Managing Weeds in Louisiana Sweet Potatoes

Linda Benedict, Smith, Tara, Miller, Donnie K.  |  6/16/2009 10:30:16 PM

Donnie K. Miller and Tara P. Smith

Weeds compete with sweet potato plants for nutrients, water and sunlight and impair crop yield and quality, making effective weed management a critical aspect to successful production. When combined with timely cultivation, herbicide applications can effectively reduce weed competition, improve harvest efficiency and increase crop productivity. Most producers rely on herbicides to manage troublesome weeds.

Herbicide labels should always be consulted for activity on weed species present, rates for soil type, potential for crop injury, specific restrictions and pre-harvest application intervals. Sweet potato growth is generally slower in cooler temperatures and, when combined with excessive rainfall, can result in increased herbicide injury.

Herbicide effectiveness is generally greatest when the chemical is applied to small, actively growing weeds. The action of herbicides with soil activity is greatly enhanced by rainfall, irrigation or mechanical cultivation within seven days after application.

In general, one-half inch of rainfall or irrigation, preferably in one occasion, is adequate for activation of most soil-active herbicides. In contrast, excessive amounts of rainfall over a short period may result in herbicide leaching and reduced efficiency, especially in lighter-textured soils. Rain-free periods required for adequate herbicide absorption following leaf contact on emerged weeds can differ depending on the herbicide used. Specific labels should be consulted concerning this and specific surfactant requirements.

Research has indicated that adding ammonium sulfate to herbicides such as glyphosate can be beneficial when they are used with hard water, which contains iron, calcium or magnesium ions. Proper nozzle selection and sprayer calibration are also key factors in maximizing herbicide activity and reducing crop injury. Information on nozzle selection is available in the "Louisiana Suggested Chemical Weed Control Guide" available at www.lsuagcenter.com or from nozzle manufacturers.

Compared with most crops grown in Louisiana, sweet potatoes have few effective herbicides labeled for use. Weed management can be aided by selecting a field without a history of serious weed problems. Frequent crop rotation with soybeans, corn and grain sorghum also can be beneficial in weed management programs by introducing alternative herbicides to increase the spectrum of weeds controlled. Because crop rotation intervals can vary among herbicides, labels should be consulted when planning initial crop rotations. Cultivation should be used to control weeds during early growth stages. In addition, sweet potato vines can effectively compete with weeds after good root systems are established.

Long-term management of perennial weeds – including Johnson grass, Bermuda grass, alligatorweed, and yellow and purple nutsedge – is best achieved through a fall fallow-ground herbicide application. Ideally, treatment should be initiated once weed growth has occurred after harvest and when actively growing plants are in the boot/heading stage but prior to first frost. The optimum recommended time to initiate treatment is September 15 to October 15. Best results occur with fall fallow-ground applications over multiple years.

Preplant Burndown

Gramoxone Inteon (paraquat) can be used for rapid control of actively growing vegetation prior to cultivation and just prior to planting. Apply at the rate of two to four pints of formulated material per acre. Use rates greater than 2.5 pints per acre only when dense weed populations exist or weed height exceeds 4 inches. Include a nonionic surfactant at 0.25 percent or crop oil concentrate at one percent by volume. Symptoms usually appear within one to two days after application. Good spray coverage is essential for maximum control because herbicide activity is strictly by contact.

Glyphosate (many formulations) can be used as a preplant treatment to control alligatorweed, pigweeds, ground cherry and numerous grass species. Apply at a rate of 0.75 to one pound of active ingredient per acre for control of annual weeds. The higher rate should be used for larger weeds. The addition of a surfactant will depend on the formulation used; therefore, consult the label. To obtain maximum benefit from preplant applications, allow seven days prior to cultivation.

In-season Management: Soil-applied Herbicides

Command 3 ME (clomazone) can be applied at the rate of 1 1/3 to four pints per acre as a broadcast post-transplant surface application. Consult the label for rate adjustments for soil type or weeds to be controlled. Command controls annual grasses and select broadleaf weeds. Command will not control nutsedge.

Valor SX (flumioxazin) can be applied at the rate of two to 2.5 ounces per acre pre-transplant. Do not mix with metolachlor (Dual Magnum). It may be tank mixed with labeled rates of clomazone (Command 3 ME) if it is applied prior to transplanting.

Dual Magnum (s-metolachlor) Consult the label prior to use. Apply at the rate of one to 1.33 pints per acre post-transplant and before target weeds emerge. Do not incorporate following application. Do not mix with Valor (flumioxazin) herbicide. Use the lower rate on coarse-textured soils. Crop injury potential may increase on sands and loamy sand soils, especially if a heavy rainfall occurs shortly after application. If irrigation is used, do not apply more than 0.5 inches of water for the first irrigation following application. This is currently labeled under section 24c of the Federal Insecticide, Fungicide and Rodenticide Act, special local needs label, 2006-2011.

In-season Management: Postemergence Herbicides

Aim EC (carfentrazone-ethyl) can be applied at the rate of 0.5 to two fluid ounces per acre broadcast with a hooded sprayer to control labeled weeds such as smooth pigweed, spiny amaranth, ground cherry and numerous morning glory species. Apply up to two fluid ounces (0.031 pound active ingredient) per acre in a minimum of 10 gallons per acre of finished spray. Hooded sprayers must be designed and adjusted to enclose the spray pattern and prevent any spray on the stems and leaves of the sweet potato. For optimal results apply to actively growing weeds up to 4 inches tall. Do not exceed 6.1 fluid ounces (0.096 pound active ingredient) per season.

Fusilade DX (fluazifop) is for control of actively growing grasses only. Apply at the rate of six to 12 ounces per acre. Use higher rates on larger grasses. Include a nonionic surfactant at 0.25 to 0.5 percent or crop oil concentrate at one percent by volume.

Poast (sethoxydim) is for control of grasses only. Apply at the rate of eight to 16 ounces peracre. Use higher rates on larger grasses. Refer to the label for specific adjuvant requirements. Select (clethodim) is for control of grasses only. Apply at the rate of six to eight ounces per acre with a one percent crop oil concentrate by volume. Use higher rates on larger grasses.

Read more in Weed Management in Sweet Potato.

Donnie K. Miller, Associate Professor, Northeast Research Station, St. Joseph, La.; and Tara P. Smith, Assistant Professor, Sweet Potato Research Station, Chase, La.

(This article was published in the spring 2009 issue of Louisiana Agriculture.)

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