Small Grain Harvest Aids and Double-Crop Soybean Weed Control

Small Grain Preharvest Aids

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 or covers. 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.

If you are close to harvesting small grains and notice seedlings in the under canopy, or have taller grass species that may affect combine efficiency, a harvest aid may be a good option. 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:

• 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.
• Harvest aids are not intended to (and usually do not) help speed up crop maturity. If applied too early, they can interfere with the natural crop maturation process. Illegal herbicide residues can result if specific application timing and other label guidelines are not followed. See the specific product label to determine the correct rate, timing, weed species controlled, and other restrictions with this type of application.
• 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 the 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. (Enlist One/Duo and Freelexx are not labeled for this use.)

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 are 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 allows 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.

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., 28 fl oz/acre Roundup PowerMax3 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 the hard dough stage and the grain is less than 30% moisture. Ground applications require 10 gallons of spray volume per acre; aerial requires 5 gallons. No rotation restriction for soybeans; 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; the 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.

Double-Crop Soybeans: Weed Management Considerations

As barley and wheat harvest occurs, double-crop soybean planting will soon follow in some areas of the state. Weeds are generally less of a problem in double-crop soybeans because of the delayed planting date. But even with fewer weeds emerging, when they are present and in combination with potential dry weather, impacts on yield can be noticeable. A weed-free start is one of the most critical aspects of weed management in double-crop soybeans. Marestail (horseweed), Palmer amaranth, and waterhemp are some of the biggest concerns in double-crop soybean fields. Most populations of marestail are both glyphosate- and ALS-resistant, so managing them can be challenging. Marestail that is cut off during small grain harvest will recover, branch out, and probably be even more difficult to control. In full-season soybeans, the use of 2,4-DLVE in the burndown program can provide control. However, most farmers are not willing to wait the necessary 7 days after 2,4-D application in double-crop, and there is potential for drift at this time of year. Palmer and waterhemp have their own set of management problems. So, what are some considerations for these types of weeds in this setting? 

• Add 1 oz of Sharpen (or equivalent rates of Zidua Pro, Optill, or Verdict) to glyphosate to control marestail stumps. Keep in mind, Sharpen at this rate helps with burndown but does not provide much residual control and soybeans can be planted immediately. For effective control, make sure to add MSO and AMS to the spray mixture when using Sharpen-containing products. Adding metribuzin improves residual control.
• Include 1 pt per acre 2,4-DLVE in the burndown and wait 7 days to plant. If planting an Enlist E3 variety, Enlist One/Duo could be applied, and then soybeans planted immediately. But be cautious of drift to sensitive areas.
• Add FirstRate (or Authority First or Sonic) or a chlorimuron-containing product (Authority XL, Canopy, Envive, Synchrony, Valor XLT, etc.) to improve control if the weed population is not ALS-resistant. (Valor, Authority, and metribuzin provide residual activity, but not much burndown activity.) Also, root uptake of Valor XLT/Envive may help kill marestail stumps. Keep in mind that if you tankmix other PPO (group 14) products that contain flumioxazin (Valor) or sulfentrazone (Authority) with Sharpen, there is a 14-day wait period before planting. Valor and Authority-containing products provide residual control of emerging Palmer amaranth and waterhemp seedlings. These types of residual herbicides also provide activity on other common broadleaf weeds such as lambsquarters, eastern black nightshade, ragweed, and velvetleaf. Cocklebur and jimsonweed can be difficult to control with just residual products.
• Use Liberty 280 or other generic version at 32 to 43 fl oz (adjust rate if using Liberty Ultra) in the burndown application. This is good on marestail, but not as effective on some other weeds such as large annual grasses and Canada thistle. You can tank-mix with other herbicides to improve performance and provide some residual control (Sharpen, metribuzin, etc.). Glyphosate (32 fl oz) can also be tank-mixed with Liberty to improve the spectrum of burndown control.
• Gramoxone is a burndown option as well and has some activity on marestail if they are <3” tall, and good coverage is essential. Adding metribuzin improves control.
• Products like Reviton and Elevore can be used for burndown, but Reviton does not provide effective marestail burndown. If using Elevore, you must wait 14 days after application to plant soybeans, although it provides excellent control of marestail.
• We do not recommend using dicamba as a burndown in double-crop soybeans since it can be risky. At this time of year, many sensitive crops and ornamentals are very susceptible to dicamba, and weather conditions favor dicamba drift. Furthermore, Engenia, Xtendimax, Tavium, and other dicamba formulations are currently not registered in this system.
• Plant a LibertyLink or Enlist E3 soybean variety and apply Liberty (or other labeled glufosinate product) post as needed. Glufosinate provides good control of marestail, Palmer, waterhemp, ragweed, cocklebur, and others. Under hot, humid, and sunny conditions in conjunction with AMS in the tank and good spray coverage, glufosinate can be a very effective herbicide option. It can be tank-mixed with Enlist One and/or glyphosate to broaden the weed control spectrum in E3 soybean fields.
• If marestail or Palmer amaranth/waterhemp are not a major problem, plant a Roundup Ready soybean and apply glyphosate post. For burndown of glyphosate-susceptible marestail, apply glyphosate at 1.5 lb ae/A (40 fl oz of Roundup PowerMax3 or 64 fl oz of a 3 lb ae/gal product).
• For farmers interested in non-GMO soybeans (or Plenish), several post herbicides are still available, but many of the broadleaf products have some potential for soybean injury, and they are not effective for control of marestail, and most are weak on Palmer/waterhemp. Keep in mind that pre-residuals should be used in these systems.
• Residual herbicides can be a critical component for weed control in double-crop beans, especially if Palmer and waterhemp are problems in your fields. However, for residuals to be effective, they need to be incorporated by at least ½” of rainfall within 7-10 days of application and before weed seed germination. However, there is a debate about which of the Group 15 herbicides provides the necessary residual for effective control of weeds like Palmer and waterhemp. In general, products that contain pyroxasulfone (Zidua, Anthem Maxx, etc.) tend to provide the most effective control of these problem weeds, but they require higher amounts of rainfall (>¾ inch) to activate them. Since rainfall can sometimes be limited for proper activation, medium and high solubility products (Group 15 and others) such as Dual Magnum, Warrant/Enversa, metribuzin, Outlook, Pursuit, Reflex, and Spartan/Authority may perform somewhat better in the summer under conditions with less moisture (⅓ to ½”).
• The use of PPO (Group 14) products like Reflex, Cobra, and Ultra Blazer applied post can be tricky. They provide effective control of Palmer and waterhemp and some other broadleaves, but do not have activity on marestail. Also, since they typically cause significant foliar soybean leaf burn, the crop may not have enough time to fully recover and yield well due to the shorter growing season. Additionally, Reflex can have carryover issues to corn next season when applied later in the season.

Much of this information is also available in an audio format through the Agronomy Highlights Podcast, specifically season 3, episode 25, post-wheat cover cropping and herbicide considerations. The Agronomy Highlights Podcast is available on all platforms and YouTube.