Controlling Weeds

Controlling Weeds

Effective weed management is a critical component of potato production. Weeds compete for light, nutrients, and moisture and can act as refuge for insects and diseases that attack potatoes. Integrating preventative, cultural, mechanical, chemical, and even biological control methods are important for effective weed management. That’s right, even biological control.

You may not realize the help Mother Nature is lending you to reduce weed populations. Predators, including field mice, sowbugs, millipedes, crickets, slugs, and carabid beetles eat weed seeds that are lying on the soil’s surface. Keeping seeds on the soil’s surface as long as possible gives predators the opportunity to eat seeds that can lead to a reduction in the soil seed bank for years. In addition, seeds can
decay even during their first winter.

Because many weeds are ubiquitous to fields, especially in the northeast, prevention by eliminating the introduction of weed seed can be difficult since several different weed seeds are most likely already present in your fields. However, if herbicide-resistant weeds or new weed problems are an issue in other fields, it is important to prevent the spread of seed from these populations into new areas. Cleaning seeds from harvest and tillage equipment used in these fields before using them in uninfested areas can help prevent the introduction of new weed problems. Also, avoid spreading manure contaminated with weed seeds into uninfested fields.

Weed management in potatoes also relies on cultural and mechanical control methods. Good crop competition by promoting early canopy development is important in preventing the germination and emergence of new weed seedlings later in the growing season. A good crop canopy also helps reduce weed growth by reducing the impact that the weeds have on the potato crop.

Cultivation and hilling operations are also effective strategies in removing weeds in potatoes. While cultural, mechanical, and biological controls are all important, the use of chemical controls or herbicides is one of the more common strategies for management of weeds in potatoes. Integrating all of these strategies provides more effective weed management than relying solely on just one of these strategies alone.

Develop An Effective Program

Developing effective herbicide programs requires careful consideration on what weeds are or will be present in the field, soil characteristics, tillage and irrigation practices, and crop rotation. Most herbicides used in potatoes have a limited spectrum of weeds in which they will control. To broaden the spectrum of weeds controlled, herbicides are often applied in tank-mixtures. Tank-mixtures can also provide more consistent performance under varying soil and weather conditions, reduce the potential for crop injury, and reduce the selection pressure for herbicide-resistant weeds.

Several of the herbicides labeled for weed control in potatoes are soil-applied. In most cases, soil-applied herbicides will not control emerged weeds. Incorporation of these herbicides by rainfall or irrigation is needed to move soil-applied herbicides into the weed seed zone for adequate control. Soil-applied herbicides in potatoes are often applied after hilling which can be two to three weeks after planting. Hilling operations will control several of the weeds that would have emerged prior to the herbicide application. If hilling and the soil-applied herbicide application are done immediately after planting, weed control usually does not last later into the season. By delaying the soil-applied herbicide application two to three weeks after planting, the length of residual weed control will be increased later into the growing season. There are also several herbicides that can be applied postemergence to control weeds that have emerged.

These herbicides can be applied after weed and potato emergence. There are currently nine different selective herbicide active ingredients labeled for use in potatoes in areas of the northeast. While there are several different trade names for these active ingredients, characteristics and use patterns are similar and will be discussed below.

- Eptam 7EC (EPTC, Gowan Co.) is a thiocarbamate herbicide typically used at 4.5 pints per acre. Eptam needs to be incorporated immediately after application; therefore it needs to be applied prior to planting potatoes. Eptam will only control weeds that have not yet emerged, and it provides good control of annual grasses and common lambsquarters and fair control of several other small-seeded broadleaf weeds. Eptam will also suppress quackgrass and yellow nutsedge at higher application rates (6.75 pints per acre). Weed control from Eptam will not generally last throughout the entire season.

- Prowl H2O 3.8ACS and Prowl/Pendimax 3.3EC (pendimethalin, BASF andDow AgroSciences) are formulations of pendimethalin, a dinitroaniline herbicide. The typical use rate of Prowl H2O is 1.6 pints per acre and the typical use rates of Prowl/Pendimax are 1.8 pints per acre. Prowl may be used either before or after crop emergence (up to 6-inch tall potatoes), but must be applied before weeds have emerged for effective control. The potential for crop injury is greater when Prowl is applied after potato emergence. Typical injury symptoms include brittle stems and leaf malformations. Tank-mixtures or sequential applications of other herbicides are needed to broaden the spectrum of weeds controlled.

- Dual Magnum/Dual II Magnum 7.6L (s-metolachlor, Syngenta Crop Protection) and Parallel/Stalwart 7.8L (MANA Crop Protection, Sipcam Agro USA, metolachlor) are chloroacetamide herbicide formulations that should be applied prior to weed emergence. S-metolachlor (Dual Magnum/Dual II Magnum) is the resolved more active isomer of metolachlor and is typically applied at 1.33 pints per acre. Labeled rates for these herbicides are also 1.33 pints per acre, but at this rate these herbicides would not provide the same amount of active ingredient as Dual Magnum/Dual II Magnum at the 1.33 pints per acre rate.

Limited research has shown that 1.33 pints per acre of Parallel/Stalwart will provide similar activity to 1.33 pints per acre of Dual Magnum/Dual II Magnum. However, these herbicides may not provide the consistency or length of control on hard-to-control weeds as Dual Magnum/Dual II Magnum.
Tank-mixtures or sequential applications of other herbicides are needed to broaden the spectrum of weeds controlled. Dual Magnum/Dual II Magnum and Parallel/Stalwart can also be applied postemergence to potatoes, but they will not control emerged weeds.

- Outlook 6L (dimethenamid-P, BASF) is a chloroacetamide herbicide that should be applied prior to weed emergence. Outlook use rates range from 12 to 18 fluid ounces per acre on coarse-textured soils and 18 to 21 fluid ounces per acre on medium- to fine-textured soils. Under cold or wet conditions, applications of Outlook may result in delayed emergence or early-season stunting.

- Lorox/Linex 4L (linuron, DuPont Crop Protection) is a urea herbicide that controls weeds by inhibiting photosynthesis. Lorox/Linex should be applied prior to potato emergence and to germinating weeds or weeds that have emerged but are very small. Lorox/Linex is typically used at 1 quart per acre and can be tank-mixed with other soil-applied herbicides, like Matrix, to broaden the spectrum of weeds controlled. If small weeds have emerged, a non-ionic surfactant at 0.125% variation of volume ratio should be included.

- Sencor 75DF (metribuzin, Bayer CropScience), a triazine herbicide, kills weeds by inhibiting photosynthesis. Sencor may be applied either before or after weed or potato emergence. The typical use rate is 0.67 pounds per acre for applications prior to potato emergence and 0.33 pounds per acre for application made after potatoes have emerged. If weeds have emerged, apply Sencor when they are less than 1 inch tall. Potato varieties can vary in their tolerance to Sencor. Red skinned varieties are more prone to injury from Sencor applications. Injury from postemergence applications is greater when Sencor is applied within three days after a period of cool, wet, or cloudy weather. Sencor will not control triazine-resistant weeds, like triazine-resistant common lambsquarters. Tank-mixtures or sequential applications of other herbicides are needed to broaden the spectrum of weeds controlled.

- Matrix 25DF (rimsulfuron, DuPont Crop Protection) is sulfonylurea herbicide (ALS-inhibitor) that can be applied either before or after weed or potato emergence. Matrix is typically applied at 1.5 ounces per acre prior to potato emergence and 1 ounce per acre after potatoes have emerged. If applied postemergence to weeds a non-ionic surfactant at 0.25% variation of volume ratio should be included, weeds should also be less than 1 inch tall. Matrix will not control ALS-resistant weeds. Tank-mixtures or sequential applications of other herbicides are needed to broaden the spectrum of weeds controlled.

- Poast 1.5SC (sethoxydim, BASF) is a postemergence herbicide controlling grasses by inhibiting the acetyl coenzyme A carboxylase (ACCase). Poast is typically used at 1 pint per acre with 1 quart per acre of crop oil concentrate. Apply to grasses up to 8 inches tall. Ammonium sulfate at 2.5 pounds per acre can be added to improve control of harder to control grasses like quackgrass.

- Select Max 0.97EC and Select/Arrow 2EC (clethodim, Valent U.S.A. Corp.) are formulations of clethodim that are used for postemergence control of grasses. The typical use rate of Select Max is 9 fluid ounces per acre and the typical use rate of Select/Arrow is 6 fluid ounces per acre. These herbicides should be applied with 1% % variation of volume ratio of crop oil concentrate and 2.5 pounds per acre of ammonium sulfate. Select Max and Select/Arrow provide better control of hard-to-control grasses compared with Poast.

For more information on weed control in potatoes, consult Michigan State University Extension Publication E-434, “Weed Control Guide for Field Crops.”  

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