Use of High-Residue, Winter-Killed Cover Crops in No-Till Organic Tomatoes

Use of High-Residue, Winter-Killed Cover Crops in No-Till Organic Tomatoes

Summary

One common criticism of organic agriculture is that it relies too heavily on tillage.  While there has been some success with organic no-till row crops, organic no-till vegetables are still a conundrum.  Inadequate weed control, narrow cover crop termination windows, and planting delays related to termination are all challenges.  This project is exploring the use of season-long managed fallow followed by high-residue, winter-killed cover crops to create a weed-free mulch that does not need exact timing or special equipment for termination. 

In 2016 we used two rounds of cover crops to smother weeds in the research plots on three cooperating organic farms in south central Wisconsin.  Those plots were then planted to three treatments and a control all dominated by sorghum sudangrass.  In the fall just before the first predicted frost, all the treatments were rolled down with a disengaged rototiller in order to align the residue and get an even mulch layer in advance of planting tomatoes in 2017.  Population counts and biomass for both weeds and cover crops were recorded at appropriate intervals.

Year one results indicate that we were able to reach our goal of  producing at least 10,000 lbs. of cover crop dry matter per acre in all but 2 of the 12 total plots .Also as predicted, Sorghum sudangrass did have a significant smothering effect on weeds.  However, the sorghum sudangrass also smothered out both the sunn hemp and the cow peas that were planted as nitrogen fixing cover crops in two of the treatments.

Objectives/Performance Targets

Trials were established on three cooperating farms in southern WI.  All the farms are certified organic and have between 8 and 12 years of experience with vegetables and cover crops.  Prior to establishing the cover crop treatment plots, we used two successions of cover crops in an attempt to draw down the weed seedbank in the trial areas.  A mix of oats and chickling vetch was seeded on 4/26/16.  Seeding rate for oats was 80 lbs./acre.  Seeding rate for chickling vetch was 50 lbs./acre. On 6/20/16 the oat/chickling vetch mixed was mowed; buckwheat was planted into the stubble at a rate of 80 lbs./acre; and the plots were tilled to kill the oat/chickling vetch mix and work in the buckwheat seed.

On 7/13/16 the treatment plots were seeded into the standing buckwheat which had only just started to flower.  The treatments are 1) sorghum sudangrass 2) sorghum sudangrass and sunn hemp 3) sorghum sudangrass and cow peas.  The control was seeded to sorghum sudangrass.  Sorghum sudangrass was seeded at 80 lbs./acre in all plots.  Cow peas and sunn hemp were also seeded at 80 lbs./acre in their respective plots.  The plots were tilled immediately after seeding to terminate the buckwheat and work in the new cover crop seeds.  On two farms, the cover crop population counts taken two weeks after seeding suggested that we would not be able to produce enough biomass to suppress weeds in the following cash crop production year.  We reseeded the treatment plots on 7/27/16 at both of those farms.

In early October, just before the first predicted frost, all the treatments were rolled down with a disengaged rotovator in order to align the residue and get even coverage in advance of planting tomatoes into the mulch in 2017.

Population counts and biomass for weeds and all cover crops were recorded just before termination of the oat/chickling vetch mix and the buckwheat.  Population counts for weeds and cover crops were taken within two weeks of germination in the final treatment plots and again just prior to rolling.

Accomplishments/Milestones

Weed Control

Weed population counts taken just before rolling the final treatment plots show that weeds were well controlled.  The average weed count on all farms over all treatments and reps two weeks after germination of the final treatment plots was 11.6 weeds/square foot.  Just before rolling, the average weed count was 2 weeds/square foot (range of 0 to 13), the indicating that the sorghum sudangrass did act as an effective smother crop.

Unfortunately, the sorghum sudangrass also tended to smother out both the sunn hemp and the cow peas that were used in treatments 2 and 3.  The average sunn hemp population count on all farms two weeks after germination was 10.3; for cow peas the number was 6.4.  Just before rolling, the average sunn hemp population count on all farms was 2.6; for cow peas the number was 1.6.

Cover Crop Biomass Accumulation

Based on previous work on organic no-till grain crops, we know that a thick mulch layer is key to suppressing weeds in the cash crop production season.  Based on that previous work, our goal was to produce enough biomass to have 10,000 lbs. of cover crop dry matter per acre in place during the cash crop production season, which meant we would need to have enough biomass at the end of this season to allow for some decomposition over winter.  The average dry matter per acre across all farms and treatments was 13,165 lbs.  All plots on all farms produced more than 10,000 lbs. dry matter per acre with the exception of two plots producing just 7,577 lbs. and 9,088 lbs.  The high end of the range was 19,693 lbs dry matter per acre.

Several weeks before rolling the plots in October, a major wind storm knocked over areas of the cover crop on all three farms.  We did our best to roll the covers in the direction that most of the stems landed, but of course the wind was not as precise a method as the rotovator.  As a result there are some pockets in the mulch layer that do not have as good of coverage as we could have achieved without the interference of the wind. 

2016 Photo of Cover Crop Rolling

Impacts and Contributions/Outcomes

Desired Impacts

• Increased grower awareness about the benefits and challenges of reduced-tillage vegetable systems
• Increased grower knowledge about how to incorporate winter-killed cover crops into vegetable systems
• Increased grower experimentation with and adaptation of reduced tillage and alternatives to plastic mulch on their farms

Impacts to Date

Year one was devoted to reducing weeds in the trial areas and establishing the cover crop treatments in preparation for vegetable crop planting in year two.  As such, we have not yet done any outreach to growers beyond those involved in the project.  Observations from cooperating farmers to date include:

• The chickling vetch did not germinate well and thus did not do much in the mix with oats.
• The sunn hemp attracted Japanese beetles, which makes it less desirable as a cover crop on vegetable farms.
• Rolling the sorghum sudangrass with the disengaged tiller in the fall was quick, easy, and seemed to do a good job of creating a uniform mat where the wind storm had not already knocked over the crop

Next Steps

The trial will continue in 2017.  Tomatoes will be punch planted into the winter-killed cover crop mulch treatments and into a black plastic control.  There will be a field day and presentation of preliminary results in September.  After all data analysis, we will produce an illustrated information sheet and short video with recommendations for using winter-killed cover crops in reduced tillage organic vegetable systems.

Collaborators: