There are several preplant and preemergence residual herbicides available for corn. These herbicide programs are key to managing glyphosate-resistant and other difficult-to-control weeds. 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 23-25 of the 2014 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland (SRP 1099). See: http://www.ksre.ksu.edu/bookstore/pubs/SRP1099.pdf
For burndown applications in a no-till system on emerged grass and broadleaf weeds, an application of glyphosate and a product containing dicamba or 2,4-D 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 preemergence herbicides often provides control of weeds for several weeks. This can greatly improve the effectiveness of a postemerge herbicide application, and give the producer more leeway on post application timing.
Soil-applied residual herbicides for corn can be grouped into several basic categories.
Acetamides and acetamide/atrazine premixes. The main acetamide products used in corn include acetochlor, S-metolachlor, dimethamid-P, pyroxasulfone, and flufenacet, and many premix products containing one of these five active ingredients. In general, these products are very effective in controlling annual grasses (except shattercane and perennial 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 and Anthem. 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/acetamide premixes are available and should be used instead of acetamides alone unless atrazine is not allowed. These premixes generally fit into two groups. A reduced atrazine rate and a full atrazine rate group of herbicide. Soil type, soil pH, and organic matter will determine whether the reduced or full rate atrazine herbicide 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 postemergence herbicide program. With the increased occurrence of glyphosate- and other herbicide-resistant weeds, the use of reduced/setup rates greatly increases the risk of unacceptable control.
HPPD-inhibitors. Examples of HPPD-inhibitors are isoxaflutole (e.g. Balance Flexx, Corvus, and Prequel) and mesotrione (e.g. Callisto, Callisto Xtra, Lexar EZ, Lumax EZ, and Zemax). These products either contain atrazine or should be applied with atrazine, and are excellent on kochia, pigweeds, velvetleaf, and many other broadleaf weeds. Lexar EZ, Lumax EZ, and Corvus+atrazine 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 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, provided the sunflower is not ALS resistant. Herbicides containing clopyralid such as Hornet, TripleFlex, or Surestart will provide very good control of sunflower.
Zemax and Callisto Xtra are new herbicides containing S-metolachor and mesotrione, and are similar to Lumax EZ or Lexar EZ less the atrazine. Control of broadleaf weeds with Zemax or Callisto Xtra will be less than Lumax EZ or Lexar EZ unless atrazine is added to the mix. Callisto, a component in Lexar EZ or Lumax EZ, has the same mode of action as Balance Flexx or Corvus but has less activity on grass weeds, thus if applied preemergence it should be applied with an acetamide and atrazine.
Triazine. Atrazine is a common component of many preplant and preemergence herbicide premixes for corn. Where weed pressure is light, a March application of atrazine with crop-oil concentrate and 2,4-D or dicamba 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 to 1.0 lb/acre atrazine with a pint of dicamba applied in early to mid-March can provide excellent control of germinating kochia. It is essential to add glyphosate 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).
PPO-inhibitors. Examples of PPO-inhibitors include flumioxazin (e.g. Valor, Fierce), and saflufenacil (Sharpen, Verdict). Valor or Fierce must be applied 7 to 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 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, length of residual is relatively short compared to other preemergence products when all are compared at full rates.
ALS-inhibitors. Examples of ALS-inhibitors for use as a soil-applied herbicide for corn include flumetsulam (Python) and Hornet, which is a premix of flumetsulam and clopyralid. 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/atrazine mix as a preemerge treatment will control the annual grasses and add considerably to large-seeded broadleaf weed control. Sunflower appears to be most sensitive to Hornet, followed closely by cocklebur and velvetleaf. Morningglory is less sensitive. Depending on weed species present, control may be improved enough that a postemerge treatment is not needed.
An additional ALS-inhibiting herbicide from DuPont is called Resolve. Also a component in Prequel and Instigate, which was previously mentioned, Resolve will provide short residual control of grass and broadleaf weeds and should be used as a setup herbicide with a good postemergence weed control program. If ALS-resistant broadleaf weeds are present, these ALS-containing herbicides often will be less effective.