Weed Control Guide for Ohio Field Crops

Weed Control Principles

Avoiding Water Contamination

Surface Water

Traces of several common herbicides have been found in the water sources for some municipal water systems in Ohio. Herbicides and other pesticides can reach streams, lakes, and reservoirs in water that runs off treated fields. Herbicides can be carried dissolved in runoff water or held on the surface of eroded soil particles. Runoff risk is greatest when heavy rains closely follow herbicide application to steep slopes. Reducing water runoff and soil erosion can protect surface water quality by keeping herbicides on treated fields.

Conservation tillage systems generally reduce water runoff and soil erosion compared to conventional tillage, and thus reduce runoff of herbicides. Runoff and erosion decrease as the amount of residue remaining on the surface increases. Incorporation of herbicides may reduce runoff potential by reducing the concentration of herbicide on the soil surface. Grass strips are effective in reducing herbicide runoff because they trap sediment carrying herbicides and slow runoff water, allowing more herbicide to fall out of solution. Leaving untreated grass strips next to streams and ponds will help protect water quality. Never clean or dump sprayers or dispose of empty containers near streams, ponds, or lakes due to the risk of contamination.

Some herbicide labels contain a surface water advisory stamement, indicating they may have a high potential for runoff into surface water under some conditions. All products containing sulfentrazone, atrazine, isoxaflutole, flufenacet, diflufenzopyr, terbacil, and cloransulam-methyl have this advisory.

Groundwater

The potential for contamination of groundwater with herbicides appears to be reduced in most Ohio, compared to many other states. However, herbicide users should be aware of the ways that groundwater becomes contaminated by herbicides. Most instances of groundwater contamination are due to leaching of herbicides from loading or disposal sites. Potential for groundwater contamination can be reduced through careful application, handling, and storage of herbicides.

Leaching. While the majority of herbicide applied generally remains in the top few inches of soil until it degrades, a small percentage of certain herbicides can leach below the root zone to possibly contaminate shallow groundwater. This is most likely to happen in sandy soils, which have a low capacity for absorbing herbicide. Herbicides that are low in solubility, strongly absorbed on soil particles, and fairly nonpersistent are least likely to leach. Conversely, herbicides that are high in solubility, weakly absorbed on soil, and very persistent are most likely to have a potential to leach.

The potential for groundwater contamination can be reduced by selecting and using herbicides with low leaching potential. This is especially important where soils are sandy. Applying lower herbicide rates and reducing the total amount applied, as in banding, can lower contamination potential.

Groundwater warning statements are required on the labels of herbicides that have been detected frequently in groundwater monitoring. Most groundwater statements have similar wording: "This product is a chemical which can travel seep or leach) through the soil and can contaminate groundwater which may be used as drinking water. This product has been found in groundwater as a result of agricultural use. Users are advised not to apply this product where the water table (groundwater) is close to the surface and where soils are very permeable, i.e., well drained soils such as loamy sands. Your local agricultural agencies can provide further information on the type of soil in your area and the location of groundwater."

All products containing any of the following active ingredients are labeled with a groundwater advisory statement:

acetochlor	alachlor	isoxaflutole
atrazine	metolachlor	flufenacet
cyanazine	clopyralid	diflufenzopyr
metribuzin	flumetsulam	terbacil
simazine	dimethenamid	hexazinone
sulfentrazone	cloransulam-methyl

Leaching from mixing or disposal areas. High soil herbicide concentrations occur through spillage or improper disposal of herbicides in small areas. These high concentrations can overload the ability of the soil to adsorb and degrade herbicides. Leaching of herbicides from these areas is much greater than in treated fields. If sprayers are dumped or washed out in the same place over the years, concentrated sources of herbicide are created. If this activity takes place near a well, risk of contamination is increased, especially if the well is not properly cased and surface runoff can enter the well. Herbicides should be stored and mixed away from the well.

The best method for disposing of excess spray mix is to use it up on a crop field. The excess can sometimes be applied at low rates on a portion of the field by increasing sprayer speed. Be careful not to exceed the total label rates for the crop, risking crop injury, carryover, or illegal applications. Applying sprayer rinsate to the field avoids similar problems with disposal.

Backsiphoning can allow large quantities of herbicide to directly enter groundwater at the depth of the well. This happens when the end of the water hose is allowed to extend into the spray solution when filling sprayers. If the water is shut off with the hose in the tank, the spray solution can backsiphon down the well or into the water system. To avoid backsiphoning, position the hose above the spray solution while filling and remove the hose prior to shutting off the water. Use an anti-backflow valve when drawing water from a well or pond. Inexpensive anti-backflow devices for hoses that are used to fill sprayers can be purchased from sprayer equipment dealers. A state regulation in Ohio requires an anti-backsiphoning device in certain sprayer systems.

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