Building resilience to extreme rainfall events: cover crop-based reduced tillage strategies for diversified organic vegetable farms

Objective 1: Facilitate a farmer-led community of practice focused on resilience to extreme weather events in small scale diversified vegetable production

See "Project Activities" section for outcomes of this objective.

Objective 2: conduct collaborative on-farm research into cover crop-based  reduced tillage strategies that integrate perennial and annual cover crops into diversified vegetable systems

In 2019-2020, we contracted with a machine shop in La Farge, Wisconsin (Brush Hollow Fabrication) to construct a 3-foot wide roller crimper compatible with a Grillo or BCS walk-behind tractor, using plans provided by Dr. Ted Kornecki, USDA-ARS Soil Dynamic Lab. This roller-crimper has been used in our on-farm and research station trials of winter rye termination. We prepared a 2 page bulletin describing initial tests of the crimper for distribution at conferences in early 2021: Small scale roller crimper.

On-farm trial establishment 2019-2020

On-farm trial plots were established in fall 2019 at Two Onion Farm (TOF), Parisi Family Farm (PFF) and High Meadows Farm (HMF). Perennial living aisles were planted with a perennial ryegrass (30 lb/A) and white clover (19 lb/A) mix, and production beds were planted with a winter rye (180 lb/A) and hairy vetch (40 lb/A) mix. At all farms, production beds were 36" wide. At TOF and HMF, living aisles were 48" wide while at PFF, aisles were 24" wide, to accommodate the scale of the available mowing equipment. In spring and summer 2020, UW-Madison restrictions on travel for research due to COVID-19 prevented researchers from carrying out on-farm trials of cover crop termination, living aisle management, and vegetable production. Based on photographs and comments provided by farmers, stand establishment was fair to good in living aisles, and good in production beds. In fall 2020, farmers mowed the trial plots ahead of rye seeding.

Cover crop establishment, termination, and vegetable transplanting 

Winter rye was hand-broadcast into production beds each fall ahead of vegetable trials. Rye was seeded at a rate of 180 lb/A across a 3 foot width for reduced tillage treatments and across a 5 foot width for the conventional tillage treatment and worked in using a Grillo walk-behind tractor and tiller attachment. 

Four separate treatments were implemented to terminate the rye cover crop in production beds. For the conventional tillage treatment, the rye cover was mowed before tillage of a 5-foot swath with a tractor-drawn rototiller (TOF and HMF) or with a Grillo walk-behind tractor and rototiller (PFF). For the strip tillage treatment, the rye cover was mowed before tillage of a 3-foot swath with a Grillo walk-behind tractor and rototiller. Tillage was repeated at least twice before planting. For the occultation treatment, the rye cover was rolled down using a large barrel (TOF and PFF) or using a brush mower with the blade disengaged (HMF) and silage plastic was secured across the 3-foot wide planting bed using soil staples. Silage plastic was in place for three weeks before planting peppers and winter squash, and for six weeks before planting cabbage.  For the roller crimper treatment, the rye cover was terminated using the roller crimper implement described above, one-two weeks past anthesis when the rye was at the milk to soft dough stage. Dates of field operations are shown in Table One.

Table One: Dates of field operations for on-farm trials of reduced tillage

Biomass samples were collected from each plot immediately before cover crop termination. One sample was collected per plot by cutting all plant material within a 1-by-1 foot quadrat at ground level. Plant material was separated into rye and non-rye categories before drying and weighing. Since a number of plots at all farms had less rye biomass than the target of 10000 lb/A (Figure 1), plots for occultation and roller crimper treatments, for which treatment success depends on sufficient biomass, were supplemented in mid-June 2021 with additional rye straw to meet the 10000 lb/A target. The rye stand at PFF was especially poor, potentially due to low soil fertility. 

Vegetable transplants for peppers (Carmen), winter squash (Bush Delicata), and cabbage (Storage #4) were purchased from a local organic farm and planted by hand as single rows in production beds with 18 inch spacing between plants. Transplants were watered by hand immediately after planting and irrigated by drip irrigation as needed through the season (by farmers). One pint of composted dairy manure was added to the planting hole for each transplant and pelleted composted poultry manure and feathermeal were applied to provide adequate nutrition for each crop as suggested by soil tests at each site (Table Two). In response to lower vegetable yields from silage plastic-occulted and roller-crimped plots in 2021, plots were supplemented with feathermeal in spring 2022 in an attempt to compensate for nitrogen tied up in rye cover crop residue. Feathermeal additions were calculated based on cover crop biomass measures from these plots and cover crop nutrient removal, estimated for rye straw at 8.6 lb per ton (Fertilizer Recommendations for Field Crops in Michigan, Christenson, et al. 1992). 

Table Two: Nitrogen, phosphorus, and potassium supplied by fertility amendments (pelleted composted chicken manure and feathermeal) at each farm. Customized amendments in 2022 are indicated by treatment code (CT = conventionally tilled, ST = strip tilled, SP = silage plastic-occulted, RC = roller crimped).

Across four weeks post-planting, transplants were replaced as needed, but transplant survival was reduced for some treatments (Figure 2). This was particularly notable for cabbage and winter squash in the roller crimper (RC) and occultation (SP) treatments at PFF, and to a lesser extent at TOF. Winter squash transplants in these treatments showed symptoms of damping-off in 2021 and bacterial wilt in 2022, while cabbage transplants were potentially killed by herbivore damage. 

Figure 2: Proportion of transplant survival for cabbage (Storage No. 4), winter squash (Bush Delicata) and pepper (Carmen) in cover crop termination treatments at three farms. Treatments are: conventional tillage (CT), roller crimper (RC), occultation with silage plastic (SP) and strip tillage (ST). Vegetables were not planted at PFF in 2022.

For the first four weeks after planting, plots were weeded using wheel- and hand-hoes for the tillage treatments, and by hand for the occultation and roller crimper treatments. Labor data for cover crop termination and vegetable crop production was collected at each farm (Table Three). 

Table Three: Labor requirements for cover crop termination and planting of vegetable crops at three farms in 2021 and 2022. Time period is between late May and mid-June, and labor is expressed as minutes to manage a 100-foot length of a 3-foot wide production bed. Total time is expressed in hours. Cover crop termination methods are CT = conventionally tilled, ST = strip tilled, RC = roller crimped, SP = silage plastic occulted. 

Cover crop termination labor requirements were relatively similar for conventional and strip tillage treatments and roller-crimping, but laying silage plastic for occultation was time-consuming. Time required in 2022 is more indicative of actual time requirements since our methods were more efficient after the first year; however, a mechanized plastic layer system could reduce the time requirement further. Rye cover crop termination included mowing (for tilled treatments), rolling and tarping (for silage plastic-occulted treatments) or roller-crimping. Bed preparation included tillage in conventionally tilled and strip-tilled plots and removal of silage plastic for the occultation treatment. In 2021, intensive hand-weeding was required in silage plastic-occulted and roller-crimped plots before planting, due to the relatively poor rye stand, and weeding through the rye debris was slow and inefficient. In 2022, hand-weeding requirements were considerably lower in the roller crimper treatments but still high in the silage plastic-occulted treatment. Planting into reduced tillage beds was slower than into tillage treatments, especially in 2021 as we refined our methods. Planting holes were initially made using a drill and soil auger, but when the drill failed towards the end of 2021 planting at HMF, we found that a hori hori or soil knife was equally effective, and we used this method exclusively in 2022. In 2021, weed barriers were added to the roller-crimped plots due to inadequate suppression of perennial weeds by sparse stands of rye cover crops: landscape fabric strips at HMF, silage plastic strips at TOF, and rye straw at PFF. This practice was repeated in 2022 at HMF and TOF to maintain comparability across years, while vegetables were not produced at PFF in 2022.

Overall, labor requirements to establish the silage plastic-occulted and roller-crimper treatments were higher than for tillage treatments, largely due to the labor requirements of pre-plant weed management in addition to the learning curve for efficient installation of silage plastic. However, the change between 2021 and 2022 in labor requirements for the no-till treatments is notable, and likely due to the intensive hand weed management in 2021 and suppression of weed growth in roller-crimped plots by addition of a fabric or plastic weed barrier as well as greater field crew efficiency for silage plastic installation. Additionally, a stronger rye stand was obtained at TOF for reduced tillage plots (which had an extra week of growth compared to other treatments) in spring 2022 and this would be expected to contribute to weed suppression.

Weed Management

At all farms in 2021, weed populations were highest in conventionally-tilled or strip-tilled plots (CT and ST) in the week after planting (Figure 3); however these weeds were relatively easy to control using wheel or hand hoes (Table Three). While weed numbers were comparatively low in plots for which the rye cover crop was terminated by silage plastic occultation (SP) or roller crimping (RC), weeding was more time consuming in these plots due to the need to hand pull weeds through the dense mat of rye residue. As described above, roller crimped (RC) treatments were supplemented with additional rye straw (PFF, 2021), landscape fabric strips (HMF, both years) or silage plastic strips (TOF, both years) due to rye biomass being lower than ideal for weed suppression and due to the high population of perennial weeds at all farms. Weed counts were not recorded for roller-crimped plots at TOF and HMF after rescue treatments were implemented since there was no longer a sufficient area to use the 1-foot quadrat, and landscape fabric and silage plastic covers effectively prevented weed emergence. While weed control in silage plastic-occulted plots (SP) was comparable to other treatments at the end of the 2021 season, weed populations in this treatment were slightly higher through much of the 2022 season at both HMF and TOF, largely due to spread of thistle and quack grass, both perennial weeds difficult to control in reduced tillage systems.

Table Four: Labor requirements for weed management of vegetable crops at three farms in 2021 and 2022, separated by crop and cover crop termination method. Labor is expressed as minutes to manage a 100-foot length of a 3-foot wide production bed. Total weeding time is expressed in hours. Cover crop termination methods are CT = conventionally tilled, ST = strip tilled, RC = roller crimped, SP = silage plastic occulted. 

Early weed management in tillage treatments used the walk-behind tractor and rototiller or wheel hoe beside plants, and hand hoes between plants. For the silage plastic-occulted and roller-crimper treatments, hand weeding was necessary and time-consuming (Table Four).  In 2021, high weed pressure in the roller-crimped plots was quickly apparent, partially due to the poor rye cover crop stand but also due to the high incidence of perennial weeds, including thistle and quack grass, at all three farms. While these weeds were most likely already present in fields in 2019, the pandemic-enforced lack of management beyond mowing in 2020 probably contributed to their spread. We trialed different methods of “rescuing” the roller-crimper treatment at each farm. At TOF, silage plastic was used to cover the sides of the roller-crimped plots, leaving a narrow strip of 4-6 inches down the center of the bed. A similar approach was used at HMF, with landscape fabric instead of silage plastic. At both locations, the physical weed barrier minimized weeding needed to the narrow planting strip. At PFF, roller-crimped plots were supplemented with rye straw at a rate of 10000 lb/A, but this did not effectively suppress the large population of thistle and quack grass which required extensive weeding. Silage plastic-occulted plots showed initial suppression of weeds, but after 3-4 weeks, weed pressure increased in these plots. In 2022, increasing populations of perennial weeds were noticeable in silage plastic-occulted plots at both HMF and TOF leading to increased time spent weeding.   

Harvest

For all crops, produce weight and number was recorded for marketable and unmarketable categories, and the top three causes of unmarketability were recorded. The total marketable yield by weight per plant for the 2021 season is shown in Figure 3. Peppers were harvested weekly when more than 50% red, or if the fruit was evidently not marketable (eg blemished, rotting). Winter squash were harvested on two dates in 2021 to capture possible differences in crop maturity between treatments, but no significant maturity differences were noted so in 2022, winter squash was harvested on a single date. Cabbage was harvested on a single date. For the final pepper harvest, full-size green fruit were harvested and yields recorded as a separate category, included in total marketable weight per plant (Figure 4). Harvest data is presented on a per-meter basis to account for variable plant survival rates. 

Several notable pest and disease issues were encountered in both years. Squash borer and squash bugs caused extensive damage to squash plants at all farms, and while pest numbers were not evaluated, squash bug numbers seemed higher in plots with more rye residue. At HMF, insect damage to green peppers was widespread and may have led to extensive rotting of ripening produce. In 2022, we were able to identify common stalk borer as a causal pest for this damage. For two dates in 2021 on which insect damage and rot was most significant in peppers at HMF, weights and numbers of unmarketable peppers with insect damage and rot were compared to determine if any cover crop treatments were more susceptible, but no significant differences were seen. At the same farm, cabbage loopers damaged cabbage wrapper leaves. This damage was not considered in categorizing cabbages as marketable.

Performance of reduced tillage systems based on yield varied across farms and crops, but in general plants in conventional tillage systems were more productive. Pepper yields for strip-tilled plots at TOF in 2021 and at HMF in 2022 approached yields for conventionally-tilled plots. Similarly, winter squash yields in strip-tilled plots at both farms in 2022, and for cabbage at TOF in 2021 and both farms in 2022 approached yields for conventionally-tilled plots. 

Soil Function Measures

Plots with high rye residue – roller-crimped and silage-tarped treatments – had generally lower soil temperatures and higher moisture levels at all locations than plots that were strip-tilled or conventionally tilled. Lower soil temperatures may have contributed to apparent nutrient deficiencies in plants in plots with high rye residue levels – plants were smaller and more likely to show chlorosis (yellowing) than plants in conventionally tilled or strip-tilled plots, although tissue testing gave inconclusive results with no clear differences between treatments.

Two physical measures of a healthy soil were measured in treatment plots at HMF and TOF in mid-October 2022, after two years of tillage or reduced tillage treatments.

Soil aggregate stability was measured on aggregates collected from the top two inches of soil in each plot. Samples were air dried for 48 hours, and 9 aggregates, each 6-8 mm in diameter, were tested for each sample using the soil stability test kit available at https://www.forestry-suppliers.com/p/073741. Aggregates were placed on sieves, immersed in distilled water, and put through two dipping cycles (raised and lowered 5 times) at 5 and 10 minutes, after which they were assigned to stability classes based on the kit protocol. Aggregate stability classes ranged from 3 (less than 10% remaining on sieve after two dipping cycles) to 6 (75-100% remaining on sieve after two dipping cycles). For both farms, soil aggregate stability was significantly lower in conventionally-tilled plots compared to roller-crimped and silage-plastic treatments, with strip-tilled plots showing intermediate soil aggregate stability (Figure 5). In strip-tilled plots, samples were taken from the tilled strip, so it is likely that the shallower tillage depth of the walk-behind tractor and rototiller compared to the full-size tractor-drawn rototiller had a reduced impact on soil aggregates.

Water infiltration into plots was assayed using a single ring approach, as follows: six-inch diameter metal pipes 6 inches in length were pounded into soil to a depth of 3 inches after removal of surface debris and plant material. A piece of plastic wrap was placed over the pipe, and 442 ml of distilled water (equivalent to one inch of water) was poured into the wrap. The wrap was removed and the time taken for water to soak into the soil was recorded. This process was repeated with a second inch of water. At TOF, soil was relatively dry on the day of the assay, and as expected there was no significant difference in infiltration time for the first inch of water infiltration. However the second inch of water infiltrated significantly more rapidly for the plots that received reduced tillage treatments, roller-crimped (RC) and silage-plastic (SP), compared to strip-tilled plots (ST), with conventionally-tilled plots (CT) showing intermediate water infiltration (Figure 6).  At HMF, heavy rains in the days before the assay was performed meant that water infiltration was slow, and no significant differences were seen.