HERBICIDE RESISTANCE MANAGEMENT

Matthew D. Schuster[1]

INTRODUCTION

Herbicide resistance management is more important than ever. Resistant weeds, such as ALS inhibitor resistant kochia, are already present. In addition, herbicide selection for various crops may be limited or mirror that of other crops, and the use of other cultural and/or mechanical management practices may not be as applicable.

Due to these reasons, rotating the mode-of-action, or chemistry, is a necessity. If modes of action are rotated, then the same herbicide family or group is not being relied upon year after year, development of herbicide resistant weed populations is prevented or delayed, and herbicide resistant weeds already present in a field are controlled more effectively.

WHAT IS HERBICIDE RESISTANCE?

Herbicides inhibit plant functions, which eventually result in the weed’s death. The herbicide’s mode-of-action relates to that specific function or process in the plant. For example, the mode of action of some herbicides is to inhibit photosynthesis, while other modes of action may be inhibition of amino acid production, cell division, or pigment development.

The trick is to not use an herbicide that has the same mode-of-action year after year. In a population of weeds there will be some naturally tolerant or resistant weeds due to genetic variation. When the same selection pressure, in other words the same mode-of-action, is used on a continual basis, the tolerant or resistant weed species is not killed and will eventually dominate the population over time (see Table 1).

Since the tolerant or resistant species survive, they produce seed and spread. Rotating the mode-of-action so that the selection pressure changes on a regular basis will help prevent, or at least delay, the development of a weed population consisting mainly of the tolerant or resistant species.

MANAGING HERBICIDE RESISTANCE

So what do you do to manage herbicide resistance?

• Understand the biology of the weed species you are trying to control in order to develop an effective weed management program. Knowledge of reproduction, seed dormancy, germination and emergence requirements, susceptible growth stages, and competitive ability compared to the crop will result in better implementation of management strategies.
• Incorporate an Integrated Weed Management approach including prevention, cultural, mechanical, biological (in some situations), and chemical control. All of these management tools are equally important and are necessary for long-term weed management. Relying on only one management option may provide acceptable short-term results, but may not allow for the transition from a reactive weed control program to a planned weed management program for the long-term.
• Keep records of your herbicide use. Do this for the past 3 to 5 years and categorize them by their mode-of-action. Is the herbicide an ALS inhibitor, ACCase inhibitor, photosynthesis inhibitor, etc? If the label doesn’t provide that information, contact your crop advisor, chemical representative, county extension agent or extension specialist. The extension bulletin Herbicide-resistant Weeds and Their Management (PNW 437) lists some of the more common herbicides used in our area, their mode-of-action, herbicide resistant weeds encountered in the PNW, and further recommendations for herbicide resistant weed management.
• After you complete a record of your herbicides for the past 3 to 5 years, note if the same mode-of-action group appears every year. If you see the same group appear in each of your crops, for control of the same weed species, then the risk of developing a resistant weed population becomes more likely.
• Formulate a plan. Look at the other herbicide options that are available and try to rotate to other modes of action. Tank-mixing herbicides with different modes of action to control the same weed species is another good tactic. Realize that different herbicide chemistries can still have the same mode of action e.g. sulfonylureas and imidazolinones. Ideally you don’t want to use the same group for controlling the same weed species more than once in a three-year period.

It may be difficult to not use the same group at least once every year or every other year, that’s reality. However, at least you will recognize what level of selection pressure is occurring in your field so that you can more carefully track your weed control effectiveness, and take appropriate action before major problems occur.

For example, you may be able to utilize more mechanical or cultural management techniques instead of an herbicide at certain times of the year. Or you may be able to alter your crop rotation to allow for the application of an alternative herbicide group. You may have to “save” the use of a particular herbicide mode of action to control your most hard-to-control weed creating the biggest challenge.

IF HERBICIDE RESISTANCE IS SUSPECTED

If herbicide resistance is suspected, note the weed species not being controlled with an herbicide application that normally should be controlling that weed. However, when doing so, be sure to rule out other possibilities for lack of control, such as environmental conditions, poor application, lack of moisture, etc.

To prevent the spread of resistant weeds, make the effort to control weeds in field borders, clean equipment before switching fields, use the most of your crop rotations to manage hard to control weeds through alternative herbicides or cultural and/or mechanical controls, and switch herbicides if the weed is confirmed to be resistant to that mode-of-action. In addition, maintain accurate records in order to evaluate your management practices.

CONCLUSION

Minimizing herbicide resistance is not easy, but with careful planning, it is manageable. It is also necessary if we want to extend the life of some of our herbicides, especially for weed species that have few herbicide options.

REFERENCES

Aldrich, R.J. and Kremer, R.J. 1997. Principles in weed management, 2^nd edition. Iowa State University Press, Ames, Iowa.

Mallory-Smith, C., Hyslop, G.R., Thill, D., Colquhoun, J., and Morishita, D. 2002. Herbicide-resistant weeds and their management. PNW 437. A Pacific Northwest Extension Publication. University of Idaho, Oregon State University, and Washington State University.

Monaco, T.J., Weller, S.C., and Ashton, F.M. 2002. Weed science: principles and practices, 4^th edition. John Wiley & Sons, Inc., New York.

Zimdahl, R.L. 1993. Fundamentals of weed science. Academic Press, Inc. San Diego, CA.

Table 1. Some common herbicides, and their corresponding mode-of-action (MOA) group number, used in a potato, sugarbeet, wheat, and alfalfa rotation. If the same group of herbicides is used every year for control of the same weed species, the risk of selecting for a resistant species that can dominate a given weed population increases (reproduced from Dr. Don Morishita’s power point presentation). Herbicides selected from the 2003 Pacific Northwest Weed Management Handbook.

Potatoes	Herbicide MOA Group	Sugarbeets	Herbicide MOA Group	Spring Wheat	Herbicide MOA Group	Forage Alfalfa	Herbicide MOA Group
Poast	1	Poast, Select, Assure II	1	Achieve 40 DG, Achieve Liquid, Hoelon, Tiller, Puma, Discover	1	Assure II, Poast, Poast Plus, Select	1
Matrix	2	Upbeet	2	Assert, Everest, Harmony GT, Harmony Extra, Glean, Finesse, Ally, Peak, Express, Amber	2	Pursuit, Raptor	2
Prowl, Treflan HFP, Treflan TR-10	3	Treflan	3	Treflan	3	Balan DF, Treflan HFP, Treflan TR-10, Kerb	3
Desicate II	4	Stinger	4	MCPA, 2-4,D, Banvel, Clarity, Starane, Stinger, Curtail, Curtail M	4	2,4-DB	4
Sencor 4, Sencor DF	5	Pyramin DF, Betamix	5			Velpar, Sencor, Sinbar	5
				Buctril	6	Buctril	6
						Karmex, Direx	7
Eptam	8	Ro-Neet, Eptam	8	Avenge, Fargo	8	Eptam	8
glyphosate	9	glyphosate	9	glyphosate	9	glyphosate	9
Rely	10
						Zorial	12
				Aim, Aim EC, Aim EW	14
Dual II Magnum, Dual Magnum	15	Outlook	15
		Nortron	16
Gramoxone Extra, Gramoxone Max, Boa, Reglone	22	Gramoxone Extra	22			Gramoxone Extra	22

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[1] Presented at the Idaho Alfalfa and Forage Conference on February 24, 2004.