Strategic Farming: Let's talk crops! session talks spray water quality's impact on herbicide efficacy
By Angie Peltier, University of Minnesota Extension crops educator, Joe Ikley, NDSU Extension weed specialist, and Greg Dahl, director of adjuvant education, CPDA
March 18, 2026’s Strategic Farming: Let’s Talk Crops session discussed how spray water quality impacts herbicide efficacy. This webinar series runs through the end of March and registrations are still being accepted: https://extension.umn.edu/courses-and-events/strategic-farming. To watch this episode visit: http://z.umn.edu/StrategicFarmingRecordings.
Weed control can be impacted by environmental conditions at the time of application, with temperature, relative humidity, rainfall and wind all capable of impacting efficacy. Similarly, the quality of the water used for spraying herbicides such as water temperature, pH, water hardness, and sediment load can impact herbicide efficacy. Specific adjuvants have been developed to overcome these water quality issues. Adjuvants can also combat foaming, reduce tank mix compatibility issues, work as surfactants or deposition agents or reduce drift.
Before he retired in 2023, Greg Dahl was asked to obtain adjuvant-related weed control research from land grant universities including University of Minnesota, North Dakota State University and Iowa State University, to determine the value of adjuvants to weed management. Herbicides alone were compared to herbicides with the recommended adjuvant and to herbicides with comparison adjuvants. When compared to the weed control observed when using a required adjuvant, weed control declines by between 30 and 90% when the same herbicide was applied without the adjuvant. Using the correct type of adjuvant is important to improve weed control and limit crop injury; the survey found that using the wrong type of adjuvant (ex. using crop oil instead of a surfactant) can reduce weed control between 5 and 50% compared to using the correct adjuvant. Adjuvant efficacy can vary considerably, with the survey finding that using a ‘premium’ product can improve weed control by 5 to 25% compared to using a product considered simply ‘good enough’.
Before the industry trade association, the Council of Producers and Distributors of Agrotechnology (CPDA), began their adjuvant certification program, anyone could produce and sell ‘adjuvants’ of questionable efficacy. The certification program (an adjuvant will be labeled as ‘CPDA Certified’), adopted a series of uniform, science-based standards for evaluating adjuvants. Adjuvants are evaluated for safety and manufacturer claims, ingredients validated and certain U.S. EPA, DOT and OSHA requirements must be met to obtain CPDA certification. Approximately 500 crop protection products with an EPA label, now have wording on their labels recommending the use of CPDA-certified adjuvants. There are currently over 230 CPDA-certified adjuvants.
Some herbicide active ingredients can impact spray solution pH on their own. In a study across Tennessee, Mueller and Steckel found that well water sources had pH ranging between an acidic 4.5 to an alkaline 8.4. Adding different glyphosate formulations alone to spray water with high pH (7.7) dropped the pH to 5.0 to 5.2.
One study by Dallas Peterson at Kansas State University tested glyphosate (RoundUp PowerMax), comparing an 8 oz rate applied alone or with 5% AMS (a little higher rate than is typically used; this rate becomes more typical the further west one goes) or 2.5% AMS (a more typical rate) plus a surfactant to glyphosate applied using 7 different water conditioning products on the market. Glyphosate alone controlled 40 to 73% of velvetleaf and volunteer sorghum, corn and sunflower. Adding AMS, increased control by 17 to 32%; adding a surfactant increased control by an additional 4 to 5%. The commercial water conditioners tested not only did not increase weed control compared to AMS alone, but many performed much worse than using AMS alone. Similar research at NDSU by Rich Zollinger showed that for velvetleaf control, 7 of 10 water conditioners tested outperformed a surfactant alone, but only 3 commercial adjuvants that contained higher levels of AMS conditioned the water and performed similarly to AMS. Close attention to the rate of some of these water conditioners (such as 0.5% volume/volume). While there are several different water conditioners marketed as AMS replacements, careful consideration should be given to product and rate choice to ensure a given product will work for a specific application or operation.
Additional Resources
Council of Producers and Distributors of Agrotechnology Adjuvant Webinars
CPDA Certified Adjuvant information
CPDA Certified Adjuvant List
North Dakota State University Weed Control Guide
War Against Weeds podcast
Zollinger, R. 2022. Spray Adjuvants: The rest of the story. NDSU Extension.
Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!
March 18, 2026’s Strategic Farming: Let’s Talk Crops session discussed how spray water quality impacts herbicide efficacy. This webinar series runs through the end of March and registrations are still being accepted: https://extension.umn.edu/courses-and-events/strategic-farming. To watch this episode visit: http://z.umn.edu/StrategicFarmingRecordings.
Why do we need adjuvants?
Before he retired in 2023, Greg Dahl was asked to obtain adjuvant-related weed control research from land grant universities including University of Minnesota, North Dakota State University and Iowa State University, to determine the value of adjuvants to weed management. Herbicides alone were compared to herbicides with the recommended adjuvant and to herbicides with comparison adjuvants. When compared to the weed control observed when using a required adjuvant, weed control declines by between 30 and 90% when the same herbicide was applied without the adjuvant. Using the correct type of adjuvant is important to improve weed control and limit crop injury; the survey found that using the wrong type of adjuvant (ex. using crop oil instead of a surfactant) can reduce weed control between 5 and 50% compared to using the correct adjuvant. Adjuvant efficacy can vary considerably, with the survey finding that using a ‘premium’ product can improve weed control by 5 to 25% compared to using a product considered simply ‘good enough’.
Before the industry trade association, the Council of Producers and Distributors of Agrotechnology (CPDA), began their adjuvant certification program, anyone could produce and sell ‘adjuvants’ of questionable efficacy. The certification program (an adjuvant will be labeled as ‘CPDA Certified’), adopted a series of uniform, science-based standards for evaluating adjuvants. Adjuvants are evaluated for safety and manufacturer claims, ingredients validated and certain U.S. EPA, DOT and OSHA requirements must be met to obtain CPDA certification. Approximately 500 crop protection products with an EPA label, now have wording on their labels recommending the use of CPDA-certified adjuvants. There are currently over 230 CPDA-certified adjuvants.
Hard Water
Water that has a high concentration of dissolved positively charged minerals -called cations- like calcium (Ca2+), sodium (Na+), potassium (K+), iron (Fe2+ or Fe3+) is called hard water. These cations interact with weak acid herbicides (ex. glyphosate) or herbicides with a negative charge in solution, rendering the active ingredient inactive as it is now unable to be taken up by plant tissue. When spray solution evaporates from plant tissue, the minerals can become visible as particles on the leaf surface. Sometimes the active ingredient can become resolubilized by dews on the leaf surface and absorbed by plant tissue; the tying up of herbicide active ingredients in hard water is often permanent, however, once the product dries on the leaf diminishing the value of one’s herbicide investment. Conditioning spray water using ammonium sulfate (AMS) or other water conditioning agents can result in hard water cations binding to the sulfate component of the molecule and ammonia aiding plant absorption.Water pH
pH, or the concentration of hydrogen ions in a liquid solution, can also impact herbicide efficacy, with high/low pH improving/decreasing efficacy, depending upon the chemical properties of the herbicide active ingredient. Testing your own spray water pH can be as simple as a trip to a spa or pool supplies business to pick up low-tech pH paper.Some herbicides need an acidic pH to stay in solution
Weak acid herbicide spray solutions (ex. glyphosate) that have a negative charge when dissociated in solution, are more water soluble in an acidic pH for the active ingredient to stay in solution. Alkaline water pH, or pH above 7, can increase the water solubility of group 14 herbicides (ex. Cobra, Resource and Valor), degrade in a spray tank, making not conditioning water a particularly risky endeavor when using these products.Other herbicides need an alkaline spray water solution to stay in solution
Other herbicides are more soluble and so stay in solution better at more alkaline pHs, including sulfonylureas (HG 2; ex. Classic, Ally, Harmony, UpBeet), group 27 herbicides (ex. Callisto, Laudis) and Select Max (HG 1), Status (HG 19 & 4), and Sharpen (HG 14). There is a class of adjuvants, called basic blends, that raise spray water pH to close to 9 designed for use with the sulfonylurea class of herbicides. High pH water sources often have higher concentration of anions such as bicarbonate, which can impact efficacy of products containing clethodim (ex. Select Max) or sethoxydim (ex. Poast), 2,4-D and MCPA.Spray solution pH and weed control
Research from Bill Johnson’s group at Purdue University showed that control of both lambsquarters and giant ragweed with saflufenacil (ex. Sharpen) increased as pH increased, with the best control observed at pHs of 7.7 or greater. Additional research tank mixing saflufenacil and glyphosate together showed that control is impacted by both spray water pH and adjuvant, with 40, 63 and 77% control of glyphosate-tolerant marestail when a non-ionic surfactant (NIS), crop oil concentrate (COC) and methylated seed oil (MSO), respectively, was added to a spray solution with a pH of 5. However, when the spray solution pH was raised to 7, control increased to 73, 86 and 94%, when NIS, COC and MSO were added, respectively. NDSU research (Zollinger), showed that when one uses low carrier volumes for a Sharpen application, it inevitably results in less total MSO applied per acre. For example, if one applied Sharpen at a spray rate of 17 gallons per acre and added 1% (v/v) MSO or 1.4 pt/A, dropping carrier volume to 8.5 gallons/acre and still adding 1% MSO, results in 0.7 pt/A MSO applied. This reduced MSO application rate per acre resulted in significantly poorer overall control of broadleaf weed species.Some herbicide active ingredients can impact spray solution pH on their own. In a study across Tennessee, Mueller and Steckel found that well water sources had pH ranging between an acidic 4.5 to an alkaline 8.4. Adding different glyphosate formulations alone to spray water with high pH (7.7) dropped the pH to 5.0 to 5.2.
One study by Dallas Peterson at Kansas State University tested glyphosate (RoundUp PowerMax), comparing an 8 oz rate applied alone or with 5% AMS (a little higher rate than is typically used; this rate becomes more typical the further west one goes) or 2.5% AMS (a more typical rate) plus a surfactant to glyphosate applied using 7 different water conditioning products on the market. Glyphosate alone controlled 40 to 73% of velvetleaf and volunteer sorghum, corn and sunflower. Adding AMS, increased control by 17 to 32%; adding a surfactant increased control by an additional 4 to 5%. The commercial water conditioners tested not only did not increase weed control compared to AMS alone, but many performed much worse than using AMS alone. Similar research at NDSU by Rich Zollinger showed that for velvetleaf control, 7 of 10 water conditioners tested outperformed a surfactant alone, but only 3 commercial adjuvants that contained higher levels of AMS conditioned the water and performed similarly to AMS. Close attention to the rate of some of these water conditioners (such as 0.5% volume/volume). While there are several different water conditioners marketed as AMS replacements, careful consideration should be given to product and rate choice to ensure a given product will work for a specific application or operation.
Turbidity in water
Another potential issue with spray water quality, although more rare, is turbidity, or particles such as silt and clay in the water. During the severe drought in 2021, some that farmed in the northern Red River Valley in North Dakota, Minnesota and Manitoba, were forced to take their spray water from the Red River, a river with a lot of turbidity. Clay particles in improperly filtered spray water bind to herbicides such as glyphosate and paraquat, making them unavailable for plant uptake.Audience questions
Ikley and Dahl fielded many audience questions, including: Does solid AMS outperform Class Act NG as an adjuvant, as AMS is the main ingredient in Class Act?; Can weed resistance be masked as merely using poor quality water when applying herbicides if there is crystallization on plant leaves?; Does going above 8.5 pounds of AMS per 100 gallons improve weed control; how much can we add?; You mentioned that some herbicide active ingredients are both more soluble at higher pH, but also break down faster at higher pH – how quickly do they break down?; How representative of our spray water sources is a 1,000 parts per million cation concentration?; Does chlorinated water impact herbicide efficacy?; How best does one test their water?; Can results of water tests vary based on how they are tested?; How should one consider making adjustments to water conditioner and adjuvant rates given the low spray volumes used for drone applications?; How does humic acid behave in tank mixes versus AMS?References
Soltani, et al. 2023. Effect of Class Act NG adjuvant on glyphosate efficacy in corn. J. of Agricultural Science. 15:1-9. https://doi.org/10.5539/jas.v15n11p1Additional Resources
Council of Producers and Distributors of Agrotechnology Adjuvant Webinars
CPDA Certified Adjuvant information
CPDA Certified Adjuvant List
North Dakota State University Weed Control Guide
War Against Weeds podcast
Zollinger, R. 2022. Spray Adjuvants: The rest of the story. NDSU Extension.
Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!
NOTE: Products listed or not listed in this article do not imply endorsement nor lack of endorsement by the authors.
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