The glyphosate-resistant marestail (horseweed) infesting Tennessee cotton fields this year required a 6X rate of glyphosate for control in university tests. That's highly resistant, according to University of Tennessee weed scientists.

How do other materials and tank mixes perform at labeled rates for burndown?

In a field test on resistant marestail conducted by University of Tennessee weed scientist Bob Hayes this year, an application of glyphosate and Direx applied on Feb. 18 provided only 65 percent control. An application of Gramoxone Max and Direx provided 99 percent control. Meanwhile Harmony Extra and Gramoxone Max provided 70 percent control. Gramoxone Max and 2,4-D provided 99 percent control.

If a plant is resistant, at normal rates “growers will see stunting and yellowing of the plant. But the growing points are green,” said Neil Rhodes, Extension weed control specialist at UT.

Hayes' research also indicated that there was little difference in control of resistant marestail among glyphosate formulations as well as among surfactants and surfactant rates. However, water volumes over 10 gallons per acre reduced control.