By Curtis Thompson, Extension Weed Management Specialist

There are several pre-plant and pre-emergence residual herbicides available for corn. These herbicide programs are key to managing glyphosate-resistant and other difficult-to-control weeds.

It’s important to use multiple modes of action when selecting herbicides. To assist growers, we have included in this article a reference number in parentheses to the herbicide’s mode of action. For example, the reference number herbicide mode of action for glyphosate is 9, and will be referred to in this article as “glyphosate (9).” There is a key to all herbicide modes of action at the end of this article.

When there are two or more numbers in parentheses, it means the active ingredients in a product have different modes of action. Also, if a herbicide is mentioned more than once in a paragraph, we include the reference number only after the first mention of the product in that paragraph.

It’s important to change herbicide programs from time to time so that you do not get hooked on any single herbicide program year after year. Thus, it’s important to know the strengths and weaknesses of each product in terms of the spectrum of weeds controlled. A table summarizing weed species response to various corn herbicides can be found on pages 24-26 of 2016 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland.

For burndown applications in a no-till system on emerged grass and broadleaf weeds, an application of glyphosate (9) and a product containing dicamba (4) or 2,4-D (4) may be critical. The choice between 2,4-D and dicamba will depend on weed species present. Dicamba products will be more effective on kochia and marestail. 2,4-D is more effective on winter annual mustards.

The use of pre-emergence herbicides, applied just before or following planting, often provides control of weeds for several weeks. This can greatly improve the effectiveness of a post-emergence herbicide application, and gives the producer more leeway on post-application timing.

Categories of Soil-Applied Residual Herbicides for Corn

Soil-applied residual herbicides for corn can be grouped into several basic categories.

Acetamides (15) and acetamide (15)/atrazine (5) premixes. The main acetamide (15) products used in corn include acetochlor, S-metolachlor, metolachor, dimethamid-P, pyroxasulfone and many premix products containing one of these active ingredients. In general, these products are very effective in controlling annual grasses (except shattercane and Johnsongrass) and small-seeded broadleaf weeds such as pigweeds.

They are much less effective in controlling small-seeded kochia or large-seeded broadleaf weeds such as cocklebur, devilsclaw, morningglory, sunflower, and velvetleaf. An exception are those products containing pyroxasulfone — Zidua (15), Anthem (15, 14) and Anthem ATZ (15, 14, 5). These products have activity on kochia and the large-seeded velvetleaf. There have been no cases of weed populations in Kansas developing resistance to the acetamides to date.

The acetamide products are most effective when applied with atrazine. Several atrazine (5)/acetamide (15) premixes are available and should be used instead of acetamides alone unless atrazine is not allowed. These premixes generally fit into two groups: products with a reduced atrazine rate (1 pound or less per acre) and products with a full atrazine rate (1-2 pounds per acre). Soil type, soil pH, and organic matter will determine whether the reduced- or full-rate atrazine product is used.

In past years, often because of cost, reduced rates of these products were applied to help manage heavy summer annual grass pressure, then followed up with a good post-emergence herbicide program. With the increased occurrence of glyphosate- and other herbicide-resistant weeds, it is essential to use the full rates of these products in conjunction with a post program.

HPPD-inhibitors (27). Examples of HPPD-inhibitors are isoxaflutole (e.g. Balance Flexx (27), Corvus (27, 2) and Prequel (27, 2)) and mesotrione (e.g. Callisto (27), Callisto Xtra (27, 5), Acuron (27, 15, 5), Lexar EZ (27, 15, 5), Lumax EZ (27, 15, 5), Acuron Flexi (27, 15), Zemax (27, 15). These products either contain atrazine or should be applied with atrazine, and are excellent on kochia, pigweeds, velvetleaf and many other broadleaf weeds.

Acuron (27, 15, 5), Lexar EZ (27, 15, 5), Lumax EZ (27, 15, 5) and Corvus (17, 2)+atrazine (5) will provide the best control of grass weeds. Corvus will also control shattercane. Balance Flexx has activity on shattercane but is less consistent than Corvus. Prequel has a low rate of Balance mixed with Resolve and will not provide the same level of residual weed control as Acuron, Lexar EZ, Lumax EZ, Balance Flexx or Corvus used at full rates.

Keep in mind, products containing Balance should not be applied to coarse-textured soils when the water table is less than 25 feet below the soil surface. Balance Flexx does not provide adequate control of sunflower. Corvus will be much better than Balance Flexx on sunflower, provided the sunflower is not ALS-resistant. Herbicides containing clopyralid (4), such as Hornet (4, 2), Resicore (15, 4, 27), TripleFlex II (15, 4, 2), or Surestart II (15, 4, 2) will provide very good control of sunflower.

A new herbicide from Syngenta called Acuron contains Lumax EZ (27, 15, 5) + bicyclopyrone (27). Bicyclopyrone is an HPPD-inhibitor herbicide that enhances large-seeded broadleaf weed control and also has grass activity. Acuron (27, 15, 5) has enhanced control of giant ragweed, common ragweed, common cocklebur and velvetleaf, along with improved morningglory control over Lumax EZ.

An herbicide just registered in 2016 is called Acuron Flexi (27, 15), which is basically Acuron without atrazine. Acuron Flexi (27, 15) and Zemax (27, 15), which is basically Lumax without atrazine (5), were developed for areas where atrazine generally isn’t used or is prohibited. Without the atrazine (5), less broadleaf weed control is expected.

Triazine (5). Atrazine (5) is a common component of many pre-plant and pre-emergence herbicide premixes for corn. Where weed pressure is light, a March application of atrazine with crop-oil concentrate and 2,4-D (4) or dicamba (4) can control winter annual weeds such as mustards and marestail and provide control of most germinating weeds up to planting.

If kochia is the key target, 0.5-1.0 pound per acre atrazine (5) with a pint of dicamba (4) applied in late February to early March can provide excellent control of germinating kochia.

It is essential to add glyphosate (9) to the mix if winter annual grasses are present. In a premix with other herbicides, atrazine adds burndown control of newly emerged grasses and broadleaf weeds present near planting time, as well as some residual control of small-seeded broadleaf weeds, such as pigweeds and kochia (except for triazine-resistant populations). Unless your situation prohibits atrazine use, always apply atrazine (5) with HPPD-inhibitor (27) and acetamide (15) herbicides.

PPO-inhibitors (14). Examples of PPO-inhibitors include flumioxazin (e.g. Valor (14), Fierce (14, 15), and saflufenacil (Sharpen (14), Verdict (14, 15). Valor or Fierce must be applied 7-30 days before corn planting in a no-till system. These herbicides provide excellent control of pigweeds; however, they are marginal on kochia.

Fierce will provide improved control of velvetleaf compared to that from Valor. The addition of atrazine (5) will enhance kochia, pigweed, velvetleaf and morningglory control, provided the populations are not triazine-resistant. Sharpen and Verdict have excellent activity on pigweeds, kochia and large-seeded broadleaf weeds.

However, the length of residual activity can be shorter than other pre-emergence products when all are compared at full rates. This will depend on the rates of Sharpen and Verdict used. Approximately 7-10 days of residual can be expected per 1 ounce of Sharpen and 5 ounces of Verdict.

ALS-inhibitors (2). Examples of ALS-inhibitors for use as a soil-applied herbicide for corn include flumetsulam, Python (2); and Hornet (2, 4), a premix of flumetsulam (2) and clopyralid (4). Both herbicides have broadleaf activity only. These products are strong on large-seeded broadleaf weeds such as cocklebur, sunflower and velvetleaf, or the small-seeded common lambsquarters.

Adding Hornet to a full rate of an acetamide (15)/atrazine (5) mix as a pre-emerge treatment will control the annual grasses and add considerably to large-seeded broadleaf weed control. These three-way premixes, acetochlor (15)+chlopyralid (4)+flumetsulam (2), include SureStart II (15, 4, 2) and TripleFlex II (15, 4, 2). Sunflower appears to be most sensitive to Hornet (2, 4), followed closely by cocklebur and velvetleaf. Morningglory is less sensitive. Resicore (15, 4, 27) is a new herbicide from Dow and is a premix of acetochlor (15)+chlopyralid (4)+ mesotrione (27). This product contains 3 modes of action as did SureStart II and TripleFlex II, only the ALS-inhibitor (2) has been replaced with an HPPD-inhibitor (27).

An additional ALS-inhibiting (2) herbicide from DuPont is called Resolve (rimsulfuron, 2). Rimsulfuron (2) is also a component in Prequel (2, 27), Instigate (2, 27), Basis (2), and Basis Blend (2), which was previously mentioned. Additional products containing rimsulfuron include Harrow (2) and Crusher (2). Resolve will provide short residual control of grass and broadleaf weeds and should be used as a setup herbicide with a good post-emergence weed control program.

If ALS-resistant broadleaf weeds are present, these ALS-containing herbicides often will be less effective.

Key to Herbicide Mode of Action Reference Numbers

The Weed Science Society of America has developed a numbered classification system based on the herbicide site of action to assist farmers and applicators in selecting herbicides with different sites of action. Most herbicide labels now prominently display the herbicide classification number at the begin­ning of the label. Herbicide premixes that contain multiple active ingredients with different sites of action will have all sites of action numbers listed.

The following list — from K-State’s 2016 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland, SRP 1126 — presents herbicides by mode of ac­tion, chemical family and the WSSA herbicide site of action number (in parentheses).

ALS-AHAS inhibitors (2):

Imidazolinone family - Arsenal, Plateau, Pursuit, Raptor, Scepter, Contain, Beyond

Sulfonylurea family - Accent, Affinity, Ally, Amber, Basis, Beacon, Cimarron, Classic, Crusher, Escort, Express, Finesse, Glean, Harmony SG, Harmony Extra, Harrow, Maverick, Oust, Peak, Permit, Spirit, Steadfast, Synchrony, Telar

Triazolopyrimidine family - Python, FirstRate, PowerFlex

Sulfonylaminocarbonyl-triazolinone family - Olympus, Osprey, thiencarbazone

EPSP inhibitors (9):

Amino acid derivative family - glyphosate, Roundup, Touchdown, and others

Auxins-synthetic (4)

Benzoic acid family - Dicamba, Banvel, Clarity, DiFlexx, Status, Vision, and others

Phenoxy family - 2,4-D, 2,4-DB, MCPA, MCPP, 2,4-DP

Carboxylic acid family - Tordon, Stinger, Remedy, Garlon, Starane, Milestone, Trycera

Quinoline carboxylic acid - Facet L, Paramount, Quinstar GT, Quinstar 4L

Auxin Transport Inhibitor (19)

Semicarbazone family - diflufenzopyr

Cell Membrane Disrupters

Bipyridilium family (22) - Gramoxone, Diquat

Diphenylether family (14) - Ultra Blazer, Cobra, Phoenix, Reflex, Flexstar, ET, Vida, Dawn, Rhythm

N-Phenylphthalimide family (14) - Encompass, Resource, Valor

Aryl-Triazinone family (14) - Cadet, Spartan, Aim

Pyrimidinedione family (14) - Sharpen, Kixor

Lipid Synthesis Inhibitors (1)

Aryloxyphenoxypropionate family - Fusilade DX, Assure II, Fusion, Targa

Cyclohexanedione family - Poast, Poast Plus, Select, Volun­teer, Section, Arrow, Tapout

Phenylpyrazolin family - Axial

Nitrogen Metabolism Inhibitors (10)

Organophosphorus family - Liberty

Photosynthetic Inhibitors

Triazine family (5) - atrazine, metribuzin, Princep, Evik, Pramitol

Phenylurea family (7) - Lorox, Karmex, Spike

Uracil family (5) - Sinbar, Hyvar

Nitrile family (6) - Buctril, Moxy, Bromox, Brox

Benzothiadiazole family (6) - Basagran

Pigment Inhibitors

Isoxazolidinone family (13) - Command

Isoxazole family (27) - Balance, Huskie

Triketone family (27) - Callisto, Impact, Laudis

Seedling Growth Inhibitors

Thiocarbamate family (8) - Eradicane, Eptam

Acetamide family (15) - Dual II Magnum, Define, Outlook, Propel, Surpass, Harness, Degree, Topnotch, Warrant

Pyrazole family (15) - Zidua, Anthem

Dinitroanaline family (3) - Treflan, Trust, Prowl, Acumen, Sonalan, Balan