Steven T. Kelly, Donnie K. Miller and Mark W. Shankle
Before Command herbicide was labeled for sweet potato production, Louisiana sweet potato growers faced numerous weeds such as annual grasses, cocklebur, morningglory and prickly sida. Although Command seemed to solve many of the sweet potato weed control issues at that time, continuous reliance on this herbicide has led to a shift in the weed spectrum in sweet potato fields.
In the last few years, pigweeds and sedges have emerged as the primary weedy pests in sweet potato production in Louisiana and Mississippi. Also, smellmelon, groundcherry and copperleaf are rapidly gaining a foothold. Since Command provides little to no control of these troublesome weeds, they have essentially gone uncontrolled, contributing to yield loss from competition and perhaps harboring yield-reducing insects.
Research to evaluate potential herbicides for use in sweet potato production began in 2001 at the LSU AgCenter’s Sweet Potato Research Station
at Chase and Mississippi’s Pontotoc Ridge-Flatwoods Branch Experiment Station near Pontotoc, Miss. The herbicides evaluated included Spartan, Valor and Sandea. Spartan, Valor
Spartan is labeled in Louisiana for broadleaf and sedge control in sugarcane while Valor is labeled in soybeans and cotton for broadleaf weed control and has been demonstrated to provide excellent pigweed, smellmelon and cop perleaf control in those crops. Sandea is currently labeled for control of annual and perennial sedges in several vegetable crops including tomato and cucurbits such as watermelons grown in Louisiana and Mississippi. In addition, this herbicide may control several broadleaf weeds if applied when the weeds are less than four inches tall.
Although the weed control spectrum of these herbicides is well-documented, the issues surrounding application timing to sweet potato are virtually unknown.
Spartan and Valor are distinctive herbicides that can be applied to both the soil surface and weed foliage (if a surfactant is included). Spartan also can be incorporated into the soil before planting. All three application methods have been investigated at both locations. When herbicides were applied to the sweet potato cuttings after planting, excessive foliar injury and some stand loss were observed with both herbicides. However, when they were applied to the soil surface before planting the sweet potato cuttings, excellent sweet potato tolerance was observed.
Valor’s manufacturer supported further investigations, and an emergency-use label was issued in 2002 and 2003 for Louisiana and Mississippi for pigweed control. A full federal label was issued in the fall of 2004, and Valor can now be used in numerous sweet potato producing states. Spartan Response
The response of sweet potatoes to Spartan, which is labeled for white potatoes but not for sweet potato, has been inconsistent. While we have observed excellent tolerance when it is applied to the soil surface before planting, if it is incorporated into the soil, it tends to reduce yield during wet years, which is a cause for concern.
In 2004, the yields from plots with incorporated treatments were reduced nearly 50 percent compared to surfaceapplied treatments. Although there were no differences in weed control among application timings, a reduction in yield of this magnitude during wet periods, coupled with unpredictable rainfall patterns, would warrant not using Spartan in this manner.
Soil carryover to other crops is also a concern. With many producers now rotating crops, a herbicide with a short life span is essential. Although soybeans can be planted at any time following a Spartan application, the rotation restriction is 18 months for cotton and 10 months for corn, grain sorghum and rice.
Sandea, which is not labeled for sweet potatoes, is best known for sedge control. Since sweet potatoes are planted to freshly prepared beds with few if any weeds present, this herbicide is an obvious choice for applications several weeks into the sweet potato growing season. Since its effects on sweet potato production were not known, Sandea was applied to different plots in seven-day intervals from 14 days to 49 days after sweet potato transplanting. Control of sedges and pigweeds was greatest when applied from 14 to 42 days after transplanting. When Sandea was applied later, the weeds were too large for effective control and had competed with the sweet potato much longer than desirable. Yield Effects
However, sweet potato yield was affected much differently. A yield reduction was observed when Sandea was applied at 14, 21 and 49 days after transplanting. The earlier applications resulted in reduced growth during the sweet potato’s critical establishment period, while the later application was during the root growth phase. Application at 28, 35 or 42 days after transplanting ensures that the herbicide is applied to smaller weeds but not to sweet potatoes during critical development stages.
Applying a herbicide to the soil surface before planting sweet potatoes does present some challenges. Sweet potatoes are unique in that a cutting is planted using a mechanical transplanter that disturbs the soil and, consequently, the herbicide barrier. Oftentimes, with a timely rainfall soon after transplanting, the herbicide barrier seems to be unaffected. But when the herbicide is not activated by rain in a timely manner, some weeds will escape in areas where the soil had been disturbed. Some producers have reduced this problem by using a roller that firms the row ahead of the transplanter, eliminating a large amount of soil disturbance and improving weed control.
These findings represent a step forward in sweet potato weed control in Louisiana and Mississippi. Although applying Valor or Spartan before planting sweet potatoes represents an additional trip across the field to apply the herbicide, it is worth it. The benefits from increased yields by controlling broadleaf weeds more than outweigh the expense. Steven T. Kelly, Associate Professor, T.H. Scott Extension, Education and Research Center, Winnsboro, La.; Donnie K. Miller, Associate Professor, Northeast Research Station, St. Joseph, La.; and Mark W. Shankle, Associate Research Professor, Pontotoc Ridge-Flatwoods Branch Experiment Station, Mississippi State University, Pontotoc, Miss.
(This article was published in the winter 2006 issue of Louisiana Agriculture.)