Glyphosate–resistant Russian-thistle was identified in Washington in 2015. Other weeds resistant to glyphosate have been identified in the state in recent years, including prickly lettuce, horseweed, kochia, and Italian ryegrass. Many people think that herbicide resistance is a recent phenomenon associated with the overuse of glyphosate-resistant crops, but as a recent article released by the Weed Science Society of America states, herbicide-resistant weeds predate glyphosate-resistant crops by 40 years.
The use of glyphosate-resistant crops (there are no glyphosate-resistant wheat varieties) is very limited in the dryland crop production systems of Eastern Washington and yet herbicide-resistant weeds are a growing concern for many Washington wheat farmers. Resistant weeds can evolve whenever a single approach to weed management is used repeatedly, whether that approach is chemical, mechanical, or cultural. A diverse, integrated approach to weed management is the first line of defense against herbicide-resistant weeds.
Washington wheat growers who suspect that they may have developed a weed that is resistant to an herbicide may want to submit a sample to the WSU Resistance Testing Program.
Water is a universal solvent that serves as the primary carrier for pesticide applications. The quality of the water used as a carrier can have a large influence on the performance of herbicides such as glyphosate. Dissolved cations such as calcium, magnesium, zinc, iron, and manganese form complexes with glyphosate that reduce its efficacy.
Ammonium sulfate (AMS) conditions water by reacting with the dissolved cations to form insoluble sulfates that will not react with glyphosate. Spray grade AMS should be added to the spray tank and thoroughly mixed before adding glyphosate.
Here is a handy calculator that uses data from a standard water quality test to determine the amount of AMS to add to your spray tank, in pounds of AMS per 100 gallons of water. The calculator uses an equation developed at North Dakota State University (Nalewaja and Matysiak, 1993) to determine the required amount of AMS needed to neutralize the effects of cations in the water on glyphosate activity. Adding more AMS than called for to neutralize the effects of cations may improve glyphosate activity by providing extra N that helps weak acid herbicides like glyphosate pass through cell membranes. The addition of 8.5 to 17 pounds of AMS per 100 gallons of water is generally recommended to improve glyphosate activity. Liquid forms of AMS are equally effective if used at equivalent rates.
Give the calculator a try and see what you think.