Thursday, 10 May 2018 11:06

How your water quality can affect herbicide applications

Written by  Shannon Chant
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With the really dry conditions last year and earlier this year, the concentration of minerals in water used for spraying herbicides may have increased. 


If you are going to apply herbicides containing glyphosate, glyphosate/dicamba, diquat, bromoxynil, 2-4-D amine, sethoxydim, clethodim and tralkoxydim the quality of the water used can affect the outcome of the application. The water quality factors that are the main concern are cleanliness and mineral ion content.  Only clear and clean water should be used for mixing these products. The same kind of inactivation can occur when these products are applied to plant surfaces that are covered with a layer of dust. Dust kicked up during the spray operation may also result in reduced control, especially directly behind the sprayer.
Many chemical elements can be dissolved in water but six major ions make up the dissolved material in most water. The dissolved chemical elements are present as ions which carry a positive or negative charge. Calcium, magnesium and sodium are positively charged (cations).  Sulphate, chloride and bicarbonate are negatively charged (anions).
Water containing calcium and magnesium can reduce the effectiveness of glyphosate and 2,4-D amine. Calcium and magnesium levels are reported as calcium carbonate (CaCO3) equivalent in parts per million (ppm) or grains per US gallon. 
For low rates of glyphosate, the maximum hardness in a water source should be 350 ppm (20 grains/US gallon).  For higher glyphosate rates, the maximum water hardness should be 700 ppm (40 grains/US gallon). Where hard water is a concern, use the maximum recommended rate of herbicide. Ammonium sulphate fertilizer (21-0-0-24) is registered for use with some glyphosate products at a rate of 3 kg of fertilizer per 100 L of water and will help to overcome the negative effect of hard water on these products.
Hard water can also reduce the activity of 2,4-D amine. If you suspect that this has been a problem with this water source in the past you should consider (in order of priority):
• use an alternate source of water if available;
• use an ester (LV) formulation if practical;
• use the maximum recommended rate of the amine formulation;
• use a non-ionic surfactant at 0.1% v/v if the amine formulation is used.
Water with a hardness of 600 ppm (35 grains/US gallon) can almost completely reduce the effectiveness of 2,4-D amine applied at 4 oz ai/ac and that the use of a non-ionic surfactant (such as Agral 90, AgSurf, Companion) at 0.1% v/v (1 L surfactant per 1000 L of spray mixture) will help to overcome this. The addition of nitrogen fertilizer to 2,4-D amine spray mixtures has not overcome hard water concerns.
Bicarbonate concentrations as low as 500 ppm have reduced the activity of products belonging to the “dim” group of herbicides (containing tralkoxydim, sethoxydim and clethodim). Bicarbonate ion levels of 1000 ppm or more may occur in some Saskatchewan water sources. The greatest reduction in activity has been when there are other factors that also reduce control at the same time, including reduced herbicide rate, late application and poor growing conditions. Where bicarbonate occurs in water, the following precautions are suggested:
If possible, avoid using water with more than 500 ppm bicarbonate when applying herbicides with tralkoxydim, clethodim or sethoxydim as active ingredients. When more than 500 ppm bicarbonate is present in the water, use the maximum recommended rate of herbicide for the target weed and apply the herbicide at the optimum growth stage of the weeds.
2,4-D amine activity is also reduced by bicarbonate ions in spray dilution water, especially when low rates (0.34 L/ac) are used. Where water containing more than 500 ppm bicarbonate is known to occur the following should be considered:
• use an alternate source of water if available.
• use an LV ester formulation if practical since LV esters are not affected by bicarbonate.
• use MCPA amine or ester rather than 2,4-D amine if MCPA is recommended for the control of the target weeds.
• use the maximum rate of 2,4-D amine.
• use a non-ionic surfactant at 0.1% v/v (1 L per 1000 L of spray mixture) if the amine formulation is used.
If you suspect that water quality may be a concern for your water source, the water should be sent to a lab that offers Agricultural Spray Water Analysis.
Shannon Chant is the Crops Extension Specialist, Regional Services Branch, Swift Current

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