About 1990, the first reports of resistance to ALS inhibiting herbicides appeared. These herbicides (e.g., Scepter, Pursuit, Classic, Pinnacle, Glean) are from two relatively new classes of herbicides (sulfonylurea and imidazolinone) that are highly effective at very low rates. These herbicides are used primarily on corn, soybean, wheat, and other field crops.

Resistance has continued to appear in many locations around the world, and there are currently about 250 known resistant weed species worldwide. Five cases have been reported in Michigan. The resistant weeds in Michigan include common lambsquarters resistant to triazines, wild carrot resistant to 2,4-D, common ragweed resistant to chloransulam, common purslane resistant to linuron, and a recent report of pigweed resistant to triazines and phenylureas.

Weed resistance to herbicides develops after repeated use of herbicides with the same mode of action. There are about 10 major modes of action which are recognized by weed scientists, with a few herbicides that have other modes of action. In most cases, if a herbicide develops resistance to one chemical, it will have cross resistance to other chemicals that attack the same site of action. For example, weeds that develop resistance to triazines, such as Aatrex or Princep, are usually also resistant to one or more phenyl ureas, such a Lorox or Karmex. The reason is that both of these classes of herbicides are photosystem II inhibitors. In some cases, resistant weeds also have cross resistance to herbicides unrelated chemically to the ones that caused the resistance.

As a general rule, the more effective a herbicide is, the more quickly resistance will develop. Thus, resistance to the ALS inhibitors developed after only about five years of large-scale use. The most likely explanation is that the herbicides kill all susceptible biotypes so effectively that the very few naturally resistant plants in a normal population reproduce and expand rapidly. Less effective herbicides leave more plants of the susceptible population which continue to predominate until their numbers are reduced to the point that the resistant biotype predominates.

Growers should be concerned about two issues in the field: resistance avoidance and resistance management. To avoid resistance, use herbicides with different modes of action. This is possible for most crops where several herbicides are registered. Rotate crops if that will allow use of different herbicides. Use herbicides at a rate sufficient to kill most weeds. This will avoid selection for moderately resistant individuals.

Once resistance occurs, a management program is needed. Herbicide and crop rotations are important. Changing to a completely different agronomic system and herbicide program also will help. Tillage equipment should be cleaned between fields to avoid moving resistant seed of plant parts to non-infested areas. In extreme situations, it may be necessary to leave a field fallow for a season and treat the resistant weeds with effective herbicides several times. In any case, the sooner a management program is initiated, the sooner the problem will be brought under control.

Weed resistance has not been a major problem in vegetable production since most growers rotate their crops regularly, and use many different herbicides. However, resistance can develop. For instance, linuron [Lorox] resistant common purslane was discovered in a field in which carrots had been grown and linuron applied for many years. Pigweeds that may be resistant to simazine and diuron have survived in asparagus fields on which those herbicides have been used for many years.

If growers observe weeds surviving that in the past would have been killed by a herbicide, they should contact their Extension agent. We can test to confirm the existence of herbicide resistance.