Background
Sweet potato is a staple crop that provides nutritional benefits to humans globally, but weed interference (Figure 1) can reduce yields by 22 to 90%. Despite increased organic sweet potato production in the United States, growers face challenges with limited weed management options and often resort to cultivation and hand-weeding.
Figure 1. Sweet potato planted on raised beds (left) at Vincennes and the same plot shown 6 weeks after transplanting with a lot of weed interference (right) (Photo by Emmanuel Cooper).
Objectives and Methodology
To address this challenge, we conducted research trials in 2022 and 2023 at Purdue Agriculture Centers at Lafayette and Vincennes to determine (1) if sweet potato cultivars differ in their ability to compete with weeds, (2) if in-row plant spacing can improve weed suppression, and (3) if a buckwheat cover crop or silage tarps can provide row-middle weed control. For the various studies, we used three different cultivars: ‘Covington’ (orange skin and flesh), ‘Monaco’ (orange skin and flesh), and ‘Murasaki’ (purple skin, cream flesh). At the start of the study, Monaco was considered for organic production systems because of its bunching growth habit and improved tolerance to soil-dwelling insect pests. Covington and Murasaki have longer vines (Figure 2). All plots were planted in early June and harvested 112 days after transplanting. Data collection in all studies included sweet potato canopy cover, weed count, weed height, and sweet potato yield by grade.
Objective 1. Do sweet potato cultivars with different growth habits differ in their ability to compete with weeds?
Year: 2022
Cultivars: Covington, Monaco, and Murasaki
Treatments: Sweet potatoes were transplanted. Weeds were removed by hand and allowed to establish and compete with the crop beginning at 0, 14, 21, 28, 35, or 42 days after transplanting.
Our findings:
As the weed-free period increased from 14 to 42 days after transplanting, sweet potato canopy cover increased from 30% to 87% for Covington, 29% to 89% for Monaco, and 34% to 97% for Murasaki (Figure 2). At 15 weeks after transplanting, Murasaki had the largest numerical canopy cover of the three cultivars for weed-free intervals of 21, 28, 35, and 42 days after transplanting. This superiority in vine canopy by Murasaki is likely due to its growth habit, which results in a rapidly formed dense canopy.
Figure 2. Effect of weed-free period on sweet potato canopy 15 weeks after transplanting (WAP) pooled across Lafayette and Vincennes in 2022. DAP = days after transplanting (Photo by Ashley Adair).
The longer weeds were allowed to exist in the crop, the more pronounced the reduction in yield became. Season-long weed interference reduced total yield by 76% for Covington, 88% for Monaco, and 65% for Murasaki relative to a weed-free control (Figure 3). To keep potential total yield loss to 10% or less, sweet potatoes needed to be maintained weed-free for 24 days after transplanting for Covington, 20 days after transplanting for Murasaki, and 33 days after transplanting for Monaco.
Figure 3. Effect of weed-free period on total yield reduction of sweet potato roots pooled across Lafayette and Vincennes in 2022. Total yield was the sum of U.S. No. 1, jumbo, and canner grades. DAP = days after transplanting (Photo by Emmanuel Cooper).
Objective 2. Can in-row plant spacing improve weed suppression?
Years: 2022 and 2023
Cultivars: Covington and Monaco
Treatments:
Our main treatment for this study was in-row plant spacing (20, 30, and 40 cm). Additionally, we evaluated two weeding frequencies: weekly from 2 to 6 weeks after transplanting or weekly for the entire 16-week growing season (Figure 4).
Figure 4. Plot layout at Lafayette in 2022 for Objective 2 (Photo by Ashley Adair).
Our findings:
Numeric trends showed that as in-row spacing decreased in 2023 from 40 to 20 cm, weed density decreased from 188 to 167 weeds m-2 at 3 weeks after transplanting. However, statistically, in-row plant spacing, weeding frequency, and cultivar had no significant effect on weed density at 3 and 6 weeks after transplanting.
In 2023, Monaco canopy cover was greater at the 20 cm spacing than the 30 or 40 cm spacings (Figures 5 and 6) at 5 weeks after transplanting. At all in-row spacings, Monaco had greater canopy cover than Covington at 5 and 16 weeks after transplanting (Figure 6).
Figure 5. Visual representation of three in-row spacings evaluated at Vincennes (Photo by Emmanuel Cooper).
Figure 6. Effect of in-row plant spacing on sweet potato canopy 5 weeks after transplanting (WAP) (left) and 16 WAP (right) pooled across Lafayette and Vincennes in 2023. Lowercase letters represent differences by in-row spacing, and capital letters represent differences by cultivar with Tukey’s HSD (p<0.05).
There were no differences in yield between our two weeding frequencies. However, as in-row spacing increased from 20 to 40 cm, sweet potato yield for U.S. No. 1, canner, and total grades decreased while the yield for jumbo grade increased (Figure 7).
Figure 7. Effect of in-row plant spacing on Covington canner, jumbo, U.S. No. 1, and total yields pooled across weeding frequency and location in 2022. Letters represent differences by in-row spacing within each grade with Tukey’s HSD (p<0.05).
Notably, in 2023, Monaco had a greater U.S. No. 1 yield than Covington, but Covington had a greater jumbo yield than Monaco (Figure 8).
Figure 8. Effect of in-row plant spacing on U.S. No. 1 (left) and jumbo (right) yields pooled across Lafayette and Vincennes in 2023. Lowercase letters represent differences by in-row spacing, and capital letters represent differences by cultivar with Tukey’s HSD (p<0.05).
Objective 3. Can a buckwheat cover crop or silage tarps be used for weed control in sweet potato row middles?
Year: 2023
Cultivar: Covington
Treatments: The main treatment was weed management method in the area between rows and included buckwheat or silage tarping established 3 weeks after transplanting and cultivation (3, 5, and 7 weeks after transplanting) (Figures 9 and 10).
Figure 9. Experimental unit of treatments. Each unit consisted of three raised beds, with only the middle one harvested and evaluated. The sweet potato was planted at 30 cm spacing. Between-row treatments were on each side of the middle-raised bed.
Figure 10. Visual representation of the three treatments in the study (Photo by Emmanuel Cooper).
Our findings:
The buckwheat and cultivation treatments had similar impacts on weed density at 6 weeks after transplanting (Figure 11), but weed height in the cultivation treatment was far less than that in the buckwheat treatment because a cultivation event at 5 weeks after transplanting removed established weeds.
Figure 11. Effect of weed management method on weed density (left) and weed height (right) at 6 weeks after transplanting pooled across Lafayette and Vincennes in 2023. Letters represent differences by weed management method with Tukey’s HSD (p<0.05).
At Lafayette, there were no differences in total yield across all treatments. However, at Vincennes, the total yield in the buckwheat treatment was less (10,798 kg ha-1) than in the cultivation (25,042 kg ha-1) and tarping treatments (21,471 kg ha-1) (Figure 12). The buckwheat treatment reduced total yield by 22% at Lafayette and 57% at Vincennes compared to the cultivation treatment.
Figure 12. Effect of row middle weed management method on total sweet potato yield at Lafayette (left) and Vincennes (right) in 2023. Letters represent differences by weed management method with Tukey’s HSD (p<0.05).
Conclusions
From our findings, we determined that:
- Sweet potato cultivars do differ in their tolerance to weed interference.
- When sweet potatoes are maintained weed-free during their critical period for weed control, decreasing in-row spacing did not improve weed suppression but did result in increased No. 1 and total yield.
- Silage tarps applied to sweet potato row middles provided excellent weed control and resulted in yield comparable to repeated cultivations.
Funding Statement: This research was supported by the U.S. Department of Agriculture- National Institute of Food and Agriculture OREI Project 1020533 and Hatch Project 7000862. The findings and conclusions in this publication have not been formally disseminated by the U.S. Department of Agriculture and should not be construed to represent any agency determination or policy.