Evaluation of Reduced and Strip-tillage Cover Crop Sweet Potato Production Systems on Soil Health, Sweet Potato Growth, and Weed Management Programs

Final report for GS18-185

Project Type: Graduate Student
Funds awarded in 2018: $16,499.00
Projected End Date: 08/31/2020
Grant Recipient: North Carolina State University
Region: Southern
State: North Carolina
Graduate Student:
Major Professor:
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Project Information

Summary:

Sweetpotato is an economically important crop for North Carolina growers. Current sweetpotato production systems rely heavily on cultivation. The lack of chemical and non-chemical weed control methods further increases reliance on cultivation. Excessive cultivation is detrimental to soil health. Reduced tillage cover crop systems have the potential to reduce reliance on cultivation and herbicides by providing an alternative weed control method. These systems also increase soil organic matter content and soil structure. A study was conducted to evaluate the effect of a reduced tillage rye cover crop system on soil, crop growth, and yield. Another study was conducted to evaluate weed control programs in this system. Sweetpotato canopy width increased at a greater rate in the conventional tillage system. Similarly, canopy height of Covington and Bayou Bell increased at a greater rate in the conventional than the reduced-tillage system. In both production systems, herbicide programs that included indaziflam, flumioxazin, and linuron tended to have greater weed control than other programs. The knowledge gained is being disseminated to growers and the scientific community through field days, extension articles, and refereed journal publications.

Project Objectives:
  1. Determine how a reduced-tillage production system influences sweetpotato growth and yield components compared to a conventional tillage system.
  2. Develop weed management programs in a reduced tillage system. Determine how a reduced-tillage system responds under intense weed pressure with and without herbicides.
  3. Disseminate lessons learned and recommendations through grower meetings, extension articles, and refereed journal publications.

Cooperators

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Research

Materials and methods:

Objective 1: Determine how a reduced-tillage production system influences sweetpotato growth and yield components compared to a conventional tillage system. Studies were conducted at the Horticultural Crops Research Station in Clinton, NC, and the Cunningham Research Station in Kinston, NC in 2019 and the Caswell Research Station in Kinston, NC in 2020. In the October prior to sweetpotato planting, conventional plots were clean-tilled while beds were formed and shaped in no till-plots. Rye was immediately broadcast seeded at 135 kg ha-1 across the entire study area. Nitrogen was applied to the rye in December (33.6 kg ha-1) and February (67.3 kg ha-1) to increase biomass and compensate for N immobilization by rye. Rye in conventional plots was killed at the boot growth stage using glyphosate at 1.24 kg ae ha-1. In late May or early June conventional beds were formed and shaped. Non-rooted sweetpotato cuttings were transplanted into conventional plots directly though the standing rye onto the raised beds using a commercial mechanical transplanter.

Within each production system multiple cultivars with varying canopy architectures were evaluated for performance. ‘Covington’ and ‘Bayou Bell’ have low-growing sprawling architectures. ‘NC04-0531’ and ‘NC15-650’ were bred for organic production and have upright architectures. The study was a split-plot design with tillage system as the whole-plot factor and cultivar as the split-plot factor. The split-plots were three rows each 6.1 m long. The first and third row were border rows. The second was a data collection row. Each whole-plot was replicated four times.

Data collected included cover crop biomass, sweetpotato canopy height and width, time to canopy closure, storage root yield and shape, soil moisture, and bulk density. Cover crop biomass was measured at two weeks after planting (WAP). Soil bulk density was measured at two and ten WAP. Soil moisture and canopy measurements were collected biweekly from two to twelve WAP. Storage roots were graded using a high-throughput optical grader into jumbo (> 8.9 cm in diameter), no. 1 (> 4.4 cm but < 8.9 cm diameter), canner (> 2.5 cm but < 4.4 cm diameter) (USDA 2005). Marketable yield was calculated as the sum of jumbo and no. 1 grades. The optical grader was also used to collect shape measurements on all roots collected in the studies. Data was subjected to analysis of variance using PROC MIXED (SAS 9.4, SAS Institute, Inc. Cary, NC).

Objective 2: Develop weed management programs in a reduced tillage system. Determine how a reduced-tillage system responds under intense weed pressure with and without herbicides. Studies were conducted at the Horticultural Crops Research Station in Clinton, NC and the Caswell Research Station in Kinston, NC in 2019 and 2020, respectively. The effectiveness of 13 weed management programs were compared in the reduced-tillage and conventional system. Herbicide programs included flumioxazin, flumioxazin tank mixed with clomazone, fomesafen, indaziflam, or bicyclopyrone pre-transplant, with or without clomazone after transplanting, and S-metolachlor with or without linuron at 2 WAP (Table 1). Flumioxazin and S-metolachlor are industry standards. Fomesafen was recently registered for use in sweetpotato under a 24(c) registration. Linuron is in the process of being registered for use in sweetpotato through the IR-4 Project. A non-treated weed-free (hand-weeded) and weedy check were included for comparison.

The study was a split-plot design with tillage system as the whole-plot factor and weed management program as the split-plot factor. The split-plots were two rows each 6.1 m long. The first row was a non-treated border row. The second and third were treated data rows. Each whole-plot was replicated 4 times. This study will also be repeated the following year. Data collected included cover crop biomass, visual weed control ratings, visual crop injury ratings, and storage root yield. Data was subjected to analysis of variance using PROC MIXED (SAS 9.4, SAS Institute, Inc. Cary, NC).

Objective 3: Disseminate lessons learned and recommendations through grower meetings, extension articles, and refereed journal publications. These studies were presented at the Organic Production Field Day in 2019 and the National Sweetpotato Collaborators Group meeting in 2020. Two refereed journal publications and an extension publication are in progress to disseminate results to the broader scientific and grower community.

Research results and discussion:

Objective 1: Determine how a reduced-tillage production system influences sweetpotato growth and yield components compared to a conventional tillage system.

Canopy growth: Early season canopy growth in conventional and reduced-tillage systems was similar regardless of cultivar. After lay-by (approximately 5 WAP) differences began to emerge between systems. Sweetpotato canopy width increased at a greater rate in the conventional tillage system (Graph 1). Similarly, canopy height of Covington and Bayou Bell increased at a greater rate in the conventional than the reduced-tillage system (Graph 2). As expected, canopy width expansion of upright cultivars was slower than that of sprawling cultivars regardless of system (Graph 3).

The differences observed in canopy growth have significant implications for weed management. Large crop canopies are better able to compete for light than small canopies. Large canopies can also serve to out compete with later emerging weeds. This suggests that conventional tillage systems result in a more competitive crop than reduced-tillage systems. Due to the reduced rate of canopy width expansion of upright cultivars, relative to sprawling cultivars, cultivation to control weeds can occur later in the season if grown in a conventional system.

Soil moisture: Volumetric water content was similar for both systems the majority of each season. At the Clinton location volumetric water content was greater for the reduced-tillage system at 10 WAP. The same trend was seen at the Cunningham location at 8 WAP. These differences were observed during extremely dry periods and suggest that during periods of drought, reduced-tillage systems may have more plant available water than conventional systems.

Soil bulk density: Soil bulk density in the reduced-tillage system was 5 and 7% higher than the conventional tillage system at 2 and 10 WAP, respectively (Graph 4). Soil bulk density for both systems at each timing was within the recommended range (<1.6 g cm-3) for root growth in a sandy soil (USDA-NRCS 2008).

Sweetpotato yield: The interaction of cultivar and production system was not significant; therefore, only main effects are presented. The conventional tillage system had 17% more marketable roots than the reduced-tillage system (Table 2), however no differences in yield weights was observed.

Bayou bell had 53 and 66% greater marketable yield than Covington and NC15-0650, respectively (Table 3). Covington and NC15-0650 had 100 and 86% greater marketable yield than NC04-0531. Bayou bell and NC15-0650 had similar no.1 yield which was greater than Covington and NC04-0531. Bayou bell also had the greatest cull yield. Storage root counts by cultivar showed a similar trend to yield weights (Table 4).

            NC15-0650 and NC04-531 had length to width ratios closest to 2 (Table 5). Covington had the lowest length to width ratio of 1.69. Storage roots with a length to width ratio of 2 are more desirable than other shapes. Yencho et al. (2006) describes Covington as having a length to width ratio of 2.

Objective 2: Develop weed management programs in a reduced tillage system. Determine how a reduced-tillage system responds under intense weed pressure with and without herbicides. 

Broadleaf weed control: The interaction of herbicide program and production system was statistically significant (p>0.05); therefore, weed control was analyzed separately by system. The interaction of herbicide program and year within the reduced-tillage system was significant therefore, weed control was analyzed separately by year for the reduced-tillage system. Weed control by herbicide programs within the conventional tillage system were combined across both years due to the lack of an interaction.

            All herbicide programs in the conventional tillage system provided over 97% weed control at 4 WAP except for bicyclopyrone without clomazone (Table 6). By 8 WAP, herbicide programs that included indaziflam, flumioxazin, and linuron tended to have greater weed control than other programs. Fomesafen with or without clomazone provided < 70% weed control after 8 WAP. Broadleaf weed control in the conventional tillage system was >60% regardless of herbicide program or year. This is due to the effectiveness of tillage in controlling broadleaf weeds. This data indicates that, along with efficacy, other factors such as herbicide resistance management and application costs should be considered when determining herbicide programs in conventional systems.

            Greater differences between herbicide programs were observed in the reduced-tillage system (Table 7). In 2019 herbicide programs with indaziflam, linuron, or flumioxazin (not tank mixed) had over 70% control at 8 WAP. Programs with bicyclopyrone or fomesafen without linuron had less than 35% broadleaf weed control. In 2020 only flumioxazin followed by clomazone, or programs containing linuron achieved greater than 70% weed control at 8 WAP. Programs with indaziflam had less than 35% weed control. The differences observed between years was likely caused by a difference in weed species. In 2019 the field was dominated by Palmer amaranth with moderate eclipta and pink purslane populations. In 2020 there was a greater diversity of weeds including morningglory, sicklepod, pink purslane, redroot pigweed, cudweed, and cutleaf evening-primrose.

            The significant production system by herbicide program interaction prevents the drawing of statistical conclusions comparing the main effect of production system on broadleaf weed control. However, the reduced-tillage system generally had much greater weed population regardless of herbicide program. This data highlights the importance of tillage in sweetpotato weed management programs. Further refinement of reduced-tillage systems is needed to increase weed suppression by the cover crop. Weed suppression by the cover crop will likely need to be equal to or greater than that of tillage before adoption by growers.

Sweetpotato yield: The interaction of herbicide program and year was statistically significant (p>0.05); therefore, storage root yield was analyzed separately by year. In 2019 the interaction of herbicide program and production system was significant, therefore yield was analyzed separately by tillage system.

            In 2020 the main effect of production system on storage root yield was significant, however, the main effect of herbicide program was not. The conventional tillage system had greater total (20%), canner (51%), and cull (68%) yield, than the reduced-tillage system (Table 8).

            In 2019 in the conventional tillage system, with the exception of bicyclopyrone followed by clomazone and S-metolachlor all herbicide programs had similar total, no.1 and jumbo yield to the weed-free check (Table 9). In 2019 in the reduced-tillage system only flumioxazin followed by clomazone and S-metolachlor with linuron had similar total yield to the weed-free check. Only flumioxazin followed by S-metolachlor and programs with linuron had a greater no.1 yield than the weedy check. No herbicide programs had greater jumbo yield than the weedy check.

The research funded by this grant answered several important questions and raised many others. These studies have shown that sweetpotato can be grown in a reduced-tillage system and that both upright and sprawling cultivars can achieve high yields in this new system. Though canopy growth varied between systems, root characteristics did not. Controlling weeds remains the greatest challenge experienced in reduced-tillage sweetpotato. An array of aggressive herbicide programs were tested and weed control was variable regardless of program.

Participation Summary

Educational & Outreach Activities

1 Curricula, factsheets or educational tools
2 Journal articles
1 Tours
4 Webinars / talks / presentations
1 Workshop field days

Participation Summary

300 Farmers
200 Ag professionals participated
Education/outreach description:

Professional outreach:

Presentations

National Sweetpotato Collaborators Group: February 2020

Southern Weed Science Society: January 2021

Northeastern Plant, Pest and Soil Conference: January 2021

Refereed journal publications

Evaluation of a reduced-tillage system in sweetpotato (expected 2021)

Herbicide programs in conventional and reduced-tillage sweetpotato (expected 2021)

Grower outreach:

Presentation

Organic Commodities Field Day: July 2019

Regional Sweetpotato Extension Meeting: February 2020

Extension publication

Weed management in reduced-tillage sweetpotato

Project Outcomes

1 Grant received that built upon this project
1 New working collaboration
Project outcomes:

The research funded by this grant answered several important questions and raised many others. These studies have shown that sweetpotato can be grown in a reduced-tillage system and that both upright and sprawling cultivars can achieve high yields in this new system. Though canopy growth varied between systems, root characteristics did not. Controlling weeds remains the greatest challenge experienced in reduced-tillage sweetpotato. An array of aggressive herbicide programs were tested and weed control was variable regardless of program.

Knowledge Gained:

Agricultural systems are complex and paradigm shifts require extensive research and outreach activities. This research project has not resulted in a new production system for growers. However, it has laid a solid foundation for future research aimed at developing more sustainable sweetpotato production systems. This project has trained me to think outside of the box when addressing issues. Increasing agricultural sustainability requires asking creative questions that illuminate the path towards creative solutions.

 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.