect to have Powell amaranth, waterhemp or Palmer amaranth in soybean fields with resistance issues, NDSU Extension would like a sample to screen for glyphosate, ALS-inhibitor (group 2) and PPO-inhibitor (group 14) resistance. If you would like to submit a sample, please contact your local Extension office.
Once you have noted which weeds are problematic where take a look back at your spray records for the last year or two to evaluate the effectiveness of the herbicides applied. Were the herbicides applied the best option for the problematic weeds you have left in the field now? Are you rotating herbicide site-of-action? If you are not sure what the site-of-action is of a herbicide, the ND Weed Control Guide or the Take Action on Weeds website (takeactiononweeds.com) are both good resources.
As you start to make plans this winter about what crops will get planted where next spring, take into consideration where the weed problems were this year. Just rotating between a grass and a broadleaf crop can help rotate herbicide site-of-action. Rank which fields should have a PRE herbicide applied first to ensure the fields with the most problematic weed problems get a PRE herbicide applied. Take time to evaluate what herbicide sites-of-action were used this year and try to figure out a way to rotate herbicide sites-of-action next year while still achieving good weed control. This can be very challenging depending on the situation but rotating herbicide sites-of-action is the best way to slow weed resistance issues.
For more information or questions about weed management, contact Alicia Harstad at the Stutsman County Extension office at 701-252-9030 or firstname.lastname@example.org.