2011 21.2 Reducing Tillage With Cover Crops in Western Washington Organic Vegetable Production

Tillage in Organic Agriculture

Organic vegetable growers rely on tillage to prepare ground for planting and for weed management. In western Washington, it is not uncommon for growers to make 10 to 20 passes each year with equipment ranging from a mold board plow to a spring tooth harrow. Tillage reduces earthworm populations, fungal based food webs, organic matter, soil aggregates, and initiates germination of many weed species. Increasingly, researchers and growers are weighing the agronomic benefits of tillage against potential deleterious effects on soil structure, soil erosion, soil compaction, fuel consumption, and contribution to greenhouse gases. For these reasons, many organic growers are looking to incorporate reduced till systems onto their farms.

Conventional agriculture producers have used no- and reduced till systems in combination with herbicides for many years and have been able to show improved soil quality and reduced weed pressure. The use of manure and compost soil amendments in organic farming can also lead to increased soil organic matter (OM) and soil quality. Researchers in Maryland found organic practices, especially manure addition, increased OM to a greater degree than conventional no-tilli. However, these researchers only compared conventional no-till systems with organic tillage systems that include manure amendments, and did not examine an organic no-till system.

Organic no- and reduced-till systems have mostly included field crops such as corn, wheat and soybeans but very little has been done with vegetable crops. In response to organic vegetable growers who wish to improve soil quality through tillage reduction and reduce costly weed pressure, Washington State University agriculture faculty began an organic vegetable no-till and reduced-till systems research and extension project. A two-day symposium and planning session was held in October 2009 where barriers were identified and direction was provided for the project. Participating growers identified specific barriers to successful implementation of no or reduced-till organic systems as: 1) inability to control fall-planted cover crops without herbicides or tillage; 2) shortened growing season due to delayed vegetable crop planting; 3) inability to manage weeds without mechanical cultivation; and 4) decrease in soil tilth (i.e. loss of the short-term benefits of tillage in preparing the seed bed).

The overall goals of this project are to: 1) identify production methods that effectively integrate cover crops and reduced tillage technologies while reducing in-season weed pressure and weed seed bank populations on western WA organic farms; 2) evaluate profitability and greenhouse gas impacts of reduced tillage cropping systems on these farms; and 3) facilitate adoption of reduced tillage technologies to a wide audience. In this article we will report on recently completed field experiments and describe future work for which funding is being pursued. Field trials in 2009 and 2010 compared the effects of reducing tillage on yield of butternut squash and carving pumpkins, soil quality, and weed
pressure. A cover crop trial to select early maturing varieties of wheat and barley was completed in spring 2010 and a second study was planted fall 2010 to barley and vetch.

No-till Research Symposium

We began planning for reduced tillage research by first meeting with Dr. Paul Hepperly, formerly of the Rodale Institute, following his key note address at the Tilth Producers conference in Bellingham, WA in 2008. Rodale has pioneered reduced tillage strategies and techniques for organic agriculture that are employed around the country. Then in October 2009, we held an organic no-till symposium in Everett, WA. Seventy-five farmers and agricultural professionals joined our invited speakers: Dr. Ron Morse, Virginia Tech; Jeff Moyer, Rodale Institute, Pennsylvania; John Luna, Oregon State University; and Carmen Fernholz, A-Frame Farm, Minnesota. These speakers shared years of knowledge and experience, and reflected on how no-till organic might fit with organic vegetable production systems in western Washington. During breakout sessions participants scrutinized current tillage practices in organic agriculture and envisioned what reduced till systems might look like in the region. Cover crops, weed management, equipment, environmental considerations, pest management, economics, cash crops, and plausible rotations were all discussed in depth. While these broad research needs were recognized, the two primary research needs identified were to: 1.) compare soil quality and weed pressure with strip tillage versus no-till, and 2.) select appropriate cover crops for reduced till systems.

Tools for Organic No-till

Reducing tillage in organic systems starts with a robust fall planted cover crop stand that will suppress summer weed growth. The second critical step is to kill the cover crop without tillage or herbicides. John Brubaker (farmer, Kutztown, PA) and Jeff Moyer (Rodale Institute) developed a roller/crimper, a metal cylinder (16.5 in. diameter) with welded metal plates (4 in. x 3/8 in.), that crimps and crushes the cover crop stems. Timing is critical as regrowth will occur if the cover crop is rolled/crimped too early. Other important no-till tools include no-till transplanters and no-till seeders for planting into the rolled cover crop. Strip tillers are commonly used in reduced-till systems; they create a narrow (10-12 in.) tilled band of soil for planting.

On-farm No-till Trial

An on-farm research experiment was performed at 21 Acres, a Woodinville, WA farm transitioning to organic, with Eric Gibson-Snyder (farm manager, Growing Washington). The experiment compared two treatments: 1) standard tillage with a rototiller, and 2) rolling/crimping the cover crop. Goals were to evaluate pumpkin yield production and selected soil quality parameters.

All plots were tilled and planted to cereal rye (Secale cereal) in September 2008. The cover crop stand was strong, and above ground cover crop biomass was estimated to be 4.7 dry tons per acre on May 31, 2009. We monitored rye development carefully to ensure sufficient crop maturity to facilitate mechanical termination without allowing viable seed development. We implemented the two tillage treatments when approximately 75% of the cover crop was at full flower. Cover crop was incorporated into the rototilled plots on 31 May, 2009 and no-till plots were rolled/crimped on June 2, 2009. The cover crop regrew in the rolled/crimped plots within 2 weeks, so these plots were rolled/crimped again on June 16, 2009. Tilled plots were rototilled again prior to transplanting pumpkins on June 27, 2009.

Soil quality parameters were measured in mid August 2009 and indicated soil in the tilled plots had significantly lower bulk density (i.e. “lighter” soil) and more rapid infiltration than in the no-till plots (Table 1). Soil compaction, as measured with a penetrometer, was also greater in the top 3 inches in the no-till plots (data not shown). These short-term improvements in soil quality from rototilling may have resulted in more immediate vigorous pumpkin growth (observed visually) and yield, although this was not significantly different between treatments (Table 1). Improvements in soil quality through tillage reduction and unincorporated cover crop residue occur over the longer term and are rarely measured within one growing season.

Cover Crop Variety Trial at: WSU-Puyallup

A winter cover crop variety trial planted fall 2009 at Puyallup indicated differences in maturity stage among barley varieties ‘Strider’ and ‘Kold’ and wheat varieties ‘Alpowa’ and ‘Stephens’ by late spring. To increase the efficacy of rolling/crimping, tillage of barley was at a later maturation stage, though this turned out to be an earlier calendar date than at 21 Acres. (where plots were rolled May 31, 2009 at flowering). By May 25, 2010 ‘Strider’ had reached late milk, ‘Kold’ was at watery ripe, both wheat varieties were at flowering, and cereal rye planted four days earlier in an adjacent field was even less mature (Table 2). There were no significant differences in above ground biomass production (dry tons/acre) among the cover crops ‘Stephens’ (3.2), ‘Strider’ (3.1), ‘Kold’ (2.8), and ‘Alpowa’ (2.3).

On-farm No-till trial at WSU-Puyallup

Three tillage systems were assessed in a barley cover crop trial to determine impact on butternut squash yield, earthworm biomass, and weed populations. Tillage (rototilling) and roller/ crimper were similar to 2009 and were performed in the ‘Strider’ plots, while flailing was performed in the ‘Kold’ plots. Barley residue was cut in a two inch wide, four inch deep swath using a custom-built tractor-drawn implement, and butternut squash was transplanted with a hand-operated Hatfield Transplanter (Johnny’s Seeds). Butternut squash yield was greater in the flailed plot (10.6 tons/acre) than in the roller/crimper (6.2 tons/acre) and tilled (6.1 tons/acre) plots (Table 3). Significant differences were observed
between plots in weed numbers but not weed biomass. There was a notable difference in earthworms between the tilled (44.7 g/m2) and roller/crimper (119.0 g/m2) plots.

Reflections on Preliminary Experiments

Tillage stimulates germination of many common summer annual weeds species. We provided early season weed control by removing tillage and adding a mulch layer by rolling/crimping or flailing winter cover crops. The amount of cover crop biomass produced in our trials was similar to that produced in no-till organic grain studies in Pennsylvania where adequate weed control was also provided (personnel communication Jeff Moyer, Rodale Institute). Weed density was not measured in our 2009 experiment but visual observations indicated good weed management early in the trial in the no-till plots. For example, approximately 4 weeks after transplanting hand weeding was necessary in the tilled plots but not in the no-till plots.

Researchers in Maryland found that rolling/crimping at the onset of flowering provided greater than 85% termination in cereal rye.ii In our 2009 study, the rye cover crop was rolled/crimped when 75% of the plants were in flower; however kill was not sufficient for transplanting so the cover crop was rolled again two weeks later. In 2010 the barley matured earlier and did not significantly delay vegetable planting. Other researchers have reported that the effective kill date for a vetch cover crop was 4 to 6 weeks after the recommended vegetable crop planting date.iii The kill date for a cover crop can significantly impact the vegetable planting date and is an important consideration in western Washington where the growing season can be limited due to poor weather conditions in the spring and fall.

In our study, tillage decreased soil bulk density and compaction and caused greater infiltration during the growing season in 2009. This “loosening” of the soil may increase root penetration and is a short-term benefit to tillage. A benefit of reduced tillage has been the increase in earthworm populations. Earthworms are known as “ecosystem engineers” because as they consume organic matter they mix it with minerals to form water-stable aggregates. Earthworm burrows also contribute to soil aeration, drainage, and porosity. To date, however, rolling/ crimping has not significantly reduced yield compared to tilling in our trials with winter squash.

Future Direction

A third year of trials was planted in fall 2010 at WSU Puyallup REC and WSU Mount Vernon NWREC to evaluate four cover crops for their suitability in reduced tillage systems. At both sites, two barley varieties, ‘Strider’ and ‘OR09913’ (OSU), and two vetch varieties, ‘Lana’ and ‘Chickling’, were planted on certified organic ground. This study includes five tillage treatments: flailing+notill, flailing+strip till, flailing+complete till, roller/crimper+strip till, and roller/crimper+no-till. Funding for this study is from the WSU Center for Sustaining Agriculture and Natural Resources’ Biologically Intensive and Organic Agriculture program, and we are seeking additional funding to further evaluate cover crop varieties including their biomass production and impact on soil quality and weed pressure in reduced tillage systems.

This project continues to be a participatory-grower research project where growers provide ideas for effectively and profitably reducing tillage in their operations. Summer field days and grower focus groups at WSU Puyallup REC and WSU Mount Vernon NWREC in 2011 will highlight these cover crop trials.

i. Teasdale, J.R., C.B. Coffman, and R. W. Mangum. 2007. Potential long-term benefits of notillage and organic cropping systems for grain production and soil Improvement. Agronomy Journal. 99:1297-1305. ii. Mirsky, S.B., W.S. Curran, D.A. Mortensen, M.R. Ryan, and D.L. Shumway. 2009. Control of cereal rye with a roller/crimper as influenced by cover crop phenology. Agronomy Journal. 101: 1589-1596. iii. Mischler,R., S.W. Duiker, W.S. Curran, and D. Wilson. 2010. Hairy Vetch Management for No-Till Organic Corn Production. Agronomy Journal. 102: 355-362.

Tags: Barley, Bulk Density, Earthworms, Infiltration, Rodale, Roller-Crimper, Soil Quality, Spader, Weeds, Wheat