There is not much more frustrating than dousing glyphosate on a weedy field and having some of the weeds laugh at you and continue growing. Weeds certainly have been proven to reduce yields, and the inability to control them with your main herbicide is nerve-wracking. However, many Corn Belt fields are populated with marestail which has become resistant to glyphosate. And if your cultivation practice is based on no-till, your soybean yields may take a significant yield hit. What do you do?
Marestail is one of those weeds that does not fit the typical schedule of other weeds. It can emerge either in late March through June or in late summer to fall. Those in the spring will form a low growing rosette before growing as tall as 6 feet. And their emerging fall cousins will be growing tall in the spring, and quite tough to control in their advanced stages. Weed specialists Mark Loux of Ohio State and Bill Johnson of Purdue have been working on a strategy for controlling marestail that is resistant to glyphosate. Part of the strategy is built on the use of LibertyLink soybeans and the use of Liberty herbicide. However, there are many alternatives that they suggest which will integrate with your weed control program. One of the benefits is avoiding an average 14 bu. per acre yield loss between a successful program and one which failed to control the marestail.
The specialists say the easiest time to control marestail is when it is in the rosette state, and less than 4 inches tall. Many of the populations have resistance to both glyphosate as well as herbicides that are ALS inhibitors such as Classic, Synchrony, or FirstRate. Their research reports that marestail is not resistant to Liberty, therefore, the use of LibertyLink soybeans are the most effective control strategy. They suggest the use of burndowns and residual herbicides, such as a post emergent application of Liberty at a 29 oz/A rate if the marestail exceeds 6 in.
If you have glyphosate resistant marestail growing this fall, your goal is to control the emerged plants with a burndown, followed by another application in the spring. Fall herbicides should not be expected to carry over to the spring, however if your marestail is not ALS resistant, you could use a product with chlorimuron. Loux and Johnson say Authority, Valor and products with metribuzin should not be used in the fall, but used in the spring. For an economical fall burndown, they suggest:
- glyphosate + 2,4-D; or
- dicamba + 2,4-D; or
- 2,4-D + a low rate of Canopy/Cloak EX or DF