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The quality and characteristics of a water source can affect performance of herbicides, insecticides, and fungicides. Water quality becomes especially important when herbicides rates are reduced, when applications are delayed to more mature and larger weeds, when weeds are more tolerant, and under poor growing conditions which affects herbicide uptake or movement within the plant.
Use of spray-tank additive, alternative water supply, or other action may appropriate when:
Cations: if sum of Ca, Mg, Na, and Fe concentrations exceeds 400 mg/L.Electrical conductivity: if EC exceeds 2,000 µmho/cm.Total dissolved solids: if TDS exceeds 1,300 mg/L.Bicarbonate: if HCO3 concentration exceeds 300 to 500 mg/L.Hardness: if hardness exceeds 350 to 700 mg/L.pH: if spray water is alkaline, especially when pH exceeds 8.0.Alkalinity: for some herbicides, if over 300 mg/L.Turbidity: if water is not clear, but noticeably murky or discolored.
Six major ions make up most dissolved materials in water.
Cations (positively charged)
Calcium, Ca+2 ii. Magnesium, Mg+2 iii. Sodium, Na+
Anions (negatively charged)
Sulfate, SO4 -2 ii. Chloride, Cl-1 iii. Bicarbonate, HCO3 -1
Other ions
Small amounts may also be present in insignificant amounts.
e.g., potassium (K+1), iron (Fe+2,+3), nitrate (NO3-1)
If sum of cation concentrations exceeds 400 mg/L, action may be necessary
Cation sum = mg Ca/L + mg Mg/L + mg Na/L + mg Fe/L
Electrical conductivity (EC)
EC is ability of water sample to conduct electric current.
EC is quick estimate of TDS in water.
Higher TDS, water can conduct more electric current
Water quality problems very unlikely if EC less than 500 µmho/cm1
Concentrations above 300 to 500 mg/L have reduced activity of certain herbicides
Especially affects tralkoxydim, sethoxydim, and clethodim group of grass killers and 2,4-D amine.
Major effects seem to be from water with elevated bicarbonate levels, but also having with low levels of other anions, like chloride or sulfate.
Options if 2,4-D amine formulations less effective:
Use alternative water source
Use LV (lo-vol)formulations; not affected by bicarbonate
Use max rate of 2,4-D and include MCPA, if recommended
Use non-ionic surfactant with amine formulation
Primary problem: iron oxidizes to form rust particles
Can settle to the bottom of the spray tank and clog nozzles and screens
High iron levels can reduce glyphosate activity
Advise using water conditioner when:
Iron levels exceed 25 mg Fe/L
Hardness plus iron concentrations exceed 400 mg/L.
Hard water does not affect all herbicides equally
Calcium and magnesium minerals are primary contributors to water hardness
Hardness minerals may bind to certain herbicides and create molecules which:
cannot enter the target pest,
enter at a much slower rate, or
precipitate out of solution
Expressed as equivalent of calcium carbonate (CaCO3)
Can be reported as “milligrams per liter (mg/L)”, “parts per million (ppm)”, or “grains per gallon (gpg)”
“mg/L” and “ppm” are considered equal
1 grain per gallon is equal to 17.1 mg/L
Hardness greater than 150 mg/L begins to reduce efficacy of glyphosate, 2,4-D amine, dicamba, chlorpyralid
Glyphosate: suggested actions with excessive water hardness (see Table 2 ):
Adding ammonium sulfate
Use spray mix immediately after mixing
Use higher herbicide rates
Hardness above 600 mg/L can almost totally deactivate 2,4-D amine
Recommended 2,4-D management strategies
Use alternative water source
Use ester formulation, if practical
Use maximum recommended rate of amine formulation
Use a non-ionic surfactant if using amine formulation
Hardness mg/L
WHO classification
< 114
Soft
114 – 342
Moderately hard
342 - 800
Hard
> 800
Extremely hard
¶ Table 2. Glyphosate and Water Hardness
Field situation Action recommended when hardness concentration
low rates: used for annual grass weed control
> 350 mg/L
high rates: used for perennial weed control
> 700 mg/L
5 gal/ac spray volume
> 700 mg/L
10 gal/ac spray volume
> 350 mg/L
Most pesticides generally perform best in slightly acidic water, pH 4.0 – 6.5 (see Table 3) . Water pH higher or lower than optimal, may cause some pesticides to begin degrading or “hydrolyzing.” (see Table 4)
Pesticide molecules may react with water (H2 O); process called “hydrolysis”
Reaction with hydronium (H+1 ) or hydroxl (OH-1 ) ions in water can change chemical charge
Can influence how long a pesticide molecule remains intact
May limit molecule’s ability to penetrate leaf cuticle and reach site of action; reduces its efficacy
Disassociation reduces herbicide efficacy
Weak acid herbicides most sensitive to dissociation.
Pesticides are normally formulated as weak acids or neutral to weakly-alkaline products.
When mixed in water, portion of the herbicide molecules will dissociate (split into two pieces); the rest will not
Dissociated molecules absorbed more slowly across plant cell membranes than whole (not disassociated) molecules
Water pH range Suggested use
3.5 to 6.0
Satisfactory for most spraying .
Satisfactory for short-term (12–24 hours) storage of most mixtures in a spray tank.
Not suitable for sulfonylurea urea herbicides.
6.0 to 7.0
Adequate for immediate spraying for most pesticides.
Do not leave the spray mixture in the tank for more than 1–2 hours, to prevent loss of effectiveness.
over 7.0
Most products mixed in alkaline water should be sprayed immediately
Bicarbonate (HCO3-1 ) ions largely responsible for alkalinity in natural waters
Carbonate (CO3-2 ) and hydroxyl (OH-1 ) ion concentrations have very minor role
Alkalinity can be problem, but not to extent of water pH and hardness
High pH plus high alkalinity can affect the solubility of imazamethabenz (Assert)
Can form porridge-like sludge in bottom of spray tank
¶ H. Suspended solids and organic matter
Suspended solids include microscopic silt, clay, and organic matter particles suspended in water
May or may not settle to tank bottom if water is left undisturbed
Turbidity defined by clarity of water
“Turbid” = water in which suspended solids can be seen floating in water.
Water may appear clear, hazy, or cloudy, or hazy depending on relative quantity of suspended solids
Can be measured by amount of light scattered by material in the water when light is shined through sample
Water turbidity known to deactivate:
Diquat
Paraquat
Glyphosate (Roundup) alone
Glyphosate formulations or combination products
Sorption coefficients are indices that reflect how strongly pesticide binds (adsorbs or sticks) to soil particles and suspended solids
“Kd” = soil sorption coefficient
“Koc” = soil organic carbon sorption coefficient
Herbicides with high Kd or Koc values bind tightly to soil and to sediment and organic matter in water
If temperature too low, some products may form sludge on bottom of tank
Effect of pH on hydrolysis or dissociation usually proceeds faster as water temperature increases
Half-life at different pH solutions*
Common Name 5 6 7 8 9
acephate
40 days
-
46 days
-
16 days
carbaryl
-
125 days
24 days
2.5 days
1 day
diazinon
31 days
-
185 days
-
136 days
dicamba
stable
stable
unstable
unstable
unstable
dimethoate
-
12 hours
-
-
48 minutes
malathion
-
8 days
3 days
19 Hours
-
paraquat
stable
stable
stable
unstable
unstable
Williamson, K. 2003. Water quality for mixing herbicides (rev.). Agdex 641-14. Alberta Ag-Info Centre. www.agriculture.alberta.ca
Whitford, et. al. 2009. The Impact of Water Quality on Pesticide Performance. Bulletin no. PPP-86. Purdue Coop. Ext. Serv., Lafayette IN. 38 pg. https://www.extension.purdue.edu/extmedia/PPP/PPP-86.pdf Tharp & Sigler. 2013. Pesticide Performance and Water Quality. MontGuide #MT201305AG. Montana State Univ. Ext., Bozeman MT
Anonymous. Water quality affects herbicide efficacy. accessed 12/18/200 9 http://oregonstate.edu/dept/nurseryweeds/feature_articles/spray_tank/spray_tank.htm
Griffin. 2009. Water Quality Effects on Pesticides. Presentation at 2009 Louisiana Agricultural Technology and Management Conference. https://laca1.org/Presentations/2009/WaterQualityEffects2009.pdf accessed 03/21/2022
[1] 1000 µmho/cm = 1.0 mmho/cm = 1000 microsiemens/cm
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