Effect of Herbicide Regimes on Weed Density and Yield in Sweet Potato

Linda Benedict, Smith, Tara, Miller, Donnie K.  |  8/16/2006 8:15:40 PM

(Photo by John Wozniak)

Photo By: John Wozniak

Tara P. Smith, Donnie K. Miller, Steven T. Kelly and Abner M. Hammond

In 2005, Louisiana producers harvested more than 16,000 acres of sweet potatoes with a total value exceeding $90 million. Weeds can affect agricultural production systems in many ways. In vegetable crop production, weeds can cause significant yield reductions by competing with crops for nutrients, light and water. Beauregard, the predominant sweet potato cultivar grown in Louisiana, is a high-yielding variety but is not as tolerant to weed interference as some of the older, lower-yielding varieties. Research suggests that the majority of yield loss in Beauregard is caused by weed interference during the first two to eight weeks after transplant.

Sweet potato producers deal with a complex of weed species, and recently, the primary problem weeds affecting sweet potato production in Louisiana are hophornbeam copperleaf, spiny amaranth (pigweed), smellmelon, and yellow and purple nutsedge. Carpetweed is also widespread but is not considered a problem. Currently, the herbicides labeled for use on sweet potato are Command (clomazone), labeled to control grasses, and Valor (flumioxazin), labeled to control broadleaf weeds. In 2004 and 2005, Sandea (halosulfuron) received emergency labeling under Section 18 of the Federal Insecticide, Fungicide and Rodenticide Act to control purple nutsedge in Louisiana sweet potato production. Since the registration of Command herbicide in sweet potato, the populations of morningglories, cocklebur and grasses have been drastically reduced, but other weeds that clomazone does not control continue as pests. Valor and Sandea have been evaluated previously, with neither having known adverse effects on growth or yield of sweet potatoes when applied as recommended.

A study conducted in 2005 investigated the effect of different herbicide regimes on weed density and yield in sweet potato. Transplants 12 inches long were cut the day of transplanting and spaced one foot apart. All herbicide treatments were applied immediately before or following transplant to weed-free beds. Treatments included Valor at 2 ounces per acre plus Sandea at 2/3 ounce per acre, Command at 2.5 pints per acre plus Sandea at 2/3 ounce per acre, Command at 2.5 pints per acre plus Valor at 2 ounces per acre, Valor at 2 ounces per acre plus Command at 2.5 pints per acre plus Sandea at 2/3 ounce per acre, and an untreated control. All Sandea applications were made according to label directions at 28 days after transplanting.

The Valor-plus-Sandea combination targeted broadleaf weeds and annual sedges, whereas Command-plus-Sandea targeted annual grasses and sedges. The combination of Valor plus Sandea plus Command targeted broadleaf weeds, grasses and sedges. The goal of the treatment regime that included all three herbicides was to establish weed-free plots with little competition.

Three random weed counts were taken 70 days following transplant in one square foot of each plot. At 115 days after transplant, two rows of each plot were harvested using a mechanical digger. Roots were graded according to U.S. Department of Agriculture standards and separated into three grade classes – U.S. No. 1 and 2, Canners and Jumbos.

Overall, yields in the study were low because of minimal rainfall in research plots in 2005. U.S. No. 1 and 2 yield – the premium yield grade of sweet potato in the United States – was significantly increased in all herbicide treated plots (up to 75 percent) compared to untreated controls. The treatment regime which included Valor and Command had the highest yield of all herbicide combinations evaluated. The majority of weed species present in the current study were broadleaf weeds, specifically spiny amaranth and carpetweed, although minimal crabgrass was also present. Differences in grass control were not detected in herbicide-treated and untreated plots in the study. All herbicide treatments reduced the number of spiny amaranth plants, and all regimes except Command-plus-Sandea – the only treatment regime without Valor – significantly reduced carpetweed numbers.

The increase of U.S. No. 1 and 2 yield in the herbicide-treated plots compared to untreated controls demonstrates the importance of practicing integrated weed management in sweet potato. Treatment regimes that included Valor and Command provided good control of spiny amaranth and carpetweed, two predominate weed species in Louisiana sweet potato production. These data indicate that greater yields may be achieved in sweet potato if herbicides are integrated into an overall pest management program.

Tara P. Smith, Assistant Professor, Sweet Potato Research Station, Chase, La.; Donnie K. Miller, Associate Professor, Northeast Research Station, St. Joseph, La.; Steven T. Kelly, Associate Professor, Scott Extension Education & Research Center, Winnsboro, La.; and Abner M. Hammond, Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.

(This article was published in the summer 2006 issue of Louisiana Agriculture.)
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