Improving Soil Health Through Cover Crop Based No-Till Organic Vegetable Production

Improving Soil Health Through Cover Crop Based No-Till Organic Vegetable Production

Summary

Cover crop-based reduced tillage (CCBRT) techniques have demonstrated positive impacts in organic row crop systems, contributing to the conservation and improvement of soil resources and the facilitation of weed management of these systems.  This technique, which uses cover crop residues as a mulch to suppress weeds, has shown more variable success in organic vegetable production systems, with data lacking to demonstrate its potential adaptation for small-scale operations.  This experiment was designed to evaluate the adaptability of CCBRT for small-scale organic vegetable production in the upper Midwestern United States, specifically evaluating weed suppression, labor inputs, and yields of vegetable crops.  Cereal rye (Secale cereale), winter wheat (Triticum aestivum), and hairy vetch (Vicia villosa) were fall-sown in 2012 and 2013 in a strip-plot design, including a control treatment with no cover crop. During the following spring, cover crop plots were strip-tilled in mid-April to establish a planting zone, with cover crops terminated at anthesis with a sickle-bar mower in late May. Bell peppers (Capsicum annuum var. ‘Revolution’), snap beans (Phaseolus vulgaris var. ‘Tavera’), and potatoes (Solanum tuberosum var. ‘Red La Soda’) were hand-planted either as transplants or seed in each treatment immediately following cover crop termination.  During each summer growing season, weeds were removed by hand approximately every 10 to 14 days as needed for management, with management times recorded for each treatment.  Vegetable crop yields and quality were measured at harvest.  Cereal rye produced greater biomass at the time of termination as compared to other cover crop treatments.  Greater numbers of weeds were counted in the wheat treatment as compared to the cereal rye, increasing the in-season labor required for weed management.  Bean yields were decreased in the all CCBRT treatments compared to control treatments in both years of the study.  Pepper yields did not differ in CCBRT treatments as compared to the control in both 2012 and 2013, although the CCBRT treatments did yield a lower harvestable weight of peppers than the straw mulch plots.  Potato tuber yields were not different in the CCBRT treatments as compared to the control in 2012, but were lower in 2013.  This data indicates that, if the CCBRT system is to have potential application for the production of vegetable crops in small-scale vegetable production, further optimization of the system must be achieved to ensure consistent and adequate weed suppression while maintaining crop yield and quality.

Objectives/Performance Targets

The objectives of this study were: 1) to compare the effectiveness of terminated cover crop mulch treatments in weed suppression throughout the cash-crop production season; 2) to assess differences in the labor efficiency of weed management using cover crop treatments as compared to typical small-scale organic vegetable production systems; and 3) to determine the impacts of the cover crop-based reduced tillage system on vegetable yields and quality.  Distinct outputs resulting from this project included a peer reviewed journal article, farmer-focused fact sheets, archived webinars, and presentations at farmer conferences and field days.  Further, the results of the research were leveraged to obtain larger federal funds through the USDA-Organic Research and Education Initiative program. 

Accomplishments/Milestones

The objective of this study was to evaluate the effectiveness of CCBRT techniques in organic vegetable crop production on small-scale farms with a low degree of mechanization, using different species of fall-sown cover crops.  Using values obtained from similar experiments conducted in row crop systems using the roller-crimper method of termination, adequate above ground biomass was produced by the cereal grain cover crops prior to termination to suggest that adequate weed suppression would be achieved.  However, in our trials, while the mowed cover crop mulch did suppress weeds early in the vegetable production season during both years of the study, weeds were established in all treatments by 6.5 WAP.  Due to termination being achieved by mowing the cover crop, as opposed to termination with rolling-crimping in the aforementioned studies, cover crop residues were unevenly distributed on the soil surface.  This led to uneven mulch cover on the soil surface, allowing weeds to establish where the soil remained bare.  These break-through weeds were continually managed throughout the vegetable production season in all treatments, in order to most accurately replicate real-world management strategies.  As the hand-weeding crews needed to work around the cover crop residue in their efforts, despite overall lower or equivalent weed pressure in the cover crop plots as compared to the control plots, the time required for weed management in the cover crop plots exceeded that of the control plots.

 Crop yields using the CCBRT technique varied by both year and by crop.  Overall, pepper plants grown with CCBRT using rye as the cover crop yielded comparably to those grown in the control plots in both years.  Conversely, snap bean yields were lower with CCBRT over both years of the study.  Potato tuber yields varied, with plants from the CCBRT plots yielding similarly to those in the control plots in 2013, but yielding less in 2014.  This variability highlights the need to test the suitability of specific cash crops for use in this system, as certain crops appear to be better adapted to this management strategy.

Further modifications to this system could lead to more consistent results from CCBRT in small-scale organic vegetable systems in the upper Midwestern region of the U.S.  With the addition of supplemental in-season fertility and drip irrigation systems, yields from CCRNT systems more closely match those resulting from typical organic management practices.  However, to reduce production risks this system and to increase the overall benefits with respect to reduction in labor needs and in-field weed populations, cover crop management must be further optimized to ensure consistent and effective weed management.  Many CCBRT agronomic systems use the roller-crimper for termination of the cover crop.  This tool more effectively places the cover crop on the ground to minimize gaps in the mulch layer that can lead to weed germination and establishment. More recently, a smaller version of this tool has been adapted to vegetable production systems, including models that allow for crimping on the tops of raised beds and that can be mounted on a hand tractor, such as the BCS.  Recent research has shown success using the roller-crimper tool in vegetable production systems (Delate et al., 2012; Ciaccia et al., 2015). While the use of the roller crimper in vegetable crops certainly holds potential advantages, management nuances of the whole-system, including transplanting the cash crop and application of supplemental fertility and irrigation need to be considered in tandem with cover crop management. Further research in upper Midwestern US organic vegetable systems, integrating rolled-crimped terminated cover crop and combination of strip-tillage and mechanical transplanting could contribute to the establishment of consistent and reliable CCBRT systems for organic farmers looking to derive production and environmental benefits.

Impacts and Contributions/Outcomes

Below describes the specific outcomes for each aspect of the experiment, helping define best management practices for no-till organic vegetable systems:

Weather

The winter of 2012-2013 experienced an average daily temperature (8.2°C) that ranged 2.4°C degrees colder than the annual average since 1981, with greater than average winter snowfall.   Precipitation in the early production season (April 1 through June 30) of 2013 was greater than average, with 257 mm more precipitation than the recorded average of 325 mm (1981-2012). From July 1 through September 30, 2013, temperatures remained close to the thirty-year average (21°C) with less than average rainfall (164 mm as opposed to 293 mm thirty year average).

           Winter average precipitation from 2013-2014 was slightly less than normal, accompanying a particularly cold winter about 3.8°C degrees below the seasonal average through December, January, and February. March and April temperatures were colder than average, with near average spring precipitation.  June precipitation was in 91 mm in excess of the 30 year average of 124 mm. Average July through September temperatures remained approximately at the 30 year average (18.8°C), with slightly less than average precipitation of 251 mm (Table 2).

Cover crop biomass

Significant year x cover variety interactions were found for many of the variables measured; therefore, results, including those for cover crop biomass, are presented by year (Table 3).  Cover crop above ground biomass production fell within the optimal ranges reported for successful outcomes using the CCRBT technique for both years of the study (Ashford and Reeves, 2003; Mischler et al., 2010). No significant differences in above ground biomass of the cereal grains were found either year (Table 3).  Due to winterkill, however, the hairy vetch variety ‘Purple Prosperity’ produced essentially no biomass during both years of the study.

Weed densities and management times

A primary objective of this experiment was to compare the weed suppression of the different fall-planted cover crops in vegetable production systems and related impacts on time required for weed management. Redroot pigweed (Amaranthus retroflexus), common lambsquarters (Chenopodium album.), common dandelion (Taraxacum officinale), ladysthumb smartweed (Polygonum persicaria), hairy vetch (Vicia villosa), shepherdspurse (Capsella bursa-pastoris), alfalfa (Medicago sativa), prostrate knotweed (Polygonum aviculare), and oxalis (Oxalis stricta) were the most numerous weedy broadleaf species, and large crabgrass (Digitaria sanguinalis), oat (Avena sativa), green foxtail (Setaria viridis) and yellow foxtail (Setaria pumila) were the most numerous grassy weed species identified in each plot during both years of the study.

A year x treatment effect and year x variety interaction were found in the data related to weed suppression; therefore, results are presented by year.  Weed suppression provided by the cover crop mulch throughout the vegetable production season showed differences by year and by cover crop variety (Table 4).  In 2013, less weed biomass was collected from the rye, rye/vetch, and straw plots versus the bare-ground control or wheat plots at 4 weeks after planting (WAP). However, during that same year, no significant differences were observed in weed pressure across treatments at 6.5 WAP.  In 2014, less weed biomass was collected from the straw mulched plots and rye plots as compared to the control treatments at 4 WAP.  These trends did not continue at 6.5 WAP planting, however; although the rye treatment did suppress weeds to an equivalent level as the control, the straw treatment, despite the presence of volunteer oat, demonstrated overall less weed pressure than other treatments (Table 4).

The following year, treatments exhibited a similar pattern in weed suppression, with the control treatment exhibiting high weed density and short management time, and wheat exhibiting high weed density and long management time. The rye treatments, both alone and planted in combination with hairy vetch, exhibited the lowest weed dry biomass early in the cash crop production season. Two weeks after cover crop termination, the control plot had the highest weed biomass while the rye, mixed rye and straw treatment had less. The data for the straw mulch plots reflected the contamination of the oat straw mulch with viable oat seed, increasing weed density at 6.5 WAP and overall management time as compared to the first year.  As in 2013, among the cover crop treatments, the rye treatment exhibited the greatest weed suppression.  No trends could be derived as to the proportion of grass versus broadleaf weeds present in the treatments either year (Table 5).

Despite early-season weed suppression benefits of the cover crop mulches, the hand-weeding time required for weed management was not decreased in the CCBRT treatments (Table 6).  In 2013, at 4 WAP, time required for weed management in the control plots (as calculated as hours person^-1 ha^-1) was equivalent to the rye, rye/vetch mix, and straw treatments, and equivalent to all CCBRT plots at 6.5 WAP.  In 2014, time required for weed management in the control plots was statistically equivalent to all CCBRT and straw mulch plots at 4 WAP, and statistically lower than the weed management time required in the wheat and rye/vetch mix plots at 6.5 WAP.

Vegetable yield and quality

Peppers

            As compared to all treatments, pepper plants grown in the straw mulch yielded the greatest amount of fruit in 2013 (26.3 Mg ha ^-1), although equivalent to yields of peppers from the control plots (21.7 Mg ha ^-1), rye plots (19.1 Mg ha ^-1), and rye/vetch mix plots (19.2 Mg ha ^-1) (Table 7). Yields from pepper plants grown in the wheat cover crop treatment were lower that the straw mulch plot (12.2 Mg ha^-1), but equivalent to yields of the rye, rye/vetch, and control treatments. In 2014, pepper plants from both rye and mixed rye plots (26.5 Mg ha^-1and 24.4 Mg ha^-1) yielded equivalent to plants grown on the control plots (30.1 Mg ha^-1).  Pepper plants grown in the wheat cover crop yielded less than the control but equivalent to the rye and rye/vetch mix treatments (21.1 Mg ha^-1).  Pepper plants grown in the straw mulch plots yielded less fruit than other treatments (13.4 Mg ha^-1), potentially due to competition from volunteer oat plants.

            In 2013, fewer unmarketable peppers were harvested from the straw mulch treatment (19.2 percent) as compared to the rye (32.1 percent), rye/vetch mix (32.2 percent), wheat (28.9 percent), and control (30.9 percent) treatments. No differences were observed in the percent of unmarketable peppers between treatments in 2014.  The primary reason for culling was disease and physiological disorders, with 86% percent of culls from 2013 and 65 percent from 2014 showing symptoms of blossom end rot, bacterial soft rot, or fungal disease.  Sunscald damage resulted in 5.1 percent of fruit culled in 2013 and 10.4 percent of fruit culled in 2014, with peppers from rye plots exhibiting less sunscald in 2013.  No significant differences were observed in numbers of peppers culled for insect damage from each treatment (data not shown).

Considering the quality of the marketable peppers, pepper fruit harvested from the rye plots tended to have larger, thicker walled fruit in both years. In 2014, the fruit harvested from the mixed rye plots were significantly greater in diameter (97 mm) than the control, straw, or wheat plots, with the straw plots averaging 79 mm. In 2013, fruit harvested from the mixed rye plot also had the largest average fruit diameter (78 mm), but was only significant different to those harvested from the wheat plot (71 mm). The harvest of marketable fruits from the rye and mixed rye plots also had the thickest wall width in both years (6.9 mm). Treatments did not affect other pepper quality characteristics such as shape, symmetry, the presence of four distinct lobes and a blocky, square shape.

Snap beans

In 2013, plants grown in the bare ground control plots yielded more beans (7.7 Mg ha^-1) than other treatments (2.7 – 4.2 Mg ha^-1) (Table 8). In 2014, bean yields from plants grown in rye cover crop (6.7 Mg ha^-1) yielded equivalent to the control plot (8.2 Mg ha^-1). As with the pepper yields, in 2014, the yields from plants grown in the straw treatment (3.3 Mg ha^-1) were lower than yields from all other treatments except wheat due to competition from volunteer oat.

In 2013, no differences were observed in the number of unmarketable beans from each treatment.  In 2014, a significantly greater number of unmarketable beans were harvested from the straw mulch plots.  Common reasons for culling were associated with size (5.6 percent in 2013, 7.5 percent in 2014), withering (7.8 percent in 2013, 1.6 percent in 2014), and insect and disease (10.1 percent in 2013, 3.7 percent in 2014) damage. Variation between treatments was observed in the average length of harvested beans, with plants grown on the bare ground control treatment producing overall longer beans than those from the wheat treatment both years.

Potato

Harvested yields of potato tubers from all treatments were not different in 2013, although yields from wheat plots tended to be lower at 15.0 Mg ha^-1 as opposed to 21.0 – 22.0 Mg ha^-1 for all other treatments (Table 9). In 2014, greater yields were obtained from the bare ground control plots (28.0 Mg ^-1ha), with all other treatments yielding similarly (19.0 – 23.0 Mg ha^-1). Plants grown in the rye and straw mulch treatments yielded a higher proportion of size Grade A tubers (over 30 percent weighing more than 113 g) as compared to the wheat treatment with only 16.2 percent of tubers produced in this size class. In 2014, potato plants from the straw mulch treatment yielded the highest percentage Grade A tubers, although equivalent to all other treatments except the rye/vetch mix.

In 2013, no differences were found in percent of unmarketable potatoes between treatments. Primary reasons for culling potatoes included: size of less than 28.3 g; greening on the potato surface; and pest damage, primarily from rodents. A lower percentage of unmarketable tubers were observed in the straw mulch treatment in 2014 (24.1 percent) as compared to the rye/vetch mix (39.5 percent).  Differences in amount of pest damage on tubers were found between treatments in both years. In 2013, control plots yielded tubers with less rodent damage (1.9 percent), as opposed to the mixed rye treatment which had 6.8 percent of potatoes affected. In 2014, 3.8 percent of tubers from control plots were affected by rodent damage, and 19.5 percent of potatoes in the rye and 14.8 percent in the mixed rye culled for this reason.

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