Marestail that escaped from pre-emergence herbicides in soybeans.

Now is the time to monitor your crops and apply cleanup weed control as needed.

As we proceed through the growing season, there will be several weed control issues that are common in corn and soybean fields. Extension agronomist Dwight Lingenfelter offers these items to consider.

Remember the June 30 application deadline for XtendiMax and Engenia. Cutoff dates for application of XtendiMax, Engenia, and Tavium are quickly approaching.

These herbicides cannot be applied to Xtend or XtendFlex-platform soybeans after June 30; other growth stage limits may apply depending on the product.

Alternative herbicides will need to be used after June 30, depending on the type of soybean that you planted.

Consider using products such as glyphosate, glufosinate, Pursuit, Raptor, Classic, FirstRate, Reflex/Flexstar, Cobra and Ultra Blazer.

However, keep in mind these products also have limits on how late they can be applied, either by soybean growth stage or days prior to harvest.

If you have Enlist E3 soybeans planted, the Enlist products can be applied through R1 (flower initiation) soybean growth stage, with no calendar date restrictions.

Be sure to scout your soybean fields for horseweed/marestail. There are populations in Pennsylvania that are both ALS- and glyphosate-resistant. In these cases, here are some suggestions.

In Xtend soybean platforms, XtendiMax, Engenia and Tavium can be used to control marestail. If LibertyLink or LLGT27 beans are planted, glufosinate (Liberty, others) at higher rates (32-36 fluid ounces) is an option and more so if tank mixed with 2,4-D choline (Enlist One/Duo) in an Enlist E3 system.

XtendFlex soybeans also are tolerant to glufosinate.

If your populations are not ALS-resistant, then FirstRate (0.3-0.6 ounces per acre) can still provide adequate control of marestail.

In a standard Roundup Ready system, a 2x rate (1.5 pounds acid equivalent) of glyphosate or a couple of in-crop applications of glyphosate usually stunts marestail.

It is always best to treat them soon after they start regrowing from the burndown application.

This will not control them but might suppress them somewhat if they are sprayed early enough.

Keep in mind, marestail plants are generally not very tolerant of shade, and most soybeans will begin to canopy over the marestail and outcompete them.

In general, the postemergence PPOs (Reflex, Cadet, Cobra, Ultra Blazer, etc.) are not effective on marestail, but the pre’s (Valor, Authority, etc.) are.

Palmer amaranth and waterhemp are also weeds of concern in soybean. Populations of these weeds continue to be found around the state.

Make sure to scout fields early to determine what kind of pigweed species are in your fields. The sooner you ID them the easier it is to manage them.

There is still plenty of time for them to emerge this season and cause problems. Once Palmer and waterhemp reach 4 inches tall they are very difficult to control, and they grow quickly.

Aside from XtendFlex or Enlist soybean systems, consider the following options.

Assuming glyphosate will still be used, the addition of a Group 14 (Reflex, Cobra, Ultra Blazer) plus a Group 15 (Zidua/Anthem Maxx/Perpetuo, Dual/Prefix, Warrant/Warrant Ultra) will be necessary for adequate control through the season.

The use of Liberty (glufosinate) in LibertyLink, Enlist E3, XtendFlex or LLGT27 varieties provides very good control of Palmer and waterhemp.

University research has shown that a tank mix of Liberty and Enlist One in E3 soybeans provides effective control of these problem pigweeds.

Pokeweed can be controlled with several herbicides in corn, including glyphosate, 2,4-D, dicamba, Status, and Callisto plus atrazine. Tank mixing provides the best control.

These herbicides can provide at least 80% control by the end of the season.

In soybean, similar control can be achieved; however, there are fewer effective options than in corn.

Glyphosate is effective (90% control) and should be used as a foundation of spray programs when controlling pokeweed in soybeans.

Applying glyphosate mid- to late summer is more effective than in the spring due to greater translocation during flowering.

The ALS-inhibitor herbicides (like Classic, Synchrony, FirstRate and Harmony) provide 60% control or less when sprayed alone and should be used in combination with glyphosate if possible.

The contact herbicides (like Reflex, Cobra and Cadet) only provide initial “burn” but then the weed recovers.

Using a residual herbicide (atrazine, metribuzin, Authority, Canopy, Python) will provide 85-90% control of pokeweed seedlings, which can emerge throughout the growing season.

Controlling volunteer corn in soybeans is a recurring problem in Pennsylvania. As the soybean crops continue to mature, volunteer corn is rearing its head in some fields.

Regarding management, we assume that most of the volunteer corn is glyphosate- and glufosinate-resistant (Roundup Ready/LibertyLink), so we will not spend time going over all the options for killing the various types of volunteer corn.

In general, the most logical choices to control volunteer corn in any kind of soybean are the postemergence grass herbicides, including Assure II, Select, clethodim and Fusilade. Remember to adjust your rate based on the height of the corn.

Quote of the Week

“Hide not your talents; they for use were made. What’s a sundial in the shade!”

— Ben Franklin

Leon Ressler is a Penn State Extension educator in Lancaster County.


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