Multi-chemistry herbicide resistance is on the rise

Multiple resistance or cross resistance of weeds to 2, 3 or even 4 classes of chemistry is steadily increasing, reports University of Missouri Agronomy Extension Specialist Wayne Crook for the Marshall Democrat-News (Marshall, MO).

According to The International Survey of Herbicide Resistance, at least 84 crops in 66 countries have weed competitors with some form of herbicide resistance, either to a single material or multiple chemistries. Also, of the 25 known herbicide action sites on weeds, 22 of them have been compromised by resistance in one or more of 155 different herbicides.

Globally, 437 unique cases of herbicide resistance have been confirmed, factoring in species and/or the herbicide site of action that has been compromised by resistance development. Weed resistance and the trend towards cross resistance does not have any barriers.

There are various tools available to deal with resistance and better yet, to prevent further resistance developing in your operation. Using multiple modes of action is the most obvious to prevent the over reliance and slow down the development of further weed resistance. Other practices of integrated weed management that can aid in reducing the development of resistance, as well as aid in controlling present resistant issues include:

1.    Plant weed free seed.
2.    Prevent weed seed production and removing any seed that is produced. This is accomplished by removing any plants that may have escaped chemical control or by removing or destroying any seed at harvest that has been produced.
3.    Include residual herbicides in your herbicide program.
4.    Select herbicides based upon multiple modes of action.
5.    Apply herbicides based upon multiple modes of action.
6.    Broad based cultural practices. Crop rotation and mechanical management practices offer other options depending on the crop and weed species. Row spacing, planting date and other practices may give opportunities to control difficult species. Chose crop species and varieties that may be more competitive in your growing environment.
7.    Control spread of resistant weed seed in your community. Weed seed from neighbors and machinery movement can spread resistant weed seed.
8.    Minimize movement of weed seed on your own operation. Knowing what resistant weed problems are on your own operation and clean machinery between fields to prevent or minimize movement of resistant weed seed.
9.    Maintain borders to avoid weed seed influx. Pay attention to vegetative areas where highway crews might spray weeds and cause resistant populations that initially go undetected.
10.  Scout fields before harvest for resistant populations. If possible, remove those weeds prior to harvest. If removal is not possible, steer around them at harvest to avoid spreading seeds down the row, then deal with them later.

The more control of resistant weeds that occurs early, the longer you can delay significant problems.