Treat Johnsongrass after fall corn harvest

COLUMBIA, Mo. – After corn harvest, plan to spray herbicides to treat Johnsongrass, a common quick-growing weed of roadside ditches, crops and pastures.

A perennial invasive grass, Johnsongrass is listed as a noxious weed in Missouri and many surrounding states. While many producers target it in the spring and summer, fall applications are far more effective and provide a better return on herbicide investment, says Kevin Bradley, a University of Missouri Extension weed scientist.

Johnsongrass spreads through both seeds and underground rhizomes. During the fall, the plant moves sugars and nutrients into its roots and rhizomes to prepare for winter. Herbicides applied at this time move with that flow, helping kill the entire root system rather than just the aboveground growth. Glyphosate is particularly effective because it travels into the rhizomes, providing more complete control, Bradley says.

If Johnsongrass foliage has been cut by a sickle mower or combine, allow plants to resume active growth before applying herbicides for maximum effectiveness. Glyphosate and/or Group 1 herbicides with the active ingredients clethodim, fluazifop or quizalofop remain among the most effective control options available. However, Bradley notes, there are a limited number of Johnsongrass populations in Missouri that are resistant to Group 1 herbicides and glyphosate, so producers should consider the herbicide use history of the field in question.

Once established, Johnsongrass can be hard to eradicate because of its extensive rhizome system. Individual plants produce as many as 80,000 seeds annually, and those seeds may remain viable in the soil for more than 10 years. Seeds spread easily through water, equipment and wildlife, including birds and deer.

According to USDA estimates, Johnsongrass infestations can reduce corn and soybean yields by up to 80%. The plant also can be toxic to cattle when grazed shortly after a frost.

Johnsongrass is often confused with shattercane, barnyardgrass and fall panicum. All of these species have prominent midveins on the leaves. However, Johnsongrass can be distinguished by its underground rhizomes, which are absent in the other species.

Shattercane, a close relative of Johnsongrass, is also becoming more common in Missouri corn and soybean fields. Unchecked plants can grow up to 8 feet tall and develop as quickly as corn.

Bradley recommends several management practices to reduce Johnsongrass populations:

• Scout fields regularly.
• Harvest infested fields last.
• Clean equipment before moving between fields.
• Drive around infested patches whenever possible to limit seed movement.
• Treat field edges and fencerows.
• When possible, mow plants before seed heads develop.
• Consult a local MU Extension agronomist about management options.

To see photos of Johnsongrass and other weeds, visit the MU Extension Weed ID Guide at weedid.missouri.edu.

Photo

Johnsongrass. Photo by John Tann. Shared under a Creative Commons license (CC BY 2.0) via Wikimedia Commons.