NDSU Crop and Pest Report

ISSUE 9  July 1, 2004


Avoid letting Valor herbicides set in the spray tank for more than 6 hours. The reason is not because of degradation in the tank but tank contamination. If allowed to sit in the tank the active ingredient will bind to hoses, plastic, rubber, and many pluming parts of the sprayer. Regular sprayer cleanout operations may not thoroughly strip the active ingredient out of the sprayer components but may dissolve in later sprayer tank loads. If these other pesticide loads are sprayed on sensitive crops, crop injury may result from the Valor solubilizing back into the spray solution.



For several years, wild oat resistance has been documented to some ACC-ase herbicides like Hoelon, Puma, Assure II, and Poast. Increase in affected acreage has been slow without much alarm or haranguing from growers. The expression of ACC-ase resistant wild oat fit the proclaimed "Bullet theory", where every time you fire a bullet or use a herbicide of the same mode of action, it tilts the scale closer to observed resistance to that mode of action. Past history has shown with ACC-ase resistance it takes about 8 to 12 herbicide applications before resistance is observed. A normal herbicide use pattern in ND would be Puma in wheat, and Assure II or Poast in sugarbeet, soybean, drybean, or other broadleaf crops. Most growers that suspected resistance by observing less than normal control changed chemical practices and used other herbicides.

In the 1990s when Select received labeling on many crops grown in ND, growers switched from Poast to Select. Reasons for switching from Poast to Select was that Select had no weaknesses. It controlled all annual grasses and volunteer cereals, gave excellent quackgrass control, and research findings found it was antagonized less than Poast or Assure II when tank-mixed with broadleaf herbicides. Research even found that Select killed wild oat plants found resistant to Puma, Poast, and Assure II which shattered the theory that when a plant becomes resistant to a herbicide it is resistant to all herbicides of that mode of action. Some growers may have thought that by switching to Select they were rotating herbicides and using a different mode of action. Other may not have cared and just wanted the end result - dead weeds. Either way, for several years Select has become the silver bullet by killing ACC-ase resistant and susceptible wild oat and other grasses.

How long could this last? How long could we expect Select to kill all the grasses especially ACC-ase resistant wild oat? This question has been looming over our headís for a few years. Could it be that this Silver Bullet is titanium plated? Well, it looks like the "Bullet Theory" proves true again. Select has enjoyed preferential use for the last 10 years or so and now this last week or two growers are reporting wild oat escaping not only Select but sequential applications of Assure II and Poast. Less than 10 sites has been reported but that means there are probably a lot more sites where resistance has happened but populations are not large enough to cause alarm to the grower or the grower might blame the escaped plants on weather, application error, skips, etc., etc., etc.

Even though our bubble has burst and Select is under the same liabilities as other ACC-ase herbicides, the sky is not falling. Just as with the case of weed resistance to glyphosate, Select has many more years of use and excellent grass control. But here is an opportunity to prove NDSU recommendations on page 107 of the 2004 ND Weed Control Guide to manage weed resistance. We give two methods, of which the first goes against hollowed weed science strategies. The first method is: Continued Herbicide Use. Growers have done just this probable without reading page 107. If the product of choice is effective, economical, compatible, and consistent - why should you change to another product? If you do change to a different product to conserve the original product then you have lost it already. Resistance will happen whether you do or donít use it so why not use it for as long as you can? There is a second part to this method that people fail to read - close monitoring for expression of resistance. This is the critical step to make Method 1 work. If you find a few plants that escape your herbicide treatment then kill them with something else. If you never let a weed go to seed you will never get resistance!

Below is information from the resistant weed section found on page 107 of the ND weed guide.

Weed resistance to herbicides cannot be prevented, but can be delayed. Herbicide and tillage rotations will only delay resistance by the length of time that the selection pressure for a given herbicide is removed by an alternative control method. The gene pool does not revert back in absence of the original selection, except if the resistant plants are poorly fit. Fitness has not been greatly different for resistant and susceptible biotypes and should not be relied on for resistant management.

There are two options relative to resistance management: one is to use the desired herbicide until resistance occurs and then change to an alternative, and the other is to rotate control methods to delay the on-set of resistance.

Method 1. Continued Herbicide Use - This approach allows for the use of the preferred treatment, but will require earlier close monitoring for resistance. The best resistance management strategy is early identification of resistant plants and then complete control (eradication) of the resistant plants while the infestation is small. Hand weeding, non-selective herbicides, cultivation or combinations of methods can be used for eradication. Identification can be best accomplished with highly effective herbicide rates so that uncontrolled plants are obvious for early eradication. Elimination of the resistant plants will allow for continuous use of the herbicide.


  • Allow use of preferred herbicide.
  • Allow for use of the herbicide best suited for weeds in a given field.
  • The above may save costs as a herbicide with a second mode of action may not be needed for the weeds present before resistance develops.
  • Disadvantages:

  • Resistance will occur sooner and require earlier monitoring for resistance.
  • Does not save the herbicide for use in crops without alternatives.
  • Method #2. Rotate Herbicides - This system will delay resistance, but may use unnecessary or less desirable herbicides in rotation or in mixture. Delaying resistance by alternative herbicides in the crop rotation is a means of keeping a herbicide for use in a crop that does not have an effective alternative.


  • Monitoring for resistance probably can be delayed.
  • Herbicide mixtures may give control of more weed species and reduce the need for scouting to choose the appropriate herbicide for the field.
  • Disadvantages:

  • May need to use herbicides other than the most desired.
  • Will select for multiple resistance.
  • Fewer herbicide options saved for future use.
  • Richard Zollinger
    NDSU Extension Weed Specialist

    NDSU Crop and Pest ReportTop of PageTable of ContentsPrevious PageNext page