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Issue No. 12, Article 8/June 10, 2005

Broadleaf Signalgrass Control in Corn

Another weed that is slowly creeping up from the south is broadleaf signalgrass (Brachiaria platyphylla (Griseb.) Nash.). The first leaf blade is linear and lays parallel to the ground. The leaves are rolled, and there is a very small ligule, consisting of a few hairs. Leaf blades are keeled (have a blunt end), are 4 to 8 inches long, and are smooth (no hairs) to rough. This lack of hair on the leaf surface distinguishes it from large crabgrass.

Dr. James Martin, extension weed specialist at the University of Kentucky, has written an excellent article on identification and control of broadleaf signalgrass. The article is reproduced here by permission of the author and can be viewed here (Adobe PDF).

Identification And Postemergence Control Of Broadleaf Signalgrass In Corn

Broadleaf signalgrass is a warm-season annual grass that can reduce corn yield by at least 25%. Growers appear to have more difficulty in managing this weed in corn compared with other row crops. The problems are associated with a number of issues including: 1) inconsistent control from soil-residual herbicides, especially in notillage, 2) timeliness of postemergence applications, and 3) antagonism of postemergence herbicide tankmixes.

Mistakes in identification are often a problem, particularly for producers who have not had any previous experience with broadleaf signalgrass. Broadleaf signalgrass looks similar to crabgrass; however, a close look at the collar region (where the leaf blade attaches to the stem) can help differentiate these two species from one another. The ligule (a tongue-like structure on the inside of the collar) of broadleaf signalgrass is a small ring of hairs compared to a large membrane-like ligule for crabgrass plants. Broadleaf signalgrass has hairs along the leaf margins near the collar, yet there are no hairs on the leaf blade surfaces. Also, the leaf sheath (the portion of the leaf surrounding the stem) of broadleaf signalgrass may have hairs, especially on the lower leaves.

The seedhead of broadleaf signalgrass is made up of 2 to 6 finger-like branches that are scattered along the upper portion of the stem and develop throughout the summer. Mature plants grow 2 to 3 feet tall and can develop roots along the lower nodes.

Some of the key identifying characteristics of broadleaf signalgrass are illustrated in Figure 1.

Postemergence control: Research has shown that postemergence applications of Accent at 0.67 oz/A can provide approximately 90% control of broadleaf signalgrass. Basis Gold, Celebrity, and Celebrity Plus should provide control similar to that achieved with Accent. However, early detection of broadleaf signalgrass and timely application of these products are necessary to obtain best results. Ideally they should be applied before broadleaf signalgrass plants exceed one to two inches in height (about 2- to 4-leaf stage of growth). Also, to achieve maximum control of emerged plants, treatments should be made before broadleaf signalgrass plants have tillered.

Antagonism can occur when these herbicides are tank mixed with other products. Reduced control of broadleaf signalgrass has been reported when Accent is combined with Marksman, Buctril, or 2,4-D. Although the antagonism has not been consistent in all studies, it may be wise to consult product labels before using tank mix combinations for broadleaf signal grass control.

The use of herbicide-resistant corn hybrids also allows several opportunities for managing broadleaf signalgrass. Lightning is registered to control up to 8-inch tall broadleaf signalgrass in Clearfield corn hybrids. Glyphosate products (e.g., Roundup UltraMAX, Touchdown, or ReadyMaster ATZ) should control 2- to 7-inch tall broadleaf signalgrass in Roundup Ready corn. Post-directed applications of Gramoxone MAX can be an effective and economical method for controlling broadleaf signalgrass in corn. However, very few corn farmers are equipped to do this method of application.

For more information on broadleaf signalgrass, please visit these Web sites:

Virginia Tech Weed Identification

University of Kentucky (Adobe PDF)

University of Kentucky

Purdue University (Adobe PDF)

--Dawn Nordby and Aaron Hager

Authors:
Aaron Hager
Dawn Refsell

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