Donnie K. Miller

In 2020, Louisiana producers harvested 6,557 acres of sweet potatoes with an estimated yield of 480 bushels per acre. Production and packing fresh market costs were approximately $4,000 per acre to $4,600 per acre, while production costs for the processing sector were approximately $2,300 per acre prior to storage. Given this high level of production costs, there is little margin for error from movement of herbicide intended for target weeds in tolerant crops to sweet potatoes grown in close proximity, which is known as off-target movement. This movement can occur through application in adverse environmental conditions, such as high wind or temperature inversions; direct application through sprayer contamination; or product volatility (changing from liquid to vapor form after application). These events have the potential to limit growth and development of the sweet potato plant through severe injury and negatively impact yield.

Two new technologies that have been commercialized allow application of 2,4-D choline (marketed as Enlist Cropping System) or diglycolamine (DGA) /N,N-Bis(3-aminopropyl) methylamine (BAPMA) salt of dicamba (marketed as Xtend Cropping System) over the top of crops that were previously intolerant to these two herbicides. Soybean, cotton and corn varieties with these traits are readily available for purchase and use by producers. These new technologies use plant genetic resistance to these products so that applications may be made directly to the transformed crops with no injury concerns. Increasing populations of weeds resistant to glyphosate herbicide and variety development techniques have shifted focus to developing these new product formulation technologies with older herbicides and also utilizing plant genetic modification to combat these weeds. Because the sweet potato is also an Ipomoea species like morningglory, a major weed targeted by these herbicides in tolerant crops, off-target movement of 2,4-D and dicamba is a major cause for concern to sweet potato producers. No research has been conducted on the potential negative impacts on sweet potatoes from reduced rates of 2,4-D (applied as Enlist Duo) and dicamba (applied as Roundup Xtend) herbicide formulations that may be encountered in off-target movement.

A field study was initiated in 2016 at the LSU AgCenter Sweet Potato Research Station near Chase, Louisiana, and repeated in 2017. Beauregard sweet potatoes were mechanically transplanted June 29, 2016, and July 5, 2017. Glyphosate in combination with 2,4-D choline or dicamba was applied at various percentages of the recommended, or labeled, use rate of each product either 10 or 30 days after transplanting (DAP) the sweet potatoes. Some sweet potatoes were left untreated (zero application), while others received applications at 1/10, 1/33, 1/66 and 1/100 of the recommended use rate. The recommended, or labeled, use rate of the herbicides used as a basis for reduced rate applications was glyphosate at 1 pound per acre plus 2,4-D choline at 0.94 of a pound per acre and glyphosate at 1 pound per acre plus DGA salt of dicamba at 0.5 of a pound per acre.

Crop Injury

Visual injury ratings at seven, 14, 28 and 35 days after treatment, presented in Tables 1 and 2, tended to follow a similar trend. In general, greater injury was observed at the later 30 days after transplanting application timing and could simply be due to plants being larger at that time and having greater leaf and stem surface area to intercept more of the herbicide spray solution. In addition, dicamba was generally more injurious than 2,4-D; this is attributed to possible inherent tolerance differences in the variety. Injury symptoms ranged from complete plant death at the highest rates to darker green/purple tinted leaves and leaf cupping/twisting and strapping (similar to appearance when leather is stretched and pulled) at both high and low rates.

Sweet Potato Yield

Sweet potato yield for each grade category is presented in Table 3. Averaged across herbicides at the 10 days after planting application timing, the U.S. No. 1 sweet potato yield was significantly reduced in comparison with no herbicide (zero rate) only with the highest 1/10 reduced rate applied. At the later 30 days after planting application timing, U.S. No. 1 yield was significantly reduced at both the 1/10 and 1/33 rates, respectively. Within each reduced rate, U. S. No. 1 yield difference in application timing was noted only with the 1/10 and 1/33 rates, where yield was lower at the later timing. With minor exceptions, trends in yield impacts observed with U.S. No. 1 were observed with other yield grades as well.

In general, glyphosate plus dicamba proved to be more injurious than glyphosate plus 2,4-D. In most cases injury was greater at the later application timing. In either case typical hormonal herbicide symptomology, such as twisting or leaf epinasty, the downward bending of leaves, was still quite evident 35 days after application. Of interest is that early season visual injury differences among herbicides were not manifested in differences in yield regardless of herbicide rate or application timing. With respect to U.S. No. 1 and total (U.S. No. 1, canner and jumbo grade) sweet potato yield, the greatest negative impact was observed with herbicide application at the upper rate range, particularly the 1/10 and 1/33 rates, and at the later application timing regardless of herbicide applied. Therefore, sweet potato producers with multicrop farming operations should follow all sprayer cleanout procedures or devote different equipment to spraying Xtend and Enlist crops. In addition, proper consideration should be given to planting these crops in close proximity to sweet potato production fields and making herbicide applications under environmental conditions that are not conducive to off-target spray movement.

Donnie K. Miller is the John B. Baker Professor for Excellence in Weed Science at the LSU AgCenter Northeast Research Station, St. Joseph, Louisiana.

(This article appears in the winter 2021 issue of Louisiana Agriculture.)

Nontreated plants of Beauregard sweet potato. Photo by Marcie Mize

Impact of 2,4-D choline plus glyphosate applied at 1/10x use rate to Beauregard sweet potato applied 10 days after transplanting. Photo by Marcie Mize

Impact of 2,4-D choline plus glyphosate applied at 1/10x use rate to Beauregard sweet potato applied 30 days after transplanting. Photo by Marcie Mize