Water Quality and Herbicides

Produced by Canada-Saskatchewan Agriculture Green Plan Agreement

Although some herbicides used in Saskatchewan are formulated as granules and are applied as dry material, most herbicides used in the province must be mixed with water and applied with some type of sprayer. The importance of using clean, clear water has been recognized for many years. Since the mid 1990s research has shown that the performance of some herbicides can be adversely affected by some types of minerals dissolved in the water used to prepare herbicide spray mixtures.

Water Chemistry

The purpose of this section is to explain water chemistry and the factors which determine water chemistry in Saskatchewan surface and ground water. It will deal only with dissolved minerals in water, not suspended materials such as silt and organic matter.

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. The major constituents are summarized in Table 1.

Table 1 - Major mineral constituents in Saskatchewan water

Small amounts of potassium (K+), iron (Fe++, Fe+++), nitrate (NO₃-) or other ions may be present, but in most natural water, the six ions listed in Table 1. are the only dissolved minerals present in significant quantities.

Measuring the Total Dissolved Minerals

The first important property to determine when considering water suitability for herbicide dilution is the Total Dissolved Solids which is usually expressed in parts per million (TDS, ppm). The TDS can be determined by evaporating a sample to dryness and weighing the minerals that remain or it can be determined by measuring the concentration of the six major ions and calculating the sum of the ions.

For example, if a water analysis showed:

The TDS by the sum of ions method is 3842 ppm.

Determination of TDS by evaporation is tedious and no longer done at most laboratories. TDS by the sum of ions requires analysis for all the major constituents and is, therefore, expensive. a much simpler method is to determine the electrical conductivity (EC) as a measure of the total dissolved mineral material. For sulphate water, which is common in Saskatchewan, EC expressed in Microsiemens per cm (uS/cm) @ 25 C is a useful first approximation of TDS in ppm. Electrical conductivity (EC) of water samples can be determined quickly and easily in the laboratory or in the field. Because EC is temperature dependent, all EC readings are standardized to 25 C.

EC of Saskatchewan Waters

The standard for comparison in Saskatchewan is the major mountain fed river systems, the South and North Saskatchewan Rivers, which have ECs of about 350 uS/cm. Other water sources and their approximate EC values are given in Table 2.

Table 2 - Approximate EC Values for Saskatchewan Waters (uS/cm)

All water sources will be subject to local variation but sloughs and dugouts cover a very wide range of EC values. Dugouts that are cut into saline water tables can be very highly mineralized. If a dugout remains full even in times of extended drought, there is a distinct possibility that it is highly mineralized.

Types of Minerals Dissolved in Water

The EC or other measure of TDS is a useful first indication of water quality for herbicide dilution. If the EC is less than 500 uS/cm, it is unlikely that the efficacy of any herbicide will be affected. For herbicide dilution interpretations, the type of minerals dissolved in the water is the most important consideration.

Cation Types – Hard and Soft Water

Hardness is a water property that is related to domestic use of water and the tendency to produce suds or curdle soap or to produce boiler scale in industrial applications. The hardness of water is determined by the amount of calcium plus magnesium present. Hardness is expressed as the amount of calcium plus magnesium present as calcium carbonate equivalent. It can be expressed as parts per million (ppm) or grains per US gallon. Grains per US gallon is an old unit but still much used in the water industry because of the ready availability of kits which can measure water hardness in the field and which are calibrated to determine hardness readings in grains per US gallon. The following example illustrates how hardness as calcium carbonate equivalent and as grains per US gallon are calculated for a water sample that contains 285 ppm Ca and 131 ppm Mg.

FOR Ca: 285 x CaCO₃/Ca = 285 x 100/40.1 = 711 ppm Ca as calcium carbonate equivalent.

FOR Mg: 131 x CaCO₃/Mg = 131 x 100/24.3 = 539 ppm Mg as calcium carbonate equivalent.

TOTAL HARDNESS (Ca CO₃) equivalent) = 711 + 539 ppm = 1250 ppm

To determine grains per US gallon divide total harness by 17.1.

Thus, 1250 ppm/17.1 = 73 grains per US gallon.

Hardness of Saskatchewan Surface Waters

The hardness of surface water is in almost direct proportion to the TDS or EC. For ground waters, the hardness is not in direct relation to the EC but depends on the type of geologic deposit the water has passed through en route to the aquifer. In bedrock deposits, the water is usually soft (sodic) despite the degree of mineralization. Thus, the general statement that glacial aquifers produce hard water while bedrock aquifers produce soft water.

Bicarbonate Content of Water

Some Saskatchewan ground waters contain relatively high levels of bicarbonate ions which usually occur in association with sodium ions. Sodium bicarbonate is a common chemical that may be more familiar to most people as baking soda.

Bicarbonate content can be a factor affecting the performance of some herbicides, particularly those in the "dim" group such as Achieve (tralkoxydim), Poast Ultra (sethoxydim) and Centurion/Select (clethodim) as well as 2,4-D amine. The major effects seem to be from waters with elevated bicarbonate levels but low levels of other anions such as chloride and sulphate. The only water source in Saskatchewan with high Bicarbonate ion but low levels of other anions is water from bedrock aquifers that are exposed or subcrop close to the soil surface. In that circumstance, the water entering the aquifer does no pass through great depths of either glacial or bedrock aquifers and hence has no opportunity to pick up significant amounts of either sulphate or chloride. The Ravenscrag aquifer in southern Saskatchewan and portions of the Judith River formation in west-central Saskatchewan produce waters with elevated bicarbonate levels. Of particular concern is the Ravenscrag aquifer in the Estevan area. In that aquifer, bicarbonate levels of 1000 ppm are commonplace.

Water Quality Effects on Herbicides

Recent research in Saskatchewan and other areas has shown that the effectiveness of some herbicides can be reduced by some water sources. The water quality factors that are of main concern are cleanliness and mineral ion content.

Cleanliness refers to freedom from suspended silt and organic matter, both of which can reduce the activity of the following herbicides:

• diquat (Reglone, Reward)
• paraquat (Gramoxone)
• glyphosate/dicamba
• bromoxynil

These products are very susceptible to inactivation by silt and organic matter so it is important to use only clear, clean water for mixing these products. It should be noted that 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.

Hard Water

Water containing calcium and magnesium can reduce the effectiveness of:

• glyphosate
• 2,4-D amine

The greatest concern is with products that contain glyphosate. Table 3 provides general water quality guidelines for glyphosate.

Table 3 - Water Hardness guidelines for glyphosate

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 antagonistic effect of hard water on these products. Reducing the water volume to the minimum required for good coverage will help to ensure maximum effectiveness of glyphosate products.

Hard water can also reduce the activity of 2,4-D amine. At the moment, no definite guidelines can be given. Where hard water is known to occur and where control with the amine formulation of 2,4-D has been less than satisfactory, the following should be considered (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

Research has shown that water with a hardness of 600 ppm (35 grains/US gallon) can almost completely antagonize 2,4-D amine applied at 280 grams ai/ha (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 antagonistic effect. While ammonium sulphate can improve the performance of glyphosate in hard water, the addition of nitrogen fertilizer to 2,4-D amine spray mixtures has not overcome hard water antagonism.

Bicarbonate

Bicarbonate ion levels of 1000 ppm or more may occur in some Saskatchewan water sources as described above. The maximum level recorded in recent surveys has been approximately 1400 ppm in water from a well near Estevan.

Bicarbonate is known to reduce the activity of 2,4-D amine and the grass killers that belong to the "dim" group of herbicides (Achieve (tralkoxydim), Poast (sethoxydim), Centurion/Select (clethodim)).

Bicarbonate concentrations as low as 500 ppm have reduced the activity of these herbicides under some circumstances.

Poast has usually performed satisfactorily at bicarbonate concentrations up to 1000 ppm. Reduced activity has been observed when other factors that also tend to reduce weed control (reduced herbicide rate, late application, poor growing conditions) have occurred in conjunction with the use of a bicarbonate water source. Control of more tolerant weeds such as volunteer barley is more likely to be affected.

In some tests, bicarbonate levels of 500 ppm have reduced the effectiveness of Achieve and clethodim. As with Poast, the greatest reduction in activity has been noticed when other factors that also tend to reduce control occurred at the same time.

Where bicarbonate occurs in water, the following precautions are suggested:

• If possible, avoid using water with more that 500 ppm bicarbonate when applying Achieve, clethodim or Poast.
• 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.

Research has demonstrated that the use of liquid ammonium sulphate fertilizer at 4 L/ha (1.6 L/ac) or 0.8 kg 21-0-0-24 dry ammonium sulphate fertilizer/ac (2 kg/ha) or 0.5 L/ha (0.2 L/ac) of 28-0-0 liquid nitrogen fertilizer as an adjuvant will overcome the antagonistic effect of bicarbonate in spray dilution water. Further research is being conducted to confirm these results and refine the fertilizer rates required.

It should be noted that field experience with Achieve, Poast and clethodim has not indicated widespread or serious problems due to water quality. When used according to label directions, these products should provide reliable weed control. In cases where reduced control occurs and does not appear to be due to more common causes such as reduced herbicide rates, late application, wash- off by rain or poor growing conditions, the possibility of bicarbonate antagonism should be investigated, especially if the water source is a deep well that provides soft water.

2,4-D amine activity is also reduced by bicarbonate ions in spray dilution water, especially when low rates (0.34 L/ac or 0.84 L/ha) are used. Where water containing more than 500 ppm bicarbonate is known to occur and where control with the amine formulation of 2,4-D has been less than satisfactory, 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.

Testing has indicated that the use of a non-ionic surfactant at 0.1% v/v (1 L surfactant per 1000 L of spray mixture) will help to overcome the antagonistic effect of bicarbonate on 2,4-D amine. Nitrogen fertilizers and acidifiers have not been effective in correcting the problem.

Iron

Iron and manganese occur in ground water in many areas of Saskatchewan. Its presence is readily detected because it produces rust (iron) or black coloured stains (manganese) on plumbing fixtures. Water containing iron or manganese compounds reduces the activity of products that contain glyphosate. An additional problem occurs when iron or manganese dissolved in ground water is exposed to air. The iron and manganese quickly oxidize producing a precipitate which can plug screens and nozzles. Iron and manganese effects cannot be corrected by additives to the spray tank, for these reasons, water sources that contain iron or manganese should not be used for mixing with herbicides.

Other Ions

Research also suggests that other metal ions such as zinc and manganese will reduce the effectiveness of glyphosate as well. Therefore the additive of micronutrient solutions to the tank when applying glyphosate is not recommended.

As long as used as directed, research on the application of glyphosate using pond or dugout water treated with copper sulphate is not a problem for glyphosate.

Summary

Evidence to date indicates that only a few herbicides are adversely affected by minerals dissolved in spray dilution water. Whether or not a problem arises will depend on the herbicide and the particular water source being used. In general, herbicides will provide satisfactory results with most water sources. Furthermore, a water source that can be antagonistic to a particular herbicide may not always result in reduced weed control. Water quality problems may surface only when some other factor or factors are also working to reduce weed control.

It can generally be assumed that water quality should not be a potential problem with herbicides other than those discussed above. Those who suspect that water quality may be a factor in the unsatisfactory performance of a herbicide are encouraged to have their herbicide dilution water analyzed by one of several laboratories offering spray water analysis such as AGAT Labs in Calgary, Alberta or A&L Labs in London, Ontario. Request the Agricultural Spray Water Analysis and use their recommendations or the information presented here to assess the suitability of the water for spraying purposes.