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

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.