Improving Spray Performance

 

After a wet and late planted spring, crops and weeds are starting to grow. Spraying weeds is important for getting good yields and reducing weed competition. Many farmers use commercial applicators to spray their fields, but a lot of farmers do it themselves or have applicators for spot spraying. There are factors that affect herbicide performance. Water quality; water pH; the temperature of the water, air, and soil; type and volume of water; size of weeds; and even time of day can impact herbicide effectiveness.

In the Midwest, Purdue University is a leader on improving herbicide performance. For anyone spraying herbicides the following publications are a must read. PPP-86: The Impact of Water Quality on Pesticide Performance. PPP-107: Adjuvants and the Power of Spray Droplets. PPP-112: Water Temperature and Herbicide Performance. PPP-115: Compendium of Herbicide Adjuvants which explains how to prevent water minerals from tying up herbicides.

Here are some tip:
Check your water quality. Water minerals affect herbicide performance because water makes up 95-99% of the spray volume. Water quality and/or water minerals is measured by water hardness. As the mineral water content increases, hardness increases. Hard water reduces herbicide solubility and plant absorption. Herbicides tied up in the spray tank are diluted, which reduces performance.

Common dissolved water minerals include calcium, magnesium, and iron. Water hardness is a measures calcium and magnesium concentration in water. The higher the number measured in parts per million (ppm), the higher the water hardness. Soft water replaces calcium and magnesium with sodium and potassium ions. Soft water is 0–114ppm, Moderately Hard 114–342ppm, Hard 342–800ppm, and Extremely Hard > 800 ppm. The ions that tie up herbicides are listed in the order of greatest potential to reduce herbicide activity: Aluminum (A1+++) > iron (Fe+++, Fe++) > Magnesium (Mg++) > Calcium (Ca++). Water conditioners soften water and reduce ions that tie up herbicides. For best herbicide performance, follow the label and use soft water.

Use clean water for sprays. Even tap water may contain small organic debris or suspended solids (soil) which may tie up herbicides. Always use a filter and void using water from ditches or ponds because it probably contains organic debris or may be higher in suspended solids.

Water and air temperature affects herbicide performance. Purdue University research found cold water (41°F) and hot water (100°F) reduced herbicide effectiveness. The best performance occurred with warm water at 72°F. Water that is too cold or too warm shocks the plant and reduces herbicide absorption. When plants are actively growing, they readily absorb the herbicide. Apply herbicides when it is too cold (below 45°F) or even too warm (above 90°F) may reduce herbicide performance. The best time to apply herbicides is between 10AM and 3PM when the sun is shining and plants are actively growing to absorb nutrients. Most herbicides are absorbed best by small weeds (LT 4 inches tall).

Check your water pH adjust pH to the desired level on the label. Most herbicides, insecticides, and fungicides work best when the pH is slightly acidic (pH 4–6.5). A pH of 7 is considered neutral. Sulfonylurea herbicides perform better in slightly alkaline (above pH 7) conditions. When the pH falls outside the upper or lower pH boundaries, performance is reduced which affect how long the herbicide remains in solution before it becomes ineffective.

The University of California at Davis have guidelines for water pH. A pH between 3.5–6 is satisfactory for most spraying and short-term (12–24 hour) storage of herbicides in a spray tank. Not suitable for sulfonylurea herbicides. A pH between 6 and 7 is 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. Most products mixed in alkaline water (GT pH 7) should be sprayed immediately since the herbicides can be tied up in minutes.

Herbicide adjuvants improve herbicide performance. Ammonium sulfate is often used to improve water conditioning to reduce water hardness. However, ammonium sulfate cannot be used with dicamba soybean herbicide. It states that clearly on the label, so always read and follow label directions because that is the law.

There are at least 13 classes of herbicide adjuvants. Some are water conditioning agents that improve water hardness and water quality. Others are surfactants that reduce surface tension and allow the herbicides to be absorbed better. Other adjuvants include oil concentrates, evaporation retardants known as humectants, ammonium fertilizer solutions, defoamers, anti-foamers, marking agents, stickers, pH adjusters, and drift suppressing agents, etc. Following these tips improves herbicide performance and reduces re-spraying. Dr. Erdal Ozkan, OSU FABE Engineer has good information on his website also.