Abstract:
Sweetpotato is an economically important crop in North Carolina, generating $332,917,000 in production value in 2020. Weed management is challenging in sweetpotato; in both organic and conventional production systems, hand-weeding is regularly used and is expensive. Our research objectives were thus to examine cultivar selection as a potential avenue of weed management in organic sweetpotato systems as well as address current grower concerns with flumioxazin and S-metolachlor (critically important herbicides for conventional sweetpotato production in North Carolina).

Field studies were conducted in 2020 and 2021 to determine whether selected sweetpotato cultivars (Covington, Murasaki, and Monaco) with different canopy architectures respond differently to cultivation and weed interference. Cultivation treatments included no cultivation weedy and weed-free checks, cultivation weekly and biweekly (every 2 weeks) from 2 to 6 weeks after planting (WAP), 6 WAP cultivation only, cultivation at 2 WAP followed by hand removal from 2 to 6 WAP, and cultivation at the first and first two weed emergences. Cultivation treatments generally provided better weed control in 2021 than in 2020, except for the 2-week followed by hand removal and one cultivation at 1st weed emergence at 10 WAP rating. For total marketable sweetpotato yield (sum of canner, No. 1, and jumbo grades), in 2020 the non-cultivated weed-free, 2 WAP followed by hand removal (2-6 WAP), and 1 cultivation at first weed emergence event treatments were not different from one another. The biweekly (2 to 6 WAP), weekly (2 to 6 WAP), 2 cultivations at 1st two weed emergences, 6 WAP only, and non-cultivated weedy treatments significantly reduced total marketable sweetpotato yield compared to the weed-free treatment. In 2021 the treatments with greater cultivation frequencies as well as the 1 cultivation at 2 WAP followed by hand removal from 2-6 WAP treatment performed better than the 1 cultivation at 1st weed emergence and 2 cultivations at 1st two weed emergences treatments compared to the non-cultivated weed-free check. Results from 2021 show that weed control benefits of regular cultivations from 2-6 WAP can potentially mitigate yield loss due to vine damage. However, data from 2020 and 2021 show differences in weed control and yield due to environmental variability. Selected sweetpotato cultivars did not express differential tolerance to cultivation nor weed interference.

Flumioxazin and S-metolachlor are widely used on conventional sweetpotato hectarage in North Carolina; however, some growers have recently expressed concerns about potential effects of these herbicides on sweetpotato yield and quality. Previous research indicates that activated charcoal can improve crop safety and reduce herbicide injury in some conditions. Field studies were conducted in 2021 and 2022 to determine whether flumioxazin applied pre-planting and S-metolachlor applied before and after transplanting negatively affect sweetpotato yield and quality when activated charcoal is applied with transplant water. The studies consisted of five herbicide treatments by two activated charcoal treatments. Herbicide treatments included two rates of flumioxazin, one rate of S-metolachlor applied immediately before and immediately after transplanting, and no herbicide. Charcoal treatments consisted of activated charcoal applied at 9 kg ha-1 and no charcoal. No visual injury was observed. There was no effect of herbicide or charcoal treatment on No. 1, marketable (sum of No. 1 and jumbo grades), or total yield (sum of canner, No. 1, and jumbo grades). Additionally, shape analysis conducted on calculated length-to-width ratio (LWR) for No. 1 sweetpotato roots found no effect from flumioxazin at either rate on sweetpotato root shape. However, both S-metolachlor treatments resulted in lower LWR of No. 1 sweetpotato roots in 2021. Results are consistent with those of prior research and indicate that flumioxazin and S-metolachlor are safe for continued use in sweetpotato at registered rates.