Corn and Soybean Weed Control in July

By Dwight Lingenfelter

As the growing season progresses, crops are growing rapidly. As a reminder, many herbicides have application restrictions related to the crop stage of growth, as well as, if the crop is being harvested for silage/forage/grazing or grain. Here are a few things to consider.

Post applications

Many of the corn and soybean crops are getting beyond the growth stage for a legal post herbicide application. Some corn herbicides can be applied up to corn that is 48 inches tall but with the use of drop nozzles; see Table 2.2-15  in the 2021-22 Penn State Agronomy Guide for more details. Many of the typical soybean post herbicides vary in their application timeframes. Products such as Classic, Reflex, clethodim and others have a rather wide window for application; however, products like glyphosate, Pursuit, FirstRate, Liberty, Enlist, and a few others must not be applied beyond the flowering stage. See  Table 2.4-14  in the 2021-22 Penn State Agronomy Guide. Keep in mind, as of July 1, Engenia, Xtendimax, and Tavium cannot be legally applied to Xtend/Flex platform soybeans. Don’t confuse late post applications and harvest aid applications. Not all herbicides can be applied up to harvest. There are only several herbicides that can be applied as a harvest aid (e.g., 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.

Harvest intervals

A number of herbicides have restrictions when harvesting corn or soybean for silage or forage. Harvest restrictions are based on the potential for illegal herbicide residues in the feed or forage. Although not generally a problem, early harvested corn may fall under some of these restrictions. Some pre corn herbicides such as atrazine, Acuron, Anthem, and Sharpen have intervals from 60 to 90 days; while others pre herbicides can be less than 45 days. For example, post applied products such as Steadfast Q and Resolve Q have a 30 day, Status a 32 day, Liberty a 70 day, Roundup a 50 day, and Impact/Armezon a 45 day harvest restriction for silage following herbicide application. See Table 2.2-18  in the 2021-22 Penn State Agronomy Guide.  Many of the pre and post applied soybean herbicides are not labeled for soybean forage but some that do include BroadAxe/Authority Elite-30 days, Boundary and metribuzin-40 days, FirstRate-25 days, and Liberty-45 days, and Enlist One/Duo-56 days. Fortunately, in-crop applications of Roundup have only a 14-day harvest restriction, while other glyphosate products may vary regarding their harvest restrictions. See Table 2.4-15  in the 2021-22 Penn State Agronomy Guide.

Post soybean options

As a reminder, if your soybean crop is still within the proper growth stages, here are some herbicide options to consider:

• Glyphosate (group 9) in Roundup Ready systems is still a viable option to control many weed species; however, it will not control glyphosate-resistant weeds such as marestail, Palmer amaranth, and waterhemp. In a standard Roundup Ready system, a 2x rate (1.5 lb ae) 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. See below for other herbicide options to control Palmer amaranth.
• ALS herbicides (group 2) such as Pursuit, Raptor, Classic, Harmony, FirstRate still provide control of several key broadleaf weeds; however, ALS-resistant marestail and pigweeds are common in the state. Classic controls cocklebur, smartweed, burcucumber, and provides suppression of certain perennials such as bindweed and pokeweed. Harmony is effective on lambsquarters, smartweed, and velvetleaf. FirstRate controls cocklebur, annual morningglory, 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.
• PPO herbicides (group 14) such as Reflex, Flexstar, Cobra, Ultra Blazer, Cadet, etc. 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/Flexstar, Cobra, Ultra Blazer provide good control of pigweeds including Palmer amaranth and waterhemp, and suppression of ragweed and eastern black nightshade, but have limited activity on marestail and are weak on lambsquarters. Cadet is great on velvetleaf but weak on other species including pigweeds, lambsquarters, ragweed, cocklebur, and marestail. In general, the pre PPOs (Valor, Authority, etc.) are effective on both marestail and Palmer. Any of these PPO herbicides can be used in conventional or GM soybeans.
• 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 LLGT27 soybean varieties. It controls marestail, Palmer, waterhemp, cocklebur, lambsquarters, ragweed, and some others.
• 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” control of weeds like Palmer and waterhemp. These weeds have a prolonged germination period during the summer and residual products will be necessary for adequate control through the season. Premixed products such as Prefix or Warrant Ultra are also available.
• Enlist E3 soybean varieties and registered 2,4-D choline (group 4) products (Enlist One and Duo) can be used in that system. These 2,4-D products cannot be used in Xtend soybean systems, otherwise major crop damage will occur. Enlist One provide control of several common broadleaf weeds, while Enlist Duo controls both broadleaves and grasses.