Managing Cover Crop and Conservation Tillage Systems To Enhance Vegetable Crop Yields, Economic Returns and Environmental Quality

2006 Annual Report for SW04-072

Project Type: Research and Education
Funds awarded in 2004: $182,438.00
Projected End Date: 12/31/2009
Region: Western
State: Oregon
Principal Investigator:
John Luna
Oregon State University

Managing Cover Crop and Conservation Tillage Systems To Enhance Vegetable Crop Yields, Economic Returns and Environmental Quality


Both on-farm and research station trials were used to evaluate cover crop selection and management for vegetable production systems in western Oregon. On-farm trials showed that cover crops could increase sweet corn yields over no cover crop treatments, however effects were inconsistent among the cover crops examined. Oat-vetch and phacelia-vetch cover crop mixture produced 3 to 4 tons of dry matter biomass and up to 200 lbs of nitrogen per acre. Phacelia-vetch cover crops were suppressed by both rolling with a cultipacker and flail mowing and were compatible with strip tillage.

Objectives/Performance Targets

1. To enhance farmers’ ability to select and manage cover crops in conservation tillage vegetable crop production systems
2. To develop and evaluate conservation tillage practices for sustainable and organic farming systems.


Participatory On-Farm Research:

On-farm trials were established on two participating commercial vegetable farms in the fall of 2005. The objectives were to continue previous year’s experiments evaluating the impact of legume-based cover crop mixtures on sweet corn yield in conservation tillage systems. These trials were conducted using strip-tillage equipment (Transtiller), since one of the overarching objectives of the trials was to evaluate suitability of cover crop mixtures with the mechanical operations of strip tillage. Too much fibrous cover crop residue can make all tillage operations difficult. The phacelia-vetch mixture, in particular, is being evaluated because of its unique physical structure and composition. The leaves are fern-like and are high in nitrogen. The stem is hollow, highly lignified, and is easily crimped by mechanical rollers. When the stems dry, they are easily shattered by the tillage equipment, which makes the plant easy to kill and incorporate into the tilled strips.

Cover crop treatments consisted of (1) oats (var = ‘Monida’), (2) oats plus common vetch (Vicia sativa), phacelia (Phacelia tanacetifolia) and vetch, and (4) no cover crop. Fields were chisel plowed and disked prior to cover crop planting in September 2005. Large treatment plots (1-3 acres) were planted using the growers’ seeding equipment. In the spring, cover crops were sprayed out with glyphosate and a Transtiller strip-till machine was used to till 12”-wide strips into the cover crop residue. Sweet corn was planted using the growers’ planting equipment.

Data Collection: Cover crop biomass was sampled in all plots in the spring prior to killing with herbicides. A 0.25 m2 quadrat sampling method (6 samples per treatment block) was used to estimate biomass at both locations. However, because of the difficulty of this method in the highly enmeshed phacelia-vetch mixtures, a hand-operated, powered 30”-wide sickle bar mower was used to cut 100 ft2 sample areas at one farm. The cover crop biomass was raked into piles and weighed in the field. Three subsamples were taken in each plot to determine moisture content and relative proportion of individual cover crop species within the mixture. Subsamples were taken to the lab and sorted by cover crop species. These were dried at 105o F for 48 hours and cover crop yields were calculated. Samples were analyzed for carbon and nitrogen content at the OSU Central Analytical Laboratory. Corn yields were estimated using the growers’ harvesting equipment. One or two truckloads of sweet corn were harvested from each plot and taken to the processing plant for weighing and grading. The harvested areas were measured to determine yield.

Results: Cover crops produced from 2 to 3.5 tons aboveground dry matter per acre, and from 107 to 201 lbs per acre of nitrogen (N) (Table 1). In both fields, the oat/legume cover crop mixture increased total N content by 80 to 100 lbs/acre compared to oats grown as a sole crop. In the Sweeney field, the legume component also increased total biomass by about 1.6 tons per acre; in the Hendricks field, the increase in total biomass by the oat-legume mixture was only 400 lbs. In both fields, the significant legume component of oat-legume mixture also produced lower C: N ratios than the straight oats, suggesting this cover crop residue would be degraded more quickly by soil microorganisms and the N released for crop uptake. In the Sweeney field, the phacelia-legume mixtures produced similar quantities of both cover crop biomass and total nitrogen as the oat-legume mixtures. In the Hendricks trial, the oat-vetch mixture produced 2 tons per acre more dry matter biomass than the phacelia-vetch mixture, and 40 lbs N more per acre.

Crop Yields. Sweet corn response to cover crop treatments varied between the two fields. Corn yield response in the Sweeney field closely matched previously reported data from 2004-2005 (See previous SARE project annual report), particularly in the ability of all cover crops to increase graded corn yield and net economic value compared to the no-cover-cropped fallow land. The phacelia-vetch mixture produced the highest economic return. However, values for all cover crop returns were similar. Although the value of cover cropping for improving soil and water quality is well known, increases in crop yields accelerate the economic adoption of cover cropping practices. In the Hendricks trial, however, corn yield was only increased by 0.5 tons per acre by oats, compared to the fallow, and net economic value was similar. In addition, in the Hendricks trial, the oat-vetch mixture produced lower yields and economic return than the straight oat cover crop. The lowest yields and economic return were from the phacelia-vetch treatment. Data from this year’s trials will be pooled with previous year’s data for statistical analysis.

Grower Demonstration Projects

Adoption of novel cover crops. Stahlbush Island Farms, near Corvallis, has expanded the use of phacelia-vetch cover crops and strip-tillage throughout their more than 4,500-acre vegetable farm. In 2006, Stahlbush planted more than 400 acres of phacelia-vetch cover crops. For 2007, they have contracted with a Willamette Valley seed grower to produce phacelia seed on 40 acres. According to Stahlbush farm manager Scott Pohlschneider, “The soil following the phacelia-vetch cover crops has much better tilth than in the soil following the oat-vetch cover crops.”

Strip-tillage Field Day: A field day was held in May 2005 at the Three-Mile Canyon farm near Boardman, OR. Eight local farm managers observed the operation of the Transtiller strip-tillage machine in several fields on this 24,000-acre farm. Growers were impressed by the strip tillage concept and by the machine, especially using GPS-guided tractors. Growers are interested in strip tillage because of the intensive wind erosion that occurs in the sandy soils of the Columbia Basin in north central Oregon. However, because of the soil compaction from heavy hay harvesting equipment in the hay field in which the strip-till machine was being demonstrated, the Transtiller produced an inadequate seedbed. Other crop rotation and cover crop sequences that would be more favorable to strip tillage were discussed by the growers.

Research Station, Replicated Trials:

A replicated experiment was conducted in 2006 at the Oregon State University vegetable research farm. The goal of his project was to evaluate cover crop selection and suppression practices for minimum tillage organic vegetable production. This was a collaborative project between Oregon State University and Stahlbush Island Farms, in which OSU provided the land (2 acres), irrigation, and research data collection, and Stahlbush provided tractors and fuel, tillage and cultivation equipment, and tractor operators. Three tillage practices (ridge till, strip till, and minimum till (disk and Roterra) were the main treatments, with split-plot treatments of with and without supplemental organic N and P through granular fish fertilizer. Plots were 150 feet long and 27 feet wide to accommodate Stahlbush Farm’s 9-row cultivator. All treatments were replicated four times in a randomized block design. Ridges for the ridge till system were created in October 2005 using a Buffalo cultivator with ridging wings (Fig. ?). An International grain drill was used to seed the cover crop, a phacelia-vetch mixture (Fig. ?). Phacelia and vetch were mixed in a 2:40 ratio by weight, and seeded at a rate of 49 total lbs/acre.

Estimating Cover Crop Biomass: Cover crop biomass was estimated on 17 May by clipping eight 10 x 10 ft quadrats with a hand-operated sickle bar mower. Plots were selected along a transect running down the center of the experiment across the replications. Corners of each plot were marked with tall flags and a 30”-wide strip mowed around the outside perimeter of the plot. The cut cover crop was raked and removed to allow the subsequent mowing of the marked plot. Cover crop biomass was raked into piles and weighed using a tarp, tripod, and hanging scale. Three subsamples were randomly selected from the mowed cover crop and placed into paper bags. These bags were placed under the piles of cover crop to retard moisture loss from the samples until all plot sampling was completed. Subsamples were then taken to the lab, and each bag separated into phacelia and vetch components. Subsample components wet weights were determined, and then the samples were placed in a crop dryer at 104o F for 72 hours to obtain dry weights. Four bags each of phacelia and vetch were taken to the OSU Central Analytical Laboratory for carbon and nitrogen analysis.

Cover Crop Suppression and Tillage Treatments: A randomized block, split-plot design was established with four replications, with three tillage treatments as the main effects and two cover crop suppression methods as the sub effects. Tillage treatments included fall ridging with spring strip till (RST), spring strip till (ST) and conventional tillage using an offset disk and Roterra (CT). Cover crop suppression treatments consisted of rolling with 15’-wide sprocket roller, or “cultipacker” (Roll) and flail mowing (Flail). Cover crops were either rolled or flailed prior to tillage, and CT plots were only flailed. Tillage plots were 27’ (9 rows @ 36” centers) x 118 ft, with a 34-ft driveway between plots for tractor and implement maneuvering. On 18 May, tillage and cover crop treatments were imposed. RST and ST plots were strip-tilled using two passes of a Transtiller, (Luna & Staben, 2002), followed by a single pass using a Howard rotospike (Luna and Staben, 2003). The Howard rotospike was followed by a basket roller and a sprocket roller. CT plots were prepared with two passes of an offset disk, followed by a single pass using a Lely Roterra and a cultipacker.

Fertilization: On 12 June, Oregon BioGrow fish fertilizer (8-5-3) was applied in 10”-wide bands over the tilled strips in the RT and ST plots, and in rows in the CT plots. Material was applied at a rate of 900 lbs/acre (70 lbs N/a). Three rows in each plot were fertilized.

Planting: Super Sweet Jubilee sweet corn
(Syngenta-Rogers® seeds) was planted 13 June using a Monesem® precision air planter. One-year-old untreated seed was planted at approx. 26,000 seed per acre. Irrigation (1” of water) was applied June 21. However, very few corn plants emerged and by June 27 a decision was made to replant the field. Rotted seeds were found in areas where corn failed to emerge, and many germinating plants died soon after emergence. OSU Plant Pathology Clinic identified Fusarium oxysporum on root samples from the field, however F. oxysporum is present in most Oregon fields. Because the corn seed was not treated with fungicides, pathogens such as pythium and rhizoctonia were likely involved in the damping off of the seedlings. The field was replanted on June 30, and new seed was used and the seeding rate was increased to approx. 30,000 seed per acre. Soil moisture was ideal for planting.

Data Collection

Cover Crop suppression: Cover crop suppression was rated by walking through each plot and estimating the total length of row that contained vetch regrowth. Phacelia regrowth was minimal, and individual plants were counted. Two observers rated the plots and ratings were averaged.

Soil Nitrate and Ammonium: Soil samples were taken in the row zone and in the between-row zone in all plots. A 1”-diameter x 12”-deep soil probe was used, with eight samples randomly taken in the unfertilized section of each plot and aggregated into a single sample. Samples were taken to the OSU Central Analytical Laboratory and placed in a dryer for subsequent nitrate and ammonium analysis. Samples were taken 23 June, just as sweet corn was beginning to emerge.
Yields: Because of the late replanting date, the sweet corn has not yet reached harvest maturity in this experiment (at the time of this report). Corn ears will be hand harvested from 100’ of row in each treatment to determine green ear weight. A subsample of 20 ears will be pulled from each of the harvested rows, shucked, weighed, and ear length measured.


The phacelia-vetch cover crop had produced approximately 7,600 lbs of aboveground biomass by May 17, and a total of 203 lbs of total nitrogen (Table 3). Of this total, 134 lbs of N came from the vetch, 69 lbs N from the phacelia.
Both the cultipacker roller and the flailed cover crop treatments gave ample suppression of the cover crop. Both treatments worked well on killing phacelia, however the flail mower was more effective in killing the vetch. Between-row cultivation with a Buffalo cultivator killed most weeds and removed the surviving cover crop. Only six person-hours were needed to hand weed the plots. Late season, a low-growing cover of henbit emerged between the rows.

New 2006 Projects

A new research project was initiated in October 2006 at two OSU research station sites, the Lewis Brown Farm near Corvallis, and the North Willamette Research and Extension Center, Aurora, OR. The same experiment is repeated at both sites. The objective of the project is to evaluate cover crop nitrogen contributions to organic broccoli production. Six cover crop treatments were selected to give a range of C: N ratios in the cover crop mixtures. These include: oats, common vetch, phacelia, oats and vetch mixture, phacelia and vetch mixture, and fallow (no cover crop). Individual treatment plots are 15 x 120’, with treatments arranged in a randomized block design. In the spring of 2007, cover crop biomass and N-content will be estimated, then the plots will be flail mowed and a seedbed prepared using a power spader and a Lely Roterra. Four rates of feather gmeal (12%N) will be applied over the crop rows within each cover crop plot in a split-plot design. Feathermeal will be used because of the high N content and low content of other mineral elements such as phosphorus and potassium. Feather meal will be lightly incorporated. Broccoli seedlings will be grown in a greenhouse and transplanted into the field using a mechanical transplanter. Soil nitrogen sampling as well as plant tissue N accumulation rates will be estimated to look at the synchrony of N release by cover crop residues with the N-uptake requirements of the crop. Broccoli heads will be cut to determine crop yields.

Impacts and Contributions/Outcomes

This project is demonstrating the potential of integrated cover crop/strip-tillage systems to increase vegetable crop yield and improve soil and water quality in both conventional and organic farming systems. Legume-based cover crop mixtures have been shown to produce more than 3 tons of dry matter biomass and up to 200 lbs N/acre. The phacelia-legume cover crop mixture was shown to be easier to mechanically suppress in organic conservation tillage systems. The feasibility of using strip-tillage for organic production systems was demonstrated. Stahlbush Island Farm, Corvallis, planted more than 400 acres of phacelia-vetch cover crop in 2006. Strip-tillage is being evaluated for use in their production of more than 1,200 acres of vegetable crops.

Current work involving N contribution of cover crops in organic vegetable systems will allow growers to account for cover crop N in preparing a fertilization program. Because of the very high cost of purchased sources of organic nitrogen ($5 to $6 per lb N), maximizing nitrogen contribution from cover crops will improve profitability.


Sam Sweeney

Country Heritage Farms
1070 Ferry Rd.
Dayton, OR 97114
Mark Dickman

Dickman Farms
15829 Mt. Angel-Scotts Mill Hwy NE
Silverton, OR 97381
Carl Hendricks

Hendricks Farms
P.O. Box 308
Stayton, OR 97383, OR 97383
Rob Heater

Farm Manager
Stahlbush Island Farms
3122 Stahlbush Island Rd
Corvallis, OR 97333
Peter Kenagy

Kenagy Family Farms
1640 Nebergall Loop
Albany, OR 97321