Plants' Resistance to Herbicides

Signs of Resistance

• Weed patches surviving after the application of chemicals that would normally control that species may have a resistance to the chemical. Look for weed patches with indistinct edges. A patch with defined edges may have been missed during the spraying process.

Preventing Resistance

• Rotating crops and chemicals prevents the weeds from developing resistance to any particular chemical. Crop rotation, planting a different crop each year in a multiple year rotation, only works if the crops use different chemicals for weed control. Always use chemical herbicides at the recommended strength. Weak chemical applications kill off the weak weeds, allowing the strongest to reproduce and increasing the vitality of the weed strain.

Controlling Resistant Weeds

• Deal with the weeds once a resistant strain is identified. Change the herbicide to something unrelated to the herbicides used in the past. Changing products doesn't accomplish anything if both contain 2,4-D as their active ingredient, for example. Physical cultivation, including hand weeding if necessary, may be the only control option. Clean any farm equipment with a pressure washer after working in the area of the resistant weeds to prevent spreading seeds.

Weeds Known to Have Resistance

• Agronomists have identified several weeds that have developed herbicide resistance in the past. Most common is pigweed although lambsquarter, ryegrass and wild oat, among others, have all demonstrated the ability to form resistance to herbicides. In most cases, these plants have developed resistance to glyphosate-based chemicals, such as Roundup.