Weed Control in Corn and Soybean in July

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-traited 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, and FirstRate still provide control of several key broadleaf weeds; however, ALS-resistant marestail and pigweeds are common in the state. FirstRate and Classic (plus glyphosate) can be applied to suppress ALS-resistant marestail and higher rates can be used in STS or Plenish systems. 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, including Plenish.
• 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, Plenish, 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 XtendFlex 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, Enversa, 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 can Not be used in Xtend soybean systems, otherwise major crop damage will occur. Enlist One provides control of several common broadleaf weeds, while Enlist Duo controls both broadleaves and grasses.

Refer to Table 2.4-11(below) in the 2025 Penn State Agronomy Guide for ratings of herbicides on selected weed species.

Weed control rating: 10 = 95–100%; 9 = 85–95%; 8 = 75–85%; 7 = 65–75%; 6 = 55–65%; N = less than 55% or no control; + = upper end of rating scale. Ratings are based on labeled application rates and weeds at 4 inches in height. Results may differ with variations in weed size, temperature, rainfall, soil moisture, and spray coverage. Ratings are based only on postemergence (POST) activity and do not reflect possible residual activity. For ratings on herbicide combinations not listed, see the component parts. Ratings assume good growing conditions and proper usage of adjuvants.
1 Herbicide is less effective on yellow foxtail compared to giant foxtail. Ratings for yellow foxtail: Assure II/Targa = 7; Fusion = 8; Liberty = 6; Pursuit = 6; Raptor = 6.
2 For use on glyphosate-resistant soybean varieties only.
3 Glyphosate will not control glyphosate-resistant corn
4 For use on glufosinate-resistant soybean varieties only.
5 Liberty will not control glufosinate-resistant corn or many hybrids with Bt traits.
6 Rate for Poast is 1 pt on annual grasses and 1.5 pt for perennial grasses.
7 For use on STS, Bolt, or non-STS soybean varieties; a lower rate must be used on non-STS varieties.

As the growing season progresses, crops are growing rapidly. 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:

Weed control rating: 10 = 95–100%; 9 = 85–95%; 8 = 75–85%; 7 = 65–75%; 6 = 55–65%; N = less than 55% or no control; + = upper end of rating scale. Crop tolerance: E = excellent (almost never any crop injury observed); VG = very good (on rare occasion is crop injury observed); G = good (seldom is crop injury observed as long as proper management practices are followed, e.g., herbicide rate and application timing, adjuvants); FG = fair to good (occasionally crop injury is observed even with proper management practices; injury is often due to herbicide interactions with environmental conditions); F = fair (some crop injury is commonly observed). Ratings are based on labeled application rates and weeds at 4 inches in height. Results may differ with variations in weed size, temperature, rainfall, soil moisture, and spray coverage. Ratings are based only on postemergence (POST) activity and do not reflect possible residual activity. For ratings on herbicide combinations not listed, see the component parts. Ratings assume good growing conditions and proper usage of adjuvants.
1 Group 2 resistance has been confirmed for this species and it is widespread in the region; Group 2 herbicides will not control these biotypes.
2 Glyphosate resistance has been confirmed for this species and is widespread in the region.
3 Common ragweed biotypes resistant to Groups 2 and 14 herbicides have been confirmed in the region. Group 14 resistance is limited to the DelMarVa region and New Jersey.
4 For use on 2,4-D resistant (Enlist E3) soybean varieties only.
5 For use on glyphosate-resistant soybean varieties only.
6 For use on glufosinate-resistant soybean varieties only.
7 For use on STS, Bolt, or non-STS soybean varieties; a lower rate must be used on non-STS varieties.
8 Excellent crop tolerance for STS or Bolt varieties, but fair or worse ratings if used on non-STS varieties.

Post Applications

In some parts of the state, corn and soybean crops are getting beyond the growth stage for a legal post-harvest 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 (see below) in 2025 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 have limits once soybean flowers are initiated. Refer to Table 2.4-14 in 2025 Penn State Agronomy Guide for additional details. Also, don't confuse late post applications and harvest aid applications. Not all herbicides can be applied up to harvest. There are only a few 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 other 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. 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, Liberty-70 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. These guidelines can be found in Table 2.2-18 for corn and Table 2.4-15 for soybean in 2025 Penn State Agronomy Guide.