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Managing Herbicide Resistant Weeds

Common pigweed (Palmer amaranth) resistant to herbicides that are ALS inhibitors and/or products that contain glyphosate is common within Craven County. Additionally, glyphosate resistant mare’s tail or horseweed (Conyza Canadensis) is becoming more widespread. Complicating this is the fact that we also have several winter weeds (such as primrose) that are simply difficult to control. Thus, for those not utilizing intensive cultivation prior to planting to eliminate weeds, it is advisable to include multiple products with differing modes of action and an herbicide with preemergence activity in a burndown herbicide application in late winter or early spring. Assuming a minimum or no-till production, the following comments offer suggestions and guidelines for management of these herbicide resistant and difficult to control weeds.

The first consideration should be to examine glyphosate rates by brand. Assuming that all rates for all products are the same may result in inappropriate rates that may be ineffective and expensive. A sample chart of products is listed below. Make sure you read the label of the specific product purchased. Add or do not add surfactant(s) based upon the label of the product. It is strongly advised to use the highest rates when weeds with known herbicide resistance are present.

Table 1. Comparison of Glyphosate Formulations and Acid Equivalence

Table 1. Comparison of Glyphosate Formulations and Acid Equivalence  (Source:  2015 Cotton Information, P123)


The second consideration should be whether or not to include a broad-spectrum broadleaf post-emergence control product such as 2,4-D, Clarity or Dicamba as part of the burndown herbicide application. Adding one of these components is strongly recommended to control emerged mare’s tail (horseweed) and primrose. However, by doing so, certain planting restrictions apply as outlined in Table 2. Planting Restrictions for Common Broadleaf Herbicides Tank Mixes with Glyphosate. Note that the exact rate of application, days of restricted planting, and other precautions will very by brand of product. Make sure to read and follow the product label.

Table 2. Planting Restrictions for Common Broadleaf Herbicides Tank Mixes with Glyphosate

Table 2. Planting Restrictions for Common Broadleaf Herbicides Tank Mixes with Glyphosate


It should be noted that mare’s tail can, and will, continue to emerge between now and May. Inclusion of any of these broadleaf weed products is needed to control emerged weeds but will not provide control of future emerging weeds. Therefore, it is advised to also include a preemergence herbicide. A partial listing and brief discussion of preemergence materials is listed below for each crop. As always, make sure to read and follow the label directions. Planting restrictions, potential replanting decisions, appropriate rates and other information will vary by product and brand. For all crops listed below, it is advised to include paraquat at plating to control any emerged weeds. Using glyphosate or ALS inhibitors (or tank mix of both) in this application may be ineffective due to herbicide resistance.

CORN – When applied well ahead of planting, two of the more common herbicide choices are flumioxazin (Valor SX) or atrazine. Both provide good control of many small-seeded broadleaf weeds but require a 7-14 day waiting period between application and planting. Flumioxazin products require an accumulation of 1/4th inch of rainfall after application and a minimum soil residue cover. Use of either of these products has limited effect on at-planting or post-emergence choices. With multiple brands of both products, make sure to follow the label of the product purchased.

If weather delays the application of a burn-down herbicides and neither of the above options is viable, other products such as Outlook, Dual, Harness, Micro-Tech, Zidua, Bicep, just to mention a few, can be used. Generally, these products do not control emerged weeds but will provide good preemergence control. Realize that the length of control normally expected from these products may be reduced simply because the burndown application may be applied 7-10 days ahead of planting. Thus, it may be necessary to scout earlier than normal and apply a post-emergence application sooner than anticipated. Whether or not this occurs will depend upon climatic factors and individual weed history. Lastly, with many generic brands available, make sure to read and follow the specific label directions regarding tank mixes, rates, cultivation and/or rotation restrictions. More information is available for corn weed management in the 2015 NC Agricultural Chemicals Manual and in the NCSU Corn Production Guide

COTTON – Assuming a timely application and the presence of glyphosate resistant mare’s tail, the preferred preemergence option to include in a tank mix of glyphosate and the broadleaf herbicide of choice (2, 4-D, Dicamba, Clarity) in an early burndown application is flumoxazin (Valor SX). However, fluometuron (Cotoran), diuron (various brands), thifensulfuron + rimsulfuron (Leadoff) or Saflufenacil (Sharpen) are also effective. However, the latter choices have one or more restriction in that they may have a greater impact on crop rotation, restrictions of application based upon soil type, rainfall requirements prior to planting or a waiting period between application and planting. Again, the later choices are good options but make sure to read and follow the specific label directions.

Following this early season burndown, aim to include another burndown material (parquat preferred) at planting. Suggested combinations provided in the 2015 Cotton Information publication include:

  • Acetochlor (Warrant) + Fomesafen (Reflex)
  • Acetochlor (Warrant) + Diuron (various brands)
  • Fomesafen (Reflex) + Diuron (various brands)
  • Fomesafen (Reflex)+ Pendimethalin (Prowl)

Specific charts, recommendations and notes can be found in the 2015 Cotton Information publication and in the 2015 NC Agricultural Chemicals Manual

SOYBEAN – If cultivation will be included, then options such as ethalfluralin (Sonalan), metalachlor (various brands), S-metalachlor (various brands), imazequin (Sceptor), dimethnamid-P (Outlook), metribuzin (Sencor), pendimethalin (Prowl), and triflluralin (Treflan) are viable options to apply as a pre-plant, incorporated material to control small seeded broadleaf weeds. Assuming, as discussion has been thus far that reduced tillage or no-till production is preferred, numerous more options are available. A complete listing of these products and combination of products is listed within the 2015 NC Agricultural Chemicals Manual on pages 261-263. Additionally, the NCSU Soybean website outlines some of the products. Rather than attempt to provide comments on these numerous products and combinations, a few suggestions and notes are offered below.

  • Products such as Valor, Leadoff and Envive provide specific instructions to avoid cultivation after application.
  • If flumioxazin (Valor) products are used in an early-season burn down application, consider that products such as Envive and Fierce also contain the same active ingredient. Fomesafen (Prefix, Flexstar) also has the same mode of action. Continued use of a single type of mode of action accelerates herbicide resistant weed development and should be avoided. Consider other choices when possible to avoid subsequent trips with the same mode of action.
  • If a product that contains metribuzin is used, make sure it is applied to appropriate soil types and tolerant varieties. The most current listing available through NCSU is available at If the variety chosen is not on this list, please contact a seed suppliers for tolerance of metribuzin for the variety(ies) chosen.
  • Products with thifensulfuron (Harmony SG) can be applied any time prior to soybean planting but products with thifensulfuron + tribenuron (Harmony Extra SG) require a 14 day waiting period prior to planting.
  • Rates for products with pyroxasulfone (Zidua, Anthem) are based upon soil type.


If control of glyphosate resistant mare’s tail (horseweed) is delayed and none of the previously discussed options will afford timely control and planting, another option is to apply Ignite or Liberty. These products will control small, emerged mare’s tail. However, this product alone may or may not provide adequate control of many other emerged weeds depending upon the size of the weed and specie. Too, utilizing this material prior to planting will limit the total amount allowed per year. Subsequent application may be prohibited for LibertyLink tolerant crops. Additionally, consider that:

  • These products will not control or may provide only limited control of larger (4” or more) mare’s tail.
  • Adequate control is best when temperature is above 750F.
  • Sequential treatments should include a burndown product with paraquat and a preemergence material at planting to control weeds not normally controlled by Liberty/Ignite as well as provide control of newly emerging weeds.


Post-emergence control should be made as early as possible to control small weeds (less than 4” tall for most but less than 2” tall under dry or less than optimum conditions). Ample data is available showing merits of early weed control to increase yield for all crops. Inclusion of a pre-emergence material is strongly advised to control late emerging glyphosate resistant Palmer amaranth. Furthermore, preemergence materials are the best option to control weeds such as Florida pusley since neither glyphosate nor glufosinate are very effective in controlling this weed post-emergence.

In addition to timeliness, make sure to use the appropriate volume for the given product. With the wide adaption of glyphosate tolerant crops, lower volume (10-15 GPA) of application is quite common. However, many of the herbicides that might be added to the post-emergence application require a higher volume (20-30 GPA) to be effective. Read and follow the direction of the product(s) added to ensure adequate control.

In some cases, especially if weeds are large or weeds are stressed from drought/heat/low soil moisture, a separate application may be warranted. Some tank mixes may result in poor control due to antagonism or timing. As example, research shows that glufosinate products should not be applied sooner than two hours after sunrise or within an hour of sunset. Another example is that tank mix of glufosinate plus glyphosate products to control perennial grasses is not recommended. Again, the point is to read and follow the pesticide label. In some cases, sequential trips are necessary to ensure adequate control.


If herbicides drift off the target site, they may damage nearby sensitive crops. Drift can occur as either spray drift or vapor drift. Spray drift is the movement of airborne spray particles and is often determined by droplet size, height of the boom, and amount of wind. Spray drift can occur with any herbicide. In contrast, vapor drift is associated with products that are volatile such as ester formulations of 2,4-D. Many herbicide labels contain specific application instructions to avoid spray drift. Follow those directions carefully. Below are general precautions to avoid spray drift.

  • Do not spray when the wind speed exceeds 10 MPH.
  • Do not spray when the wind is blowing toward a sensitive crop.
  • Avoid nozzles and pressures that create fine droplets.
  • Use drift-reducing nozzles such as Spraying Systems Turbo TeeJets or XR-type nozzles.
  • Use the highest spray volume and highest flow rate practical.
  • Use a pressure within the lower recommended pressure range recommended for a particular nozzle.
  • Keep your boom as low as possible. However, make sure to properly overlap of the spray pattern for the particular nozzle.

The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the N.C. Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conform to the product label. Be sure to examine a current product label before applying any product.

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