There is still time to control weeds in fields that were not planted to crops this year due to wet weather. A major goal of any control implemented at this time should be prevention of seed production by summer annual weeds.
It’s not always necessary or practical to kill weeds, especially large ones, to prevent seed production. Summer annual weeds that are mowed now, or substantially affected by herbicides, should produce few seed, even if they are able to still reach maturity.
The choices for control are probably limited to mowing or herbicides at this point. Tillage is certainly an option, but control of the large weeds in many fields would require more than a shallow tillage pass.
As much as we would like for growers to avoid glyphosate applications in order to minimize further selection for glyphosate-resistant weeds, a glyphosate-based herbicide program may still make the most sense here.
Large weeds are most effectively controlled by systemic herbicides and the low cost allows glyphosate to be used at 1.5 to 3 lbs ae/A in these fields. It’s important not to apply glyphosate alone, however. A mixture of glyphosate and 2,4-D may be the most cost-effective approach. This should control most grass and broadleaf weeds, and injure the glyphosate-resistant marestail enough to prevent or greatly reduce seed production.
Use 2,4-D amine and avoid applying near areas that could be damaged by 2,4-D movement. It’s also possible to apply a mixture of glyphosate and dicamba, or a premix of 2,4-D and dicamba, but the potential volatility of dicamba at this time of year is a major concern.
Where it’s not possible to use a mixture of glyphosate and 2,4-D, use of either Ignite or a mixture of glyphosate and Sharpen can achieve the goal of minimizing weed seed production, especially for marestail.
It may be possible to accomplish this with Gramoxone also, but only at the higher labeled rates and in combination with 2,4-D or a high rate of metribuzin. Use a spray volume of 15 to 20 gpa when applying Sharpen, Ignite, or Gramozone, and avoid use of nozzles that produce primarily large droplets.