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Spray Water Quality

By Samantha Marcino, BSA, P.Ag, CCA, Crops Extension Specialist, Yorkton

March 2021

Sprayer in a field
Sprayer in a field

Spring herbicide season is quickly approaching and producers should start thinking about their water source quality. Clark Brenzil, Provincial Weed Specialist, recently did a webinar presentation on spray water quality and reviewed some very important topics.

Brenzil encouraged producers to get their water tested to determine where levels are at instead of just assuming its fine to use. Depending on the source, quality can vary throughout the season and for shallow, above-ground sources, producers may need to test more than once throughout the growing season.

One of the first things to look for in a water quality test is the level of bicarbonates. These are generally an issue in water from deep wells near the southern and western borders of the province. Bicarbonate levels can affect the performance of certain herbicides, especially the ones in the Group 1 dim as well as 2,4-D amine. Concentrations as low as 500 ppm can reduce the effectiveness of these herbicides under certain situations. Bicarbonate antagonism is easy to correct for Group 1 dims. You can add in ammonium sulphate (AMS) at one per cent v/v or UAN (28-0-0) liquid as an adjuvant to overcome the antagonistic effects in spray dilution water. The addition of AMS to overcome antagonism in 2,4-D is less effective. If bicarbonate levels are higher than 500 ppm it might be worthwhile to find a different water source. If you are unable to find another water source, make sure you are using the maximum recommended rate of the herbicide and applying it at the optimum growth stage of the weeds you are targeting. 2,4-D esters and MCPA are not antagonized by ions in spray water so switching to those is an effective fix as well.

Other minerals that commonly cause issues are calcium and magnesium. These both can reduce the effectiveness of glyphosate as well as 2,4-D amine. The following table shows the maximum water hardness for glyphosate use. If water hardness is a concern, use the maximum recommended rate of herbicide and add in AMS at 1.2kg active per acre to the water before adding glyphosate. Reducing the water volume to the minimum required for good coverage can help the effectiveness of glyphosate products by reducing the overall amount of antagonistic ions in the tank relative to the glyphosate. Keep in mind, however, that correcting for hard water rates above the 700 ppm CaCO3 equivalent can result in the formation of precipitates in the water that can block nozzles. You can use this calculator to source AMS rates which can add refinement to the rates of AMS used to save costs.

Water Hardness guidelines for glyphosate

Glyphosate use rate Maximum Water Hardness
Grains/US gal. ppm CaC03equivalent
Low Rates for Annual Grass Weeds 20 350
Higher Rates for Perennial Weeds 40 700

Iron and manganese can also cause issues with glyphosate efficacy. These are very common minerals in Saskatchewan water and cannot be corrected for by using AMS. Ammonium citrate and citric acid have shown promising results in trials for decreasing the antagonism caused by iron. Many wells have water with iron present, but rarely at high enough levels to actually cause antagonism of glyphosate.

Suspended clay or organic matter in spray water can also cause issues with herbicides like glyphosate and diquat based desiccants such as Reglone. The active ingredients bind strongly to these particles, reducing their activity by preventing them from entering the plant.

Water quality is only one part of the spraying equation but an important one at that. If you have any questions about water quality, please contact your local crops extension specialist or the Agriculture Knowledge Center at 1-866-457-2377.

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