FARGO - Weeds resistant to herbicide used to control them are a potential new problem for North Dakota farmers, according to Dallas Peterson, extension weed control specialist at North Dakota State University.

Herbicide resistance, Peterson said, occurs when a weed becomes tolerant to a chemical to which it was previously susceptible. While this has previously been a problem in other parts of the country, he said now kochia populations resistant to Glean have been confirmed in the state, while resistant Russia thistle and prickly lettuce populations have developed elsewhere.

Peterson said resistance to a herbicide can develop Iwben weeds that contain a low frequency of genetically resistant plants are continuously exposed to the chemical; while the herbicide controls the susceptible plants, the frequency of resistant plants gradually increases. Poor weed control doesn't necessarily indicate development of a resistant weed population for other factors influence weed control, such as amount of herbicide applied, stage of growth of the weeds and environmental conditions at applicauon, Peterson said, but ineffective control with a herbicide that has done a good lob in the past may indicate resistance.

Factors which can contribute to development of herbicide resistant weeds include repeated use of a herbicide or use of chemicals that have the same mode of action, using highly effective herbicides that give nearly complete control of susceptible plants by affecting a single metabolic process, a high turnover rate, limited lseed dormancy in a species and a diverse weed populations with plants that have the genetic trait of resistance.

Resistance of kochia and Russian thistle to Glean confirms to this scenario, said Peterson. Glean, a very persistent herbicide often remaining in the soil for two to three years, is the most cost effective treatment for broad-leaved weed control in continuous wheat and wheat-fallow-wheat production systems.

Glean, he said, controls weeds by acting on a specific enzyme site and is very effective against kochia and Russian thistle, but both weeds are highly variable with many different biotypes.

Kochia resistant to Glean developed in fields where Glean was used continuously for four to five years, according to Peterson. He said native kochia populations are estimated to contain about one resistant plant per million susceptible plants and resistance gradually becomes more predominant.

Weeds which are resistant to Glean also have increased tolerance other other chemicals with the same mode of action, including the suuonylurea herbicides, Ally, Harmony, Express, classic, Amber and Oust, and the imidazloinone herbicides, Assert, Scepter, Pursuit and Arsenal, and several new experimental herbicides.

Development of weeds resistant to herbicides with this mode of action could have a profound effeclt on development of new herbicides, Peterson said.

Buildup of resistant weed populations he said, can be minimized or prevented by using alternative methods of weed control to reduce the selective pressure for resistant types, including crop rotations, integrated pest management, avoiding repeated herbicides that have the same mode of action, using combinations of herbicides that have different modes of action and preventmg the spread of resistant weeds to new area.

Glean, Ally and Harmony labels are heing modified to use these techniques for avoiding buildup of herbicide resistant weeds, said Peterson, and the revised labels will limit the frequency of applications and emphasize tank mixtures with other herbicides.

Failure to follow the label guidelines where resistance has occurred, he said, may eventually lead to loss of the effectiveness of otherwise useful herbicides.

Dallas Peterson

Extension Weed Control Specialist

North Dakota State University

Fargo, N.D.

December 1988