By Gary Hartwig and Liz Morrison

Later this summer, you’ll finish applying herbicides for the season. Then you’ll be about to evaluate how well your weed control program worked in 2017 — and how you could improve it in 2018.

“August is a good time to reflect on weed management,” says Lisa Behnken, University of Minnesota Extension educator and weed scientist.

If you had failures, what could you have done differently? What worked well this year, and how can you repeat that success to be successful next season? However, you should be prepared to change a successful, effective program to combat herbicide resistance.

As you review 2017, ask yourself if you achieved the goals of a successful and sustainable weed management program. Did you:

• Identify the weed spectrum in each field?

• Use practices that discourage the development of herbicide-resistant weeds?

• Use multiple, effective herbicide modes of action?

• Apply effective preemergence residual herbicides?

• Make well-timed postemergence applications with residual activity that included multiple, effective herbicide modes of action?

• Include non-chemical tactics?

• Limit the number of weed seeds entering the soil?

• Prevent yield losses from weed competition?

Weeds that escaped
Which weeds escaped? The first step in assessing your weed control program is to identify surviving weeds so you can target them for future control. Also, document which fields had significant weed escapes this year and will therefore contribute most to the weed seed bank. Next season, give these fields extra attention.

Why did weeds survive? It’s important to understand why some weeds survived, in order to avoid the same problems next year.

The most obvious way that weeds survive is by avoiding herbicide exposure altogether:

• Did survivors emerge after your preemergence (pre) herbicide had degraded to ineffective concentration, nontoxic levels in the soil, due to a low application rate, too-early application or excess rainfall?

• Did weeds sprout after your postemergence (post) application? Pigweeds such as tall waterhemp and Palmer amaranth, for example, have long germination periods. They can grow under the crop canopy and produce seeds.

• Did you apply the right herbicides? Were the products highly rated for control of the problem weeds in that field?

• Was your pre effective? Two of the main factors that influence the performance of a pre are moisture and soil type:

• Was your pre application rate correct for the soil type? Always apply the full labeled rate. “Setup” rates are no longer recommended.

• Was rainfall adequate to activate the pre herbicide? Even if moisture levels are unfavorable after a pre application, you will still get some residual weed control — which decreases weed density and extends the window for timely post applications.

• Was your post effective? Post performance is influenced by weed size, spray coverage and environmental conditions.

• How big were weeds at application time? In the glyphosate era, we were able to kill tall weeds, but those days are over. Today, we must attack weeds when they are “matchbook size,” as Behnken puts it — 2 to 3 inches tall.

Review possible mistakes
Spraying weeds that are too big tall is one of the most common weed management mistakes. Remember that profitable weed control is all about when you kill weeds. Weed competition, especially early in the season, can cost you dearly. U-M trials at Rochester in 2014 showed that corn yields suffered by 40 bushels per acre when weed control was delayed by just five days beyond the optimum time for removal.

• Were your application methods rigorous? Did you use the correct water volume, spray pressure, spray additives and nozzles? Remember that with contact herbicides, such as Liberty, you need to cover every growing point on the weed to get complete control.

• Did you spray when environmental conditions were unfavorable — too cold or too windy? Follow the herbicide label.

Herbicide resistance
Are weeds in the field becoming herbicide-resistant? Resistance to glyphosate (Group 9) is common in hard-to-control weeds such as giant and common ragweed, tall waterhemp, lambsquarters (lambsquarters is not confirmed resistant in Minnesota) and kochia, Behnken says. Stacked herbicide resistance is also on the rise, she says.

Some Minnesota populations of ragweed and tall waterhemp, for example, are also resistant to ALS-inhibitors (Group 2) such as Classic FirstRate (we don’t use a lot of Classic — use FirstRate instead) and Pursuit; and PPO-inhibitors (Group 14), such as Cobra and Flexstar.

When weeds in a field are first becoming resistant to an herbicide, you’ll often find surviving plants right next to dead plants of the same species. That’s a clue to change your management:

• Did you apply multiple modes of effective action against target weeds? To slow resistance development, you should try to hit target weeds with more than one effective mode of action with every spray pass. Tankmixing is more effective than sequential attacks.

• Did you follow a multiyear weed control plan? Mapping out a two- or three-year herbicide plan will help you rotate modes of action and avoid overusing any one herbicide group, Behnken says.

• Did you include non-chemical tactics? Delayed soybean planting, effective preplant tillage and timely in-season cultivation are all feasible ways to control herbicide-resistant weeds. Hand-pulling weed escapes, mowing field edges and cleaning up fencerows helps avoid weed seed bank deposits.

“The time you invest in making a weed control plan, evaluating the plan, walking your fields is well worth it,” Behnken says.

In the era of resistant weeds, what you do on your farm really matters. So take control and take action.

Hartwig is a Bayer Crop Science agronomist from Le Center and chairman-elect of the Minnesota Certified Crop Advisers. Morrison is a writer based in Morris. Find information and links to Minnesota CCAs at mcpr-cca.org.