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Options for tackling problem weeds

Talkin’ Weeds: Your herbicide options will decline as the season progresses, but options still exist.

5 Min Read
A soybean field
PROTECT YOUR BEANS: Several options exist to control mid- and late-season problem weeds in soybeans, but be aware of application time frames and harvest restrictions. Dwight Lingenfelter

Certain types of weeds will become even bigger problems as the growing season progresses.

Weeds such as marestail, Palmer amaranth, waterhemp, burcucumber, late-season annual grasses (foxtail, crabgrass, panicum), and perennials like thistle and dogbane can easily plague a crop later in the season. But the later it gets, fewer herbicide options will be available to provide adequate control.

Many herbicides have application restrictions related to crop growth stage — and if the crop is being harvested for silage, forage, grain or will be grazed. The first thing is to check the herbicide label or other references such as the Penn State Agronomy Guide for a list of maximum crop stage limits and weed heights.

Post soybean options

If your soybean crop is still within the proper growth stages, here are some herbicide options to consider:

Glyphosate (group 9). Glyphosate (group 9) in Roundup Ready systems is still a viable option to control many weeds. However, it will not control glyphosate-resistant weeds such as marestail, Palmer amaranth and waterhemp.

In a standard Roundup Ready system, a two-times rate (1.5 pounds an acre) of glyphosate, or a couple of in-crop applications, will usually stunt marestail. It is always best to treat it soon after it starts regrowing from burndown. This will not control it, but it might suppress it somewhat if it is sprayed early enough.

Keep in mind that marestail plants aren’t very shade-tolerant. Most soybeans will start to canopy and outcompete them.

ALS herbicides (group 2). ALS herbicides (group 2) such as Pursuit, Raptor, Classic, Harmony and FirstRate will provide control of several key broadleaf weeds. However, ALS-resistant marestail and pigweed are common in Pennsylvania and other parts of the region.

Classic can control cocklebur, smartweed, and burcucumber, and can suppress certain perennials such as bindweed and pokeweed.

Harmony is effective on lambsquarters, smartweed and velvetleaf. FirstRate can control cocklebur, annual morning glory and ragweed.

Pursuit and Raptor are good on cocklebur, eastern black nightshade and velvetleaf, as well as annual grasses. These herbicides can be used in conventional or GM soybeans.

Rizky Triandi Pamungkas/Getty Images -  Palmer Amaranth plant

PPO herbicides (group 14). PPO herbicides (group 14) such as Reflex, Flexstar, Cobra, Ultra Blazer, Cadet and more have activity on certain broadleaves. But keep in mind that not all of them have the same effectiveness on all broadleaf weeds.

For example, Reflex, Cobra and Ultra Blazer provide good control of pigweeds, including Palmer amaranth and waterhemp, and can suppress ragweed and eastern black nightshade. But they have limited activity on marestail and are weak on lambsquarters.

Cadet is great on velvetleaf, but weak on other species such as pigweed, lambsquarters, ragweed, cocklebur and marestail.

In general, the pre-PPOs (Valor, Authority) are effective on both marestail and Palmer. Any of these PPO herbicides can be used in conventional or GM soybeans.

Glufosinate (group 10). Glufosinate (group 10), the active ingredient in Liberty, Cheetah, Interline, Scout and other generics, can be used over-the-top in LibertyLink, Enlist E3 and XtendFlex soybeans. It can control marestail, Palmer, waterhemp, cocklebur, lambsquarters, ragweed and some others.

Acetamide herbicides (group 15). Acetamide herbicides (group 15) such as Dual, Zidua/Anthem Maxx, Outlook, Warrant and a few others do not control existing weeds, but these should be included in the post spray mixture to provide “overlapping residual” of weeds like Palmer and waterhemp. These weeds have a prolonged germination period during summer, and residual products will be necessary for adequate control throughout the season. Premixed products such as Prefix or Warrant Ultra are also available.

For Enlist E3, 2,4-D choline (group 4) products (Enlist One and Duo) can be used. These 2,4-D products can’t be used in Xtend soybean systems. Otherwise, major crop damage will occur. Enlist One will provide control of several common broadleaf weeds, while Enlist Duo controls both broadleaves and grasses.

For XtendFlex soybeans, you can still spray with glyphosate or glufosinate if they are not beyond the flowering stage. Registered dicamba products (Xtendimax, Engenia, Tavium) can’t be legally applied after the June 30 cutoff date.

Late-season considerations

Some corn herbicides can be applied to corn that is 48 inches tall, but only with the use of drop nozzles. Many soybean post herbicides vary in their application time frames. Products such as Classic, Reflex, clethodim and others have a wide window for application. However, products like glyphosate, Pursuit, FirstRate, Liberty, Enlist and a few others have limits once soybean flowers are initiated.

Don’t confuse late post applications with harvest aid applications. Not all herbicides can be applied up to harvest, and only a few can be applied as a harvest aid (2,4-D, dicamba, glyphosate, Gramoxone, Aim and a few others), and these must be sprayed within a certain time period when the crop is mature and ready to harvest.

Glyphosate, for example, is labeled as an in-crop application that can be sprayed in corn up to 48 inches tall and then after corn reaches full maturity or harvest. Spraying glyphosate during tassel is an off-label application.

Remember harvest restrictions

Many herbicides have restrictions on harvesting corn or soybeans. Harvest restrictions are based on the potential for illegal herbicide residues in feed or forage.

Although not generally a problem, early-harvested corn may fall under some of these restrictions. Some pre-corn herbicides like atrazine, Acuron, Anthem and Sharpen have intervals of 60 to 90 days, while others can be less than 45 days.

For example, post-applied products like Steadfast Q and Resolve Q have a 30-day restriction; Status has a 32-day restriction; Liberty has a 70-day restriction; Roundup has a 50-day restriction; and Impact/Armezon has a 45-day harvest restriction for silage following herbicide application.

Many pre- and post-applied soybean herbicides are not labeled for soybean forage, but some are — including BroadAxe/Authority Elite, 30-day restriction; Boundary and metribuzin, 40-day restriction; FirstRate, 25-day restriction; Liberty, 70-day restriction; and Enlist One/Duo, 56-day restriction.

Fortunately, in-crop applications of Roundup have only a 14-day harvest restriction, while other glyphosate products may vary regarding their harvest restrictions.

Lingenfelter is an Extension associate of weed science with Penn State Cooperative Extension.

About the Author(s)

Dwight Lingenfelter

Dwight Lingenfelter is an Extension associate of weed science with Penn State Cooperative Extension. He can be reached by email at [email protected].

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