Have you asked, or been asked, that question in the past couple of years? I think we all have in the Eastern Cornbelt. Never the easiest of weeds to control, herbicide resistance has made them that much tougher. Multiple flushes and a lack of leaf surface area add to the challenge. Here are a few things we have learned.
1. Spray small weeds
Marestail, like most weeds, are easier to kill when they are small. Our goal is to spray them while they are still in the rosette stage, before the central shoot starts to elongate.
2. Use Enough Gun
That’s the title of one of the best books written about African safari hunting. It’s also sound advice for marestail and other resistant weeds. Once marestail have been injured, from an ineffective herbicide application or an incomplete tillage pass, they are almost impossible to kill. Don’t skimp on the herbicide rates. Use the correct adjuvants at the full recommended rate.
3. Tillage Timing
Some new minimum tillage tools are popular in some areas. They are designed to create holes in the top few inches of soil with a minimum of disturbance. The problem is they also can damage weeds without killing them. This can make the weeds less susceptible to a herbicide application. If used before a burn-down, make sure surviving weeds heal completely before spraying. If tilling after spraying, give the weeds time to die before disturbing them. From a weed control stand-point, you are probably better off tilling completely or not at all.
4. Multiple Modes of Action
This is solid advice to delay weed resistance. If resistance is present, you must use this strategy. Some weed populations are developing cross-resistance to more than one class of herbicide. Try to apply at least 2 classes of herbicide that are still effective on the weed species present. If glyphosate resistant marestail are present, don’t count glyphosate as one of the 2 modes of action. Even if you are applying 2 or 3 modes of action, if only 1 mode is controlling the weeds, you will hasten resistance to the effective class of chemistry.
5. Multiple Applications
Multiple weed flushes mean multiple applications. Marestail used to be primarily a winter annual. It germinated in the fall and matured in late spring/early summer. Now it seems to germinate almost all year long. No one application will provide season-long control. We recommend an application in the fall after harvest, using a high rate of 2,4-d and/or dicamba. You may want a residual herbicide in the fall as well. If you are only going to use a residual product once, though, do it in the spring. A fall residual will run out at, or shortly after, planting, leaving you no protection from late germinating weeds. If marestail are present again in the spring, a burn-down treatment containing one of the Kixor products may be combined with a strong residual product to extend control.