Evaluation of Cover Crops and Conservation Tillage for Conventional and Organic Sweetpotato (Ipomoea batatas) Production in North Carolina
A three-year study on the effect of management of a cover crop mixture of hairy vetch and ‘Wrens Abruzzi’ rye in organic sweetpotato was compared to a conventional check using a systems approach. The organic systems include 1) no cover crop, 2) cover crop incorporated prior to transplanting, and 3) reduced tillage. All three years, experiments were conducted at the Center for Environmental Farming Systems (CEFS), Goldsboro, NC. Cover crops were planted in November 2000, 2001, and 2003 at a rate of 32 pounds of hairy vetch and 50 pounds of rye to the acre. All three organic treatments received nine tons to the acre of compost. Compost was applied in spring 2001 and fall 2002 and 2003 based on North Carolina Department of Agriculture and Consumer Services (NCDA & CS) nutrient analysis. Prior to application, it was estimated that approximately 80 pounds of nitrogen were available to the crop each year. Ammonium nitrate was applied to the conventional treatment 28 days after sweetpotato planting to supply 50 pounds of nitrogen. Although nitrogen inputs were greater in organic treatments, sweetpotato leaf tissue nitrogen analysis at 30 and 60 days after plant were similar among treatments. Sweetpotato slips were planted in 2001 with a conventional transplanter. In years 2002 and 2004, a reduced-tillage transplanter designed by Dr. Ron Morse of Virginia Polytechnic Institute in Blacksburg, VA. This transplanter has contributed to the success of this system by minimizing disturbance of surface residue at planting. System performance was assessed by collecting data on weed density and biomass, soil moisture, temperature, and soil nutrients including inorganic nitrogen, plant growth parameters including leaf tissue analysis, sweetpotato vine biomass and sweetpotato yield by market grade and quality; and wireworm larval density throughout the season. In addition, production costs for each system have been documented, and a comparative analysis on the relative costs of each system will be calculated.
Both years, weed biomass interfered with crop biomass in the reduced tillage treatment. Monocot biomass was greater in reduced tillage than other treatments, and was reflected in a reduction in sweetpotato vine biomass at harvest in 2001 and 2002. Dicot weed biomass was greater than conventional and similar among organic treatments in 2001. In 2002, reduced tillage organic was greatest, but no differences were noted among conventional, organic cover crop incorporated, or organic no cover crop.
Between sweetpotato planting and harvest, treatments were baited for wireworm using imbibed untreated corn and wheat seed in a one to one ratio. Corn and wheat seed were installed with a soil core two inches in diameter, and removed following seed germination with a four-inch diameter core. Both cores are four inches deep. Larvae were counted and identified by species. In 2001, wireworm larval density (species pooled) was greatest in the organic cover crop incorporated treatment and lowest in the organic reduced tillage and conventional treatments. Although additional analysis is needed to accurately interpret the data, it may be possible that the increased soil moisture in the cover-incorporated treatment contributed to wireworm activity compared to the conventional and reduced tillage treatments. In 2002, low soil moisture predominated over much of the season. Few wireworms were baited, and no treatment differences in larval density were observed.
Except for a reduction in yield in the reduced tillage treatment the second year, yields of organic treatments were not different than conventional yields both years. To assess marketability, roots were examined for insect damage, and rated based on frequency and severity of insect feeding scars. In 2001, yield quality of No.1's was dramatically improved in reduced tillage (76 percent marketable) verses conventional (49 percent) treatments. In 2002, yield quality was improved over 2001, and no treatment differences occurred.
Sweetpotato is one of the most economically important crops in North Carolina. In 2002, N.C. supplied 37% of the nations' market and generated over 64 million dollars for the state. There are 37,000 acres in production in N.C., and over 98% are managed conventionally. Sweetpotato has a variety of insect, weed and plant pathogen pests that reduce quality and yield. Soil-dwelling insects such as wireworm (Melanotus communis Gyllenhal and Conoderus spp.) pose the greatest threat. Despite repeated applications, conventional insecticides are only marginally effective; as evidenced by the fact that 30-40% of harvested yield suffers extensive root damage and is not suitable for sale. These insecticidal materials are currently being reviewed under the Food Quality Protection Act because of their potential for environmental harm. Additionally, growers have few chemical options for weed control. Little research has been conducted to provide growers with innovative alternative practices that will preserve or improve environmental health while maintaining economic viability. Therefore, the objectives of this trial as follows:
To examine differences in soil dwelling insects, soil physical properties, crop growth parameters, and weed competition on crop yield in organic and conventional sweetpotato production.
To investigate the impact of conservation tillage on sweetpotato quality and yield.
Evaluate the economics of conventional verses organic sweetpotato production in terms of cost effectiveness and product return to serve as a guide for growers who are interested in producing organic sweetpotatoes.
To participate in outreach and education events for growers, buyers and extension agents.
Sweetpotatoes 'Beauregard' were transplanted July 2001 and June 2002, and harvested October 2001 and September 2002. Transplanting is planned for mid June 2004. Soil nitrogen, soil moisture, seasonal wireworm larval densities, weed densities throughout the season, weed and crop biomass at harvest, plant tissue nutrient analysis, and yield quantity and quality will be measured as in 2001 and 2002.
A laboratory trial to investigate the maximum rate of movement of M. communis in a range of bulk densities and/or soil moisture for a given substrate is planned for early summer 2004. This data may provide insight on the potential for this species to travel in a variety of environmental conditions that reflect those imparted by treatments in the main field trial. An understanding of this pest’s biology will guide future management decisions.
Impacts and Contributions/Outcomes
Due to the postponement of year three until the 2003-2004 field season, results will be presented for all three years following completion of the trial in fall 2004. Presentations at national scientific meetings including the National Sweetpotato Collaborators meeting in February 2005, Southern Region American Society for Horticultural Science, February 2005, and the American Society for Horticultural Science National Meeting, 2005 are anticipated. Presentation to growers at the Carolina Farm Stewardship Association’s Sustainable Agriculture Conference, November 2004 is also planned.