Weed Control: Mid-season Issues in Soybean, Corn, and Small Grains

By Dwight Lingenfelter

As we proceed through the growing season, there are several issues surrounding weed control and herbicide use in corn, soybean, and small grain fields. Below are some items to consider:

June 30 application deadline approaching for XtendiMax and Engenia

Cut-off dates for application of XtendiMax, Engenia, and Tavium are quickly approaching. These herbicides cannot be applied to Xtend/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, Ultra Blazer and several others. 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. (On a sidenote, 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.)

Horseweed/marestail in soybean

There are populations in PA 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 fl oz) is a viable option and more so if tankmixed 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 oz/A) can still provide adequate control of marestail. 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. In general, the post PPOs (Reflex, Cadet, Cobra, Ultra Blazer, etc.) are not effective on marestail, but the pre’s (Valor, Authority, etc.) are.

Palmer amaranth and waterhemp 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 or waterhemp reaches 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, etc.) plus a Group 15 (Zidua/Anthem Maxx/Perpetuo, Dual/Prefix, Warrant/Warrant Ultra, etc.) will be necessary for adequate control through the season. The use of Liberty (glufosinate) in LibertyLink, Enlist E3, or XtendFlex, LLGT27 varieties provides very good control of Palmer and waterhemp. University research has shown that a tankmix of Liberty and Enlist One in E3 soybeans provides effective control of these problem pigweeds.

Pokeweed management

Pokeweed can be controlled with several herbicides in corn, including glyphosate, 2,4-D, dicamba, Status, and Callisto + atrazine. Tankmixing 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 (Classic, Synchrony, FirstRate, Harmony, etc.) provide 60% control or less when sprayed alone and should be used in combination with glyphosate if possible. The contact herbicides (Reflex, Cobra, Cadet, etc.) only provide initial "burn" but then the weed recovers. Using a residual herbicide (atrazine, metribuzin, Authority, Canopy, Python) will provide about 85-90% control of pokeweed seedlings, which can emerge throughout the growing season.

Controlling volunteer corn in soybeans

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 post-grass herbicides (e.g., Assure II, Select, clethodim, Fusilade, etc.). Below are some rates to consider depending on the height of the volunteer corn:

Always include the necessary adjuvants otherwise control will be impacted. In most cases, this will require the addition of a crop oil concentrate (COC) or methylated seed oil (MSO). If crop injury is a concern, a nonionic surfactant (NIS) may be used in place of an oil with Select Max only. Keep in mind if you plan to tank-mix a broadleaf-specific herbicide, the rate of the grass herbicide may need to be increased to overcome possible antagonism.

Small grain preharvest aids

Wheat in Pennsylvania nearing maturity. Photo: D. Lingenfelter, Penn State Weed Science

Below is an overview of some options to consider for preharvest weed control in small grains. However, before applying any product, here are a few considerations:

• This may be a great opportunity to control perennial weeds prior to double crop soybean or prior to establishment of a late summer hay crop. Although late summer/early fall is still the best time for perennial weed control, this is certainly better than early summer applications and may help fit into certain crop rotations.
• Don’t expect miracles with harvest aids. It is still easier to control smaller weeds (<4 inches tall), than larger ones (2-3 feet). In some cases, the lower portions of the weeds may still be green and unaffected.
• It takes time to bring down a large weed. It usually requires about a week or more to obtain good control or dry-down of some weeds. This time may be increased if cool/wet weather conditions exist.  Some of the small grain herbicides labeled for preharvest are systemic and slower acting than a contact herbicide.
• Harvest aids are meant to help facilitate harvest and reduce harvest loss. Any yield loss likely has already occurred from weed competition throughout the growing season. Also any weed seeds produced will contribute to next year’s weed problems.
• Herbicide drift from these applications at this time of year will cause injury/death to sensitive crops/plants in surrounding areas.
• Adhere to all restrictions regarding grazing and feeding of straw for each of these products.

2,4-D

Some formulations of 2,4-D ester are labeled for preharvest applications in wheat, barley, oat, and rye. (Only certain 2,4-D products have all of these small grains listed on their label under preharvest, so consult the specific label before application.) Use 1 to 2 pints per acre depending on the product (no adjuvants are recommended). Applications should be made after the small grain is in the hard-dough stage and at least 14 days before harvest. Do not apply to small grains with a legume underseeding. If a preharvest application of 2,4-D ester is made, double-crop soybeans can be planted 7 days later for 1 pint or 30 days for 2 pints. 2,4-D is volatile and can potentially damage nearby sensitive crops. Avoid use if temperatures are expected to exceed 75°F.

Aim

Can be used as a preharvest application in small grains. Use as a harvest aid to desiccate a limited number of broadleaf weed species. Application of 2 fl oz/A Aim shall be made when the crop is mature and the grain has begun to dry down. Ensure spray coverage is sufficient, otherwise poor control will result. Methylated seed oil or crop oil concentrate at 1 to 2 gal/100 gal is required; liquid nitrogen fertilizer at 2 to 4 gal/100 gal or ammonium sulfate at 2 to 4 lb/A can be added to the methylated seed oil or crop oil. Allow at least 7 days between application and harvest.

Clarity/dicamba

Clarity or other registered dicamba products can be used at 8 fluid ounces per acre preharvest when wheat and barley is in the hard dough stage and the green color is gone from the nodes (joints) of the stem. Apply at least 7 days prior to harvest. Do not use if legumes are underseeded. Do not use preharvest-treated wheat for seed unless a germination test is performed on the seed with an acceptable result of 95% germination or better.  Although the Clarity label allow double crop soybeans following their application (with adequate rainfall and time), the potential for injury may preclude this use. See a Clarity label for specific guidelines on rotational crops. Be cautious of drift. (Engenia and Xtendimax are not registered for this use.)

Glyphosate

Glyphosate products may be applied to wheat and feed barley. Apply after the hard dough stage of grain (20% or less grain moisture for feed barley and 30% or less for wheat) and at least 7 days before harvest. Do not apply more than 1 lb ae (i.e., 32 fl oz/acre Roundup PowerMax or 48 fl oz/acre 3S products), and do not treat wheat being grown for seed as a reduction in germination or vigor may occur. Additionally, do not apply to wheat with a legume underseeding. There are no double-crop soybean planting restrictions if glyphosate is used as a preharvest treatment. Be cautious of drift.

Sharpen

Sharpen may be applied to wheat, feed barley, and triticale only. Apply after hard dough stage and grain is less than 30% moisture. Ground applications require 10 gallons spray volume per acre; aerial requires 5 gallons. No rotation restriction to soybean; 1 to 5 months for vegetables. Apply with methylated seed oil (1 gal/100 gal) plus ammonium sulfate (8.5 lb/100 gal) or urea ammonium nitrate (UAN) (1.25 to 2.5 gal/A). Apply at least 3 days before harvest; label recommends 10 days for optimum desiccation effect.

Other herbicides

Other herbicides such as Gramoxone, Harmony Extra, Peak, Maestro/Buctril, etc. are not labeled for use in small grains as a preharvest aid.