Sweet Corn Growers Must Be Wary Of Annual Grasses, Resistant Species, And Emerging New Weeds

 

Sweet corn growers need to be wary of annual grasses as these weeds can be very competitive with the crop.  Photo credit: Dwight Lingenfelter, Penn State University
Sweet corn growers need to be wary of annual grasses as these weeds can be very competitive with the crop.
Photo credit: Dwight Lingenfelter, Penn State University

Sweet corn growers have many weed management tools at their disposal compared to 10 or 15 years ago. But even with these new products, weed control in sweet corn can still be a challenge. This crop is generally planted at lower plant populations than field corn, and is also slower growing, shorter, and produces a less-dense canopy.

As a result, additional light penetrates to the soil surface and lower canopy, favoring weed growth. These factors make sweet corn less competitive with weeds than field corn, and thus, more susceptible to losses due to weeds. In addition, weeds interfere with the operation of mechanical pickers.

This article will cover issues related to annual grass control, the increase in resistant weed species, and a new weed to the Mid-Atlantic area: Palmer amaranth.

Problem Grasses
Annual grasses such as foxtails, crabgrass, fall panicum, and shattercane can be common problem weeds in sweet corn. They can be very competitive with the crop and difficult to control postemergence.

Crop rotation usually helps reduce the weed seedbank in the soil. Herbicides like Harness (Monsanto), Outlook (BASF), Impact (Amvac Chemical Corp.), Accent Q (DuPont Crop Protection), Option (Bayer CropScience), and Dual II Magnum and Lumax (Syngenta Crop Protection), will control most annual grasses.

Keep in mind, crabgrass, yellow foxtail, and fall panicum can be especially difficult to control postemergence. In general, two-pass systems, which are those that include a soil residual followed by a postemergence application, provide the best and most consistent control of annual grasses.

Resistance Issues
As more weed species become resistant to herbicides, certain precautions such as tank-mixing, crop rotations, and a combination of weed management techniques must be implemented to prevent resistance. Understanding herbicide modes of action is a key factor in this process. The Weed Science Society of America developed a grouping system to help with this process.

Herbicides that are classified as the same group number kill weeds using the same mode of action.
Thus, it is best to select or combine herbicides that provide at least two different modes of action against the same weed. Group numbers can be found on many herbicide product labels and can be used as a tool to choose herbicides in different mode of action groups, so mixtures or rotations of active ingredients can be planned to better manage weeds and reduce the potential for resistant weed species.

Two-pass herbicide programs are also good for managing resistant species. Having residual herbicides in the program broadens the spectrum of weed control, improves weed control consistency throughout the growing season, and allows timely post applications (over-the-top applications to weeds and sweet corn) by widening the application window.

Postemergence herbicides should only be used in sequence after a soil-applied herbicide. In Penn State research, a two-pass system provided more effective weed control overall compared to a single application. For best results if spraying a post treatment, apply when the weeds are small (less than 3 inches tall).

Total post weed control is not recommended because sweet corn seedlings are very non-competitive with weeds, and weather conditions that prevent postemergence herbicide application may delay weed control until it is too late to prevent loss. Having a soil-applied herbicide down improves overall weed control, provides additional herbicide modes of action for resistance management, and provides some insurance in case postemergence herbicides cannot be sprayed on time.

New Weed Pests In The East

If Palmer amaranth is present in your field, do not remove the weed plants. Instead, destroy the weeds by burying or burning near the infested field. Photo credit: Dwight Lingenfelter, Penn State University
If Palmer amaranth is present in your field, do not remove the weed plants. Instead, destroy the weeds by burying or burning near the infested field.
Photo credit: Dwight Lingenfelter, Penn State University

During the last several months, there have been a number of potential Palmer amaranth (Amaranthus palmeri) infestations in Pennsylvania. These have ranged from one or more soybean fields with severe infestations to a small number of plants that are relatively isolated.
Waterhemp is another pigweed species that is creeping into the Northeast.

This new outbreak could likely be associated with the problems to the south and west with these two species. It is uncertain how Palmer amaranth was introduced to some farms, but equipment, hay, feed, and manure are all suspected. Paying close attention to potential pigweed seed contamination could really be important in preventing the introduction of these two weeds in the Northeast.

Like other pigweeds, Palmer amaranth and waterhemp seeds are very small, round, and black. Other identifying features include smooth stems, leaf architecture around the stem that resembles a poinsettia, a singular hair in tip of the leaf notch when it is small (less than 6 inches tall), a long petiole (a stalk that attaches the leaf to the stem), and a watermark pattern on some leaves — just to name a few.

Management Tactics
There are certain herbicides in sweet corn that provide control of Palmer including atrazine, acetochlor-products, Prowl (BASF), Impact (Amvac Chemical Corp.), Callisto and Lumax (Syngenta Crop Protection), Laudis and Liberty 280 (Bayer CropScience), 2,4-D and a few others. Again, two-pass systems work best with Palmer since it has a long germination period.

If these weeds are present on your farm, do not transport them from fields. It is best to keep the plants/seeds in the area in which they are identified. There is concern that seeds could be spread if any kind of movement occurs outside the field boundaries. Instead, destroy weed plants by either burying or burning near the infested field.

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