Monday, February 6, 2023

Early Season Weed Control Options

03-11SScvr-smGlyphosate-resistant weeds, especially Palmer amaranth, have soybean growers in the South once again pondering their early season weed control options.

By Bob Scott

Before the introduction of Roundup Ready soybeans, weed control in soybeans began with the selection of a residual herbicide that best fit the grower’s needs.

At that time, total postemerge weed control in soybeans was possible, but much more difficult than a two-pass Roundup system later would become. Residuals all but went the way of the dinosaurs during the 2000s.

The occurrence of glyphosate-resistant weeds, especially Palmer amaranth or Palmer pigweed, has switched that focus back on early season options. The need to start clean and stay clean has never been greater.

Residual herbicides are key

I am telling every grower group I talk to that the key for success in the face of resistance is to be proactive. It requires planning. This means using a residual up-front.

It does not matter if you are growing conventional, Roundup Ready or LibertyLink, the use of a residual to get things started will make for a much better program than trying to go total post.

In the face of resistant pigweed, the worst thing you can do is use the first glyphosate application to find out if you have resistance or not. We recommend the use of a residual product across the board in LibertyLink soybeans.

We are also recommending much earlier post applications than what have been going out for the past few years. For many weeds, including and especially pigweed, we want to see the first application go out no later than on two- to three-inch weeds. If a residual is not used, this is probably going to be 10-14 days after soybean emergence.

It will be hard for us in Arkansas to spray all these soybeans 10-14 days after they come up. This is where a residual product may help. When activated by rainfall, I expect to get at least two to three weeks of residual control of pigweed, for example. Hopefully, this will buy me some time to make my post applications as the residuals break and not all at once.

Burndown prior to planting is a good time to add a residual product to the mix. We will use a lot of Gramoxone (paraquat) prior to planting soybeans in no-till and stale seedbed areas, especially where pigweed is a problem. Valor herbicide, for example, has a great fit here to provide some residual activity prior to planting soybeans.

Be wary of ‘salvage’ treatments

Where a residual is not used, I recommend a product like Dual, Prefix or Warrant be added to the first post application of Roundup or Ignite for resistance management and residual grass and pigweed control.

Flexstar (fomesafen) and Ignite (for LibertyLink soybeans only) are two key products in the fight for pigweed control. They will provide control of two- to three-inch pigweeds when applied post. Any larger, and it will be a miss on some.

This worries me because it could lead to the development of resistance to these products. Last year, for example, a lot of Flexstar was used as a “salvage” treatment on pigweed, and it will not kill big pigweeds. I am afraid that some of the surviving plants may be more tolerant to Flexstar. In other words, late applications of Flexstar or Ignite will lead to selection for resistant biotypes.

Prevention of the further development of resistant weed bio-types is another good reason for using a residual up-front on at least some of your soybean acres this year.

Residuals: Return on investment

How about money? Residual herbicides are not free; however, in study after study, what weed guys like me see in our plots year after year is a consistent advantage to early season weed control.

Starting clean and keeping troublesome weeds out of the field for the first three to four weeks of the growing season will usually result in an additional three to four bushels per acre of yield.

This is not that much if beans are at $4 to $5, but it gets my attention a little more if they are more than $10.

If you have glyphosate-resistant pigweed or horseweed, then you are probably already using a residual herbicide. It is not really a choice but a requirement for clean fields.

The addition of a residual applied pre or post will help prevent the development of resistant weeds on your farm. It’s true it can add cost to your weed control program; however, it could also eliminate the need for sequential post applications in-season and increase yields.

For more information on soybean weed control and weed control in other crops, check out the “MP44,” Arkansas’ Weed Control Guide online at

Bob Scott is an Extension weed scientist with the University of Arkansas Division of Agriculture.

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