By Mark Loux

We have received more questions about post-emergence marestail control in corn this year than the past 10 years combined. We do not currently show effectiveness ratings on marestail for post-emergence corn herbicides in the “Weed Control Guide for Ohio and Indiana” because of the previous lack of questions.

Dicamba, Liberty, and atrazine are really the only effective options here, and atrazine alone may not kill larger plants. The HPPD inhibitors (group 27), Callisto, Impact/Armezon, and Laudis, and the PPO inhibitors (group 14), Cadet, Aim, and Resource, do not have adequate activity on marestail. Use of ALS-inhibiting herbicides (group 2) is also not advised due to the prevalence of ALS-resistant marestail populations. A few other reminders:

• Status can be applied broadcast to corn up to 36 inches tall or V10 (rates up to 5 oz/A).
• For Clarity and other 4 lb/gal dicamba products, the general recommendations are: Up to 16 oz product/A — apply broadcast up to the 5-leaf stage or 8-inch corn; 8 oz product/A or less — apply broadcast or as a directed spray up to 36 inches tall or 15 days before tassel emergence, whichever occurs first. Corn larger than about 8 inches tall is more sensitive to dicamba, and use of a directed spray will reduce the risk of injury.
• Effective activity of Liberty and atrazine requires spray volumes of at least 15 gpa, and nozzles that promote adequate coverage with contact herbicides.
• For any herbicide, use of drop nozzles to deliver spray below the canopy of taller corn can improve control of weeds that are smaller than the corn.