Weed control decisions now crucial to season-long clean fields

• By Wesley Everman •

arkansas palmer pigweed
Your weed control program before planting may affect weed control later on.

As we start wrapping up corn planting, many people are looking toward planting some soybeans. While many other inputs are important considerations at planting, weed management decisions this time of year can be critical to maintaining clean fields throughout the season.

Starting with clean fields at planting and applying a residual to keep them clean for the first part of the season will allow flexibility when making the first postemergence application, regardless of what postemergence herbicide program you have planned. With that being said, your postemergence program should be considered when choosing your preemergence herbicides in order for them to be complementary.

There are many excellent preemergence herbicides available to use in soybean, however understanding their strengths and weaknesses will allow you to choose the appropriate products for your system. Consult the the Weed Management section of the portal and the NC Ag Chem Manual for specific recommendations for rate and use considerations.

While postemergence herbicide program should be considered when planning a preemergence program, there are several options that are effective regardless of postemergence use. A strong preemergence program utilizing two modes of action should be applied to help take selection pressure off the postemergence herbicides. Where pigweed or ragweed are the primary concern, a PPO-inhibitor (Group 14) such as flumioxazin (Valor, Envive, Fierce, etc), sulfentrazone (Spartan, Authority products), and others will provide an excellent foundation for control.

In addition to a Group 14 herbicide, we should be adding in a Group 15 herbicide (Dual Magnum, Outlook, Warrant, Zidua). Many people are using the Group 15 herbicides to control Palmer amaranth, quite effectively. It will be important to mix them with Group 14 or metribuzin where possible to take selection pressure off them.

There have been reports of pigweed resistant to Group 15 herbicides in other parts of the country, and we need to avoid selecting for this resistance.

Sicklepod on the uptick

The past five years or so, we have observed a resurgence of sicklepod. There really are only a couple of preemergence options that are effective on sicklepod. Top of the list is Scepter (imazaquin). Other broadleaf preemergence herbicides will give partial control or suppression, but in areas with heavy sicklepod infestations, Scepter should be applied. This is primarily a concern in conventional soybean, other systems have effective postemergence options.

Sicklepod is on the uptick in soybean fields — photo by Rebekah D. Wallace, University of Georgia; Bugwood.org

If you are planting conventional soybeans this year, remember excellent grass control can be achieved postemergence, however, broadleaf control will be limited to PPO-inhibitors or ALS-inhibitors, depending on weeds present. Realize timing will be critical, and PPO- and ALS-resistant biotypes of several species exist in North Carolina.

Therefore you should keep this local information in mind when choosing your preemergence program. Our research has shown that a PPO-inhibitor applied preemergence still has activity on pigweed or ragweed. This is not necessarily true for ALS-resistant biotypes. If you are planning to plant Roundup Ready2 soybean with no stacked traits, I would plan my preemergence program like conventional soybean where Palmer amaranth and common ragweed are present. There are still people planting straight Liberty Link soybean with no stacked traits.

A strong preemergence program will be essential for preemergence grass and Palmer amaranth control. This is especially true to allow time to make timely postemergence applications of Liberty. Utilize Group 14 and 15 herbicides to control a broad spectrum of weeds and set up the system for success. Again, metribuzin may be utilized where soil and variety allow to help take selection pressure off the other modes of action.

Those who are planting either of the auxin traits (Enlist or Xtend/Xtendflex) have the greatest flexibility with regard to postemergence weed control. However we should not become complacent at planting and take the preemergence herbicide for granted. A strong preemergence program utilizing two modes of action should still be applied to help take selection pressure off the postemergence herbicides.

The programs described above should be sufficient to provide excellent early season weed control. While I mentioned the Group 14 and Group 15 herbicides often, and they are widely used, I want to stress there are other options to consider. The yellow herbicides still are excellent grass control products, work on our weed spectrum in North Carolina, and can be a place we can save a few dollars while not sacrificing grass weed control.

They can also be effective on Palmer amaranth when activated properly. I have mentioned metribuzin several times, and I think this is an excellent option for several of our difficult to control broadleaf weed species. Additionally, there are many premixes available, consult the NC Ag Chem Manual and your local extension agent for recommendations that fit your specific mix of species and soils.

Dr. Wesley Everman is Extension weed specialist and associate professor, Crop & Soil Sciences, at North Carolina State University. He may be reached at wesley_everman@ncsu.edu

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