Cover crops can provide many benefits. They can suppress weeds, prevent erosion, increase water infiltration and water-holding capacity, improve nutrient cycling and create pollinator habitat.
A lot of information about cover crops is available in academic literature and from seed companies. However, only anecdotal evidence about cover crop performance in central Oregon is available. A wide variety of species, varieties and planting times also makes it hard to know which cover crops are right for a farm.
In response to grower interest, Clare Sullivan (Small Farms Extension, Deschutes County) designed and planted three trials in 2018 to test the performance of various cover crop species planted at different timings in central Oregon. Trials were planted at the times during which growers may have a cropping window: spring, summer and fall.
All three trials were planted at the Central Oregon Agricultural Research and Extension Center (COAREC) in Madras using small-plot research equipment. The trials were fertilized prior to planting based on soil test results, using feather meal and perfect blend. No pesticides were used.
Weed control is a primary concern for organic farmers, so the trials focused on crop emergence and weed suppression.
Spring trial
The spring trial was planted on April 16, 2018 with six replicated treatments:
- Anaconda radish
- Attack mustard
- Caliente mustard
- Crimson clover
- Horizon spring pea
- Cereal rye
Six demonstration plots were also planted without replication:
- Austrian winter pea
- Cereal rye/pea mix
- Mustard/radish mixtures.
All but two of the species (Caliente mustard and crimson clover) had good emergence within one week of planting. The Caliente mustard seed was determined to be old and not viable, so its poor emergence wasn't representative of the variety. Crimson clover was very slow to establish.
The cereal rye and pea species grew the most quickly while soils were cool (about 55–58°F). Mustard and radish species caught up as soils warmed later in May. The Caliente mustard and crimson clover plots became overrun with weeds and were discounted as treatments.
Cover crop biomass samples of the remaining plots were collected at full flower. Mustard and radish plots were sampled eight weeks after planting (June 12, 2018), while cereal rye and pea plots were sampled nine weeks after planting (June 19, 2018). Both biomass samples and weed counts were taken from two 0.5 m2-quadrats per plot.
Of the four replicated treatments, Horizon spring pea produced the most biomass (2,691 lb/acre), followed closely by radish and Attack mustard. Cereal rye produced the least biomass (Table 1).
All four species were competitive against weeds (fewer than two weeds/ft2). Radish, with its broadleaf canopy, excelled at weed suppression (Table 1). The Attack mustard and Horizon spring pea plots had more upright growth habits that allowed for more weed growth.
Although not replicated, biomass produced by the Austrian winter pea in the demonstration plots (2,668 lb/acre) was similar to that of spring pea. Winter pea (0.5 weed/ft2) provided greater weed suppression due to its prostrate growth habit. The cereal rye/pea and mustard/radish mixtures did not produce as much biomass as the single species plots (data not shown).
| Cover crop | Seeding rate (lb/a) | Seed cost ($/lb) | Seed cost ($/a) | Biomass (lb/1,000 ft2) | Biomass (lb/a) | Weed count1 (plant/ft2) |
|---|---|---|---|---|---|---|
| Attack mustard | 10 | $2.60 | $26.00 | 52 | 2,267 | 1.7 |
| Cereal rye | 70 | $0.29 | $20.30 | 31 | 1,335 | 1.1 |
| Radish | 10 | $2.35 | $23.50 | 53 | 2,322 | 0.6 |
| Spring pea | 70 | $0.50 | $35.00 | 62 | 2,691 | 1.8 |
Summer trial
The summer trial was planted on June 6, 2018 with six replicated treatments:
- Attack mustard
- Buckwheat
- Cow pea
- Phacelia
- Sorghum
- Sudangrass.
As in the spring trial, additional demonstration plots were planted without replication:
- Anaconda radish
- Caliente mustard
- Buckwheat mixtures
- Saia oats
Emergence was less successful in the summer planting than in the spring trial, and several species struggled to establish well. The sorghum plots had very low germination rates and established very poorly. The cow pea and sunn hemp plots also established poorly, likely due to cool temperatures. The sorghum and cow pea plots were discounted as treatments, leaving four replicated species for evaluation.
Cover crop biomass samples were collected seven weeks after planting (July 25, 2018) at full flower. Both biomass samples and weed counts were taken from two 0.5 m2-quadrats per plot.
Of the four replicated treatments, buckwheat and sudangrass produced the most biomass (about 2,100 lb/acre). Attack mustard and phacelia produced a smaller amount (Table 2).
Summer annual weed pressure was very heavy, especially from redroot pigweed (Amaranthus retroflexus). All four species struggled to be as competitive against weeds as the spring plantings were, with phacelia providing the most weed suppression (2.8 weeds/ft2).
Although not replicated, the Caliente mustard in the demonstration plots produced the most biomass (2,850 lb/acre), and the Anaconda radish provided the most weed suppression (fewer than two weeds/ft2).
| Cover crop | Seeding rate (lb/a) | Seed cost ($/lb) | Seed cost ($/a) | Biomass (lb/1,000 ft2) | Biomass (lb/a) | Weed count2
( plant/ft2) |
|---|---|---|---|---|---|---|
| Attack mustard | 10 | $2.60 | $26.00 | 41 | 1,810 | 5.3 |
| Buckwheat | 60 | $0.85 | $51.00 | 49 | 2,141 | 4.1 |
| Phacelia | 5 | $5.00 | $25.00 | 40 | 1,750 | 2.8 |
| Sudangrass | 35 | $0.65 | $22.75 | 48 | 2,096 | 4.0 |
Fall trial
The fall trial was planted on September 14, 2018 with eight replicated treatments of legume, brassica and cereal combinations. These cover crops were overwintered for evaluation in the spring.
Thoughts so far
In selecting a cover crop, it's important to know what benefit you want it to provide. Potential ecosystem benefits from cover crops include, for example, nitrogen (N) fixation, weed suppression and biomass production. Local data on cover crop performance can help you choose a species and variety that will fit in your rotation.
If weed suppression is the primary goal, Anaconda radish and Austrian winter pea seem to be good choices for a spring cover crop. Both species establish quickly and have prostrate growth habits that help suppress weeds. Planted at only 10 lb/acre, radish costs approximately $10/acre less than pea (Table 1). However, if N fixation is a goal, either spring or winter peas would be a better choice.
Summer cover crops can fit into a tight growing window (about 6 weeks). Of the cover crops tested, buckwheat and sudangrass produced the most biomass in the shortest period of time.
At the planting rates used in this study, a buckwheat cover crops costs more than double the cost of sudangrass (Table 2). However, buckwheat offers pollinator benefits that sudangrass does not. While phacelia produced less biomass than the other species, it provided the best weed suppression and excellent pollinator habitat.
Based on the spring and summer trials, crimson clover does not make sense as a short-season cover crop, since it establishes slowly and is not competitive against weeds. Also, based on these trials, there were no biomass or weed suppression benefits to cover crop mixes.
