Late Season Herbicide Applications: Are They Possible Now?

Every year we are asked about how late is too late to spray herbicides to control weeds such as marestail, Palmer amaranth, waterhemp, burcucumber, and others. As usual, the answer depends on the product being sprayed, the crop, the setting, and the weeds. Most products have cutoff restrictions based on crop growth stage and/or days before harvest. Whether using "high-boy" sprayers, aerial or drone sprayers, or some other contraption, the herbicide guidelines must be followed. In most cases, the crops are currently in "no-man's land" since most are beyond the maximum growth stage and/or pre harvest interval (PHI) for a standard broadcast application and are not yet mature enough for a harvest aid. Also keep in mind that late-applied herbicides are usually not as effective since the weeds are well beyond the recommended stage of growth for best control. Most labels require that weeds be sprayed before they reach 6 inches tall. Furthermore, be aware of special application restrictions if the crops are being used as a forage. Also, illegal herbicide residues in the commodity can result if specific application timing and other label guidelines are not followed resulting in possible load rejection and penalties.  Also consider the amount of crop damage and potential yield loss from sprayer wheel tracks when spraying this late in the season.

Harvest Aids: As we move into harvest season, farmers may notice some problem weed situations. Also, as soybean leaves start to turn yellow, problem weeds may become more noticeable. At this point in the season, it is difficult to do much except for a harvest aid prior to corn or soybean grain production. In corn, this type of application requires high clearance equipment or aerial application, so hopefully these fields are the exception and not the norm. Keep in mind, herbicide applications made during this late timeframe are used primarily to help desiccate green weed tissue in order to improve the harvesting process. Contact herbicides are usually better at this process; however, it can vary by weed species. Some of these herbicides are not that effective on large weeds or certain species. Harvest aids are not intended to (and usually do not) help speed up crop maturity. Below is a brief list of products available for harvest aid applications in corn and soybeans.

Corn

• Aim 2EC — May be applied 3 days before harvest at 2 fl oz/acre. Use as a harvest aid to desiccate certain broadleaf weeds. Apply in 10 gal/acre water. Include necessary adjuvants and make sure spray coverage is sufficient otherwise poor control will result.
• Defol 5L — Can be applied to desiccate problem weeds in early maturing corn. Apply 4.8 qt/acre, 14 days before harvest in 10-20 gal/acre water and include appropriate adjuvants. Some universities recommend tank mixing Defol with Aim or Gramoxone for improved activity. Do not graze treated fields or feed fodder/forage until 14 days after application.
• Glyphosate — Up to 0.75 lb ae/acre (32 fl oz of a 3 lb ae/gal formulation) to corn a week or more prior to harvest. Include necessary adjuvants to improve performance. Must be applied to grain when moisture is 35% or less and after maximum kernel fill.
• Gramoxone 3SL — Apply 0.8 to 1.3 pt/acre after black layer and at least 7 days before harvest. The higher rate can be used to desiccate mature broadleaves and grasses over 18 inches tall. Be sure to include a nonionic surfactant.
• Sharpen 2.85SC — Apply 1 to 2 fl. oz/acre after physiological maturity and at least 3 days before harvest. However, it’s best to allow up to 10 days for optimum desiccation effect depending on environmental conditions. Include MSO plus AMS or UAN to improve performance. Desiccation-treated field corn stover (stalks) may be grazed or fed to livestock.
• 2,4–D LV4 — Apply 1 to 2 pt/acre after the hard dough or dent stage. Use higher rates on larger weeds and those under stress. Do not forage or feed corn fodder for 7 days after application.

Soybean

• Aim 2EC — Apply 1.5 fl oz/acre as a harvest aid to desiccate certain broadleaf weeds. Application shall be made when the crop is mature and the grain has begun to dry down and at least 3 days before harvest. Apply in 10 gal/A water. Include necessary adjuvants and make sure spray coverage is sufficient otherwise poor control will result. Do not feed treated soybean forage or hay to livestock.
• Clarity 4S— Apply 8 fl oz to 2 quarts after soybean pods have reached a mature brown color and at least 75% leaf drop has occurred. Wait at least 7 days before harvest. Use a non-ionic surfactant or crop oil concentrate plus nitrogen solution in the spray solution. Do not feed soybean fodder or hay following a preharvest application.
• Defol 5L — Can be applied to desiccate problem weeds in early maturing soybean. Apply 4.8 qt/acre, 7-10 days before harvest in 20 gallons/acre of water. No adjuvant is recommended. Do not graze treated fields or feed treated fodder.
• Glyphosate — In Roundup Ready soybeans, glyphosate may be applied through flowering (R2 stage) for late-season control. For pre-harvest, glyphosate may be applied to Roundup Ready and conventional soybeans after 80% leaf drop (loss of all green color). Apply up to 0.75 lb ae/acre (32 fl oz of a 3 lb ae/gal formulation) in 10-20 gallons of water/acre to control weeds that may interfere with harvest or to control perennials such as quackgrass or Canada thistle, but will not control glyphosate-resistant weeds or dry down Roundup Ready varieties. Allow a minimum of 7 days between application and harvest. Use of a non-ionic surfactant plus ammonium sulfate in the spray solution may improve control. Do not graze or harvest the treated crop for livestock feed within 25 days of application. Do not use on soybeans grown for seed.
• Gramoxone 3SL — Apply 5.4 to 10.7 fl. oz/acre plus nonionic surfactant (1 qt/100 gallons of spray) to soybean plants that are mature (65% or more of the seed pods have reached mature brown color or seed moisture is 30% or less. Do not apply within 15 days of harvest and do not graze or harvest for forage or hay.
• Sharpen 2.85SC — Apply 1 to 2 fl. oz/acre after physiological maturity when greater than 50% leaf drop has occurred, and remaining leaves are yellow and at least 3 days before harvest. Include MSO plus AMS or UAN to improve performance.

Palmer amaranth and waterhemp infestations: If your fields look like the picture at the beginning of this article, there is no herbicide that will control Palmer or waterhemp this late in the season. The best thing to do in these situations would be to mow down the field immediately to prevent further weed seed production. If these fields were to be harvested, the weed seeds would be spread back onto the field, causing increased problems for years to come. Furthermore, the weed seeds would contaminate the combine, thus spreading the problem to other fields. Fields that are contaminated with Palmer and/or waterhemp need to have special management tactics employed to slow their growth and spread. Using thick cover crops (e.g., cereal rye) combined with effective 2-pass (pre followed by post) residual herbicide programs in corn and soybeans can improve your chances of controlling these problematic weed species. The main goal is to eliminate the population by not allowing these weeds to set seed each year.