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College of Food, Agricultural, and Environmental Sciences


How to Control Weeds in Continuous Corn

March 1, 2007

COLUMBUS, Ohio -- Planting continuous corn may promote greater weed control problems, but growers can stave off any serious issues as long as they follow the best management strategies for their situation.

"Rotation of crops has historically resulted in greater diversity in herbicide use. Planting a monoculture of corn, or even soybeans, can result in greater weed problems over the long term, due to the propensity for overuse of certain herbicides that weeds can become resistant to," said Mark Loux, an Ohio State University Extension weed specialist. "However, we are not convinced that the planting of continuous corn really poses more serious issues for weed management, compared with a crop rotation, as long as appropriate weed management strategies are used."

Loux offers the following considerations for weed management in a continuous corn system to most effectively control tough weeds and minimize selection for herbicide resistance.

• Use tillage or pre-plant burndown herbicides to ensure that corn is planted into a weed-free seedbed. Loux said doing so may help growers avoid issues of glyphosate resistance that have impacted other crop systems. "Planting soybeans into weedy fields, and delaying the first glyphosate application until sometime after soybeans emerge, contributed to the development of glyphosate resistance in giant ragweed and marestail, and also the general increase in winter annual weeds and dandelions," said Loux, who also holds an Ohio Agricultural Research and Development Center appointment.

• Select an herbicide program that is appropriate for the weeds in the field. "A total pre-emergence herbicide program can control many annual weeds, but frequently fails to provide season-long control of weeds that can emerge in mid-season, such as dense annual grass infestations, giant ragweed, burcucumber, and perennial weeds," said Loux. "A combination of pre-emergence and post-emergence herbicide applications is more effective for these type of weeds, and in any field with a history of poor weed control."

• Use a diversity of herbicide sites of action to reduce the risk of herbicide resistance. "This is more easily accomplished where a combination of pre-emergence and post-emergence herbicides are used, compared with a total pre-emergence or post-emergence approach," said Loux. "For example, avoid using atrazine as the sole broadleaf herbicide in continuous corn, and avoid continued use of ALS inhibitors for control of the same weeds every year."

• Use other herbicides in conjunction with glyphosate programs to reduce the chance of glyphosate resistance. "Some examples: apply pre-emergence herbicides at the time of planting to reduce the weed population that will need to be controlled with post-emergence glyphosate applications; in a tilled field where a total post-emergence program is planned, mix glyphosate with other herbicides that can help control emerged weeds and provide residual weed control; and where glyphosate is applied late post-emergence following pre-emergence herbicides, mix it with low rates of other post-emergence herbicides," said Loux.

• Manage post-emergence glyphosate applications appropriately, and apply when weeds are small.

• Consider using Liberty Link herbicide in post-emergence applications instead of glyphosate when planting corn hybrids with the BT Herculex trait, or when planting stacked hybrids that are resistant to both glyphosate and BT Herculex. "There is about a $6 per acre difference in cost between glyphosate and Liberty, but use of Liberty breaks the cycle of continuous glyphosate use, and this can have long-term benefits," said Loux.

• Consideration should be given to the type of corn that is used each year in a continuous corn system. "From a herbicide use standpoint, growers have a choice of conventional hybrids; glyphosate-resistant hybrids; Liberty Link hybrids; and stacked-trait hybrids," said Loux. "Rotation of these types of corn from year to year, and corresponding rotation of herbicides, should reduce the risk of resistance to glyphosate and other herbicides."

• Consideration should be given to the rotation of corn hybrids to minimize the risk from volunteer corn, which could be herbicide-resistant and pose a weed problem.

For more information on managing weeds in continuous corn systems, or to learn more about continuous corn production, log on the OSU Extension Agronomy Crops Team Web site at

Candace Pollock
Mark Loux