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Basics of Fall Cover Cropping for Hemp in Oregon

  • Gordon Jones
  • Valtcho D. Jeliazkov
  • Richard J. Roseberg
  • Sam D. Angima

Photo: Edwin Remsberg, U.S. Department of Agriculture
Cover crops such as these radishes can help break up compacted soil layers.

Photo: Edwin Remsberg, U.S. Department of Agriculture
Figure 1. Cereal grain and radish cover crop.

Photo: U.S. Department of Agriculture
Figure 2. Austrian winter pea.

Photo: Tracy Robillard, U.S. Department of Agriculture
Figure 3. A diverse winter cover crop including crimson clover, hairy vetch, cereal grains and brassicas.

About this publication

Hemp may only be grown in compliance with applicable state and federal law, including the 2014 and 2018 farm bills and anticipated U.S. Department of Agriculture regulations. The following information is for educational purposes only to inform licensed growers operating in compliance with applicable state and federal laws. Consult your local authorities, Department of Agriculture representatives, or personal attorney for questions regarding the legality of growing hemp in your jurisdiction.

Introduction: Why plant cover crops?

With careful management, cover crops can provide multiple benefits for hemp production systems. Cover crops build soil organic matter through decomposition of their biomass and by feeding the soil microbial community through root exudates. Increased soil organic matter can improve soil aeration, cultivation, and the infiltration rate of rain and irrigation water. Legume cover crops fix atmospheric nitrogen and supply it to subsequent crops, reducing or replacing fertilizer requirements. Cover crops also suppress weeds and can reduce nematode populations in the soil.

Winter cover crops have significant environmental benefits. They limit soil erosion by holding onto soil and cushioning the erosive impact of rain. Cover crops also take up soil nutrients remaining at the end of the growing season, reducing leaching losses of nitrogen and protecting surface and groundwater quality.

Note: Oregon Department of Agriculture water-quality rules prohibit discharging wastes, including soil and fertilizers, into state waters. Contact the Oregon Department of Agriculture for more information on water-quality rules: 503-986-4700 or https://oda.direct/AgWQ

Selecting cover crop species

Winter cover crops generally fall into one of three categories: grasses, legumes and brassicas.

Grasses such as oats, cereal rye and annual ryegrass produce significant amounts of biomass and have extensive root systems that can dramatically reduce soil erosion and take up available soil nutrients.

Legumes, such as clovers, vetches and peas, fix nitrogen from the atmosphere and supply it to subsequent crops. Brassicas, including turnips, mustards and radishes, can break up compacted soil with their deep roots and reduce soilborne diseases and nematode populations as they decompose.

Grasses, legumes and brassicas are often planted together as mixtures for winter cover crops to confer the various benefits of each group (Figure 1).

Growers should choose a cover crop carefully, taking into consideration possible shortcomings, such as:

  • Some cover crops may be difficult to establish.
  • Cover crops may utilize the nutrients and reduce water availability to the subsequent crop.
  • Some may establish as weeds.
  • Some cover crops could be difficult to terminate; annual grasses, such as those described below, are easier to terminate than perennial grasses.
  • Cover crop residue may complicate seedbed preparation.
  • Some cover crops may act as a host for subsequent crops’ disease or insect problems.

Consider the full, multiyear crop rotation when selecting specific cover crops in relation to other crops in the rotation. Hemp growers also should consider the specific requirements of their crop. Is the hemp for fiber, grain or the production of secondary metabolites? Soil preparation; seeding or planting rates; and timing, nutrient requirements and management may differ for various hemp varieties and strains.

Potential cover crops for hemp

Cereal rye, oats, wheat, barley and triticale

Cereal grains often are used as the base grass in cover crop mixtures, with cereal rye being the most commonly used in the Pacific Northwest. Cereal grains provide the best winter cover when planted between Sept. 15 and Oct. 15.

Planting rates should be between 60 and 120 pounds of seed per acre. Depending on time of planting, higher densities ensure quick ground cover from fall rains.

Annual ryegrass

Annual ryegrass is a fast-establishing cover crop suited to a range of conditions. It performs better than cereal grains on wet sites. Annual ryegrass vigorously competes with weeds and scavenges free nitrogen from the soil.

Annual ryegrass, if allowed to set seed in the spring, can become weedy in subsequent crops. When planted in monoculture, use a seeding rate of 30–40 pounds per acre is recommended. Use lower rates of annual ryegrass when planted in mixtures with legumes or other species.

Hairy vetch

Hairy vetch is a winter-hardy annual legume that produces significant biomass and weed suppression through its viny growth habit. Seeding rates for hairy vetch range from 15–40 pounds per acre; use lower rates when vetch is grown in a mixture with a cereal grain. A crop of hairy vetch can fix 90–200 pounds of nitrogen per acre, and about half of that nitrogen will be available for the subsequent crop after incorporation in spring. Hairy vetch can become weedy and should be terminated before it sets seed.

Field peas

Field peas or Austrian winter peas (Figure 2, page 2) are less tolerant of saturated soils than vetches or clovers. Field peas are seeded at 50–100 pounds per acre; use lower rates when planting in a mixture with other species. Field peas can fix 90–150 pounds of nitrogen per acre and decompose quickly when incorporated into the soil.

Clovers

Several annual clover species are used in winter cover crop mixtures. Crimson clover is most commonly planted. It grows well in many soils but does not tolerate saturated conditions. Crimson clover should be seeded at 15–30 pounds per acre; use lower seeding rates when planting in a mixture with grasses. Crimson clover fixes 70–130 pounds of nitrogen per acre, and about half of that becomes available to the subsequent crop after incorporation in spring. Other clover species used in mixtures in winter cover cropping in Oregon include berseem, balansa and Persian clovers.

Note: Legume roots need to be colonized by rhizobia bacteria in order to fix atmospheric nitrogen. This is most successfully accomplished by planting seed that has been inoculated with the appropriate strain of rhizobia.

Brassicas

Brassica crops are often included in cover crop mixtures. Daikon radishes alleviate compacted soil and improve aeration. Brassicas, especially mustards, are sometimes planted as a “biofumigant” crop.

The compound that makes brassicas taste spicy, glucosinolate, breaks down into a biocidal compound called isothiocyanate, which reduces nematode and soilborne diseases in crop fields. Brassicas can be planted in mixtures with annual grasses and legumes. Seeding rates for brassicas are 5–15 pounds per acre. Use higher seeding rates for broadcast seeding and planting brassica monocultures.

Note: The Oregon Department of Agriculture has developed rules related to growing brassicas to prevent the spread of black leg, a fungal pathogen of brassicas. All brassica seed planted in Oregon must be certified as free from black leg, and brassica crops should be monitored for the disease. More information can be found in Protect Oregon’s Brassica Crops: ODA’s Black Leg Rules Aim to Control a Disease Outbreak (https://extension.oregonstate.edu/pests-diseases/plant-diseases/protect-oregons-brassica-crops-odas-black-leg-rules-aim-control).

Planting method and planting date

Cover crops can be seeded with a grain drill or broadcast onto the soil surface. Use higher seeding rates when broadcast seeding or if the seedbed is rough. Small seeded cover crop species, including annual ryegrass, clovers and brassicas, should be planted from ¼- to ½-inch deep. Cover crops with larger seed, including cereal grains and field peas, should be planted 1–2 inches deep. Successful establishment requires good seed-to-soil contact; use a grain drill to place the seed at the proper depth or broadcast seed over the soil surface and then roll the field with a cultipacker or other roller to ensure the seed comes firmly into contact with the soil.

Cover crops will be most successful when planted between Sept. 15 and Oct. 15. The earlier end of this range is better for regions with an earlier fall frost date. Irrigate if weather conditions are dry after planting and irrigation infrastructure and water supply allow. Hemp harvest requirements, weather conditions, and access to equipment can delay planting. If cover crop planting is delayed, increasing seeding rates can ensure good groundcover prior to winter.

Terminating the cover crop in spring

Most growers terminate cover crops in spring prior to planting their hemp crop. Some cover crops can be terminated by mowing, but many require tillage to ensure the crop has been adequately killed. Some cover crop species, such as hairy vetch, become weedy if allowed to set seed, so terminate the cover crop prior to seed set.

Cover crops allowed to grow alongside the hemp crop will likely cause competition for water and nutrients and may cause competition for light. If this competition is not managed or alleviated with additional irrigation and fertilizer, the productivity and yield of hemp may suffer.

The OSU Extension Catalog is the source for current, peer-reviewed, research-based learning materials published by OSU Extension.

Sunn Hemp: Forage and Soil-Building Superhero

By Anne C. Randle

Sunn hemp, a tropical plant primarily grown as a cover crop or green manure, has increased dramatically in popularity over the last decade. Originally from India, it’s easy to understand what makes it so popular among vegetable and row crop farmers in the United States.

Sunn hemp possesses many soil-building traits, including high rates of biomass production — over 20 percent greater than crimson clover and hairy vetch in research trials. It is not only resistant to plant root nematodes but actively suppresses them. In as little as 60 to 90 days it can produce 120 pounds of nitrogen per acre and can suppress weeds up to 90 percent.

Sunn Hemp is adapted to a wide variety of soil and environmental conditions, thriving through hot, dry summers and continuing to grow until the first frost. But sunn hemp isn’t just a soil builder — it also offers benefits as a forage producer. Recent on-farm grazing trials have yielded an abundance of information on using this crop for grazing.

Sunn hemp growing in a field.

What is Sunn Hemp?

According to the USDA NRCS Plant Guide, sunn hemp originated in India where it has been grown since the dawn of agriculture. It has been utilized as a green manure, livestock feed and as a non-wood fiber crop. It is a member of the legume family. It is a branched, erect, herbaceous shrubby annual growing 3 to 9 feet high with bright green simple, elliptical leaves.

Sunn Hemp has deep yellow terminal flowers (open raceme to 10 inches long), and the light brown pods are small (1 inch long and a half inch wide) and inflated. It has a well-developed root system with a strong taproot. The number of seeds per pound is 15,000.

Grazing Sunn Hemp

Sunn hemp is highly palatable and recovers quickly from grazing. In its leaves, the neutral detergent fiber (NDF) reaches 22-28 percent, acidic detergent fiber (ADF) 22-27 percent and crude protein 25-30 percent. These numbers rival the nutritive value of other forage legumes, including crimson clover. Its stems are of lower forage quality, so the key to sunn hemp management is grazing it early before the lower leaves begin to drop. Removing the top shoot also promotes branching, which increases leaf production.

Plants can be grazed when they reach 1.5 to 3 feet tall and can be eaten down to within about a foot of the ground without suffering mortality. After four to six weeks, forage quality declines rapidly. As long as animals can still reach its leaves, sunn hemp remains suitable for grazing until flowering.

This crop isn’t without drawbacks, however. It is a member of the Crotalaria genus, notorious for seeds that are high in toxic pyrrolizidine alkaloids. Ingesting the seeds at a high rate can cause damage to the liver, lungs, heart and nervous system. Susceptibility depends on the animal species: pigs are most vulnerable, followed by chickens, horses, cattle and sheep. Goats have the lowest risk.

Although the total content of toxic compounds in sunn hemp is much lower than other Crotalaria species, the alkaloids are still present in amounts that warrant special management.

Plants begin to flower five to six weeks after planting. At this point animals should be removed to avoid exposure to toxic compounds in the seeds, especially if the livestock are prone to grazing seed heads. This isn’t a major loss, as forage quality begins to drop at this point. It also isn’t an issue in northern U.S. climates: because plants are photoperiod-sensitive, flowering in response to shorter days, a killing freeze will usually occur before the plants are able to produce seeds in these areas.

However, one farmer in Alabama found that the plants began to flower while animals were still grazing in the late summer. It’s unclear if sunn hemp was directly responsible for any ill effects on his small ruminants, but it’s not a bad idea to proceed with caution once plants begin to flower. (It should be noted that leaves and stems do not contain any of the toxic alkaloids.)

Growing Considerations

Sunn hemp is easy to grow and amazingly productive. Plant when soils reach above 50°F and at least four to five weeks before frost. Plants will be killed when temperatures dip below 28°F. Optimal soil conditions include a pH between 5 and 7.5 and good drainage. Seed can be treated with cowpea inoculant to increase nitrogen fixation. For forage production, a seeding rate of 30-50 pounds per acre is recommended.

Seed should be drilled at ½-inch depth for best germination. Because plants can reach 4 to 6 feet in height, wide spacing between rows (6 inches is recommended) may make plants susceptible to lodging. With adequate moisture, temperature and fertility, researchers have recorded a growth rate of 1 foot per week. Plants can return to or exceed this growth rate if slowed by temporary drought.

Sunn hemp should be used sparingly, if at all, in mixes with other cover crop species. It has a tendency to hog nutrients and sunlight, suppressing the growth of other plants. The benefit of its growth rate is that it is highly competitive with weeds, even outpacing crabgrass in on-farm trials. Sunn hemp itself has a low potential to become a weed, unlike other Crotolaria species.

Even after grazing, sunn hemp leaves a substantial amount of organic matter in the field, unlike many other forage crops. It may be necessary to cut and chop up the fibrous stems before the pasture can be replanted.

While sunn hemp seed cost has in the past been a barrier to farmers, costs now compete with other legume cover crops, averaging around $70 per 50 pounds. This is slightly higher than current cowpea and crimson clover prices ($50 and $60 respectively) and lower than hairy vetch ($90).

Sunn hemp has the potential to fill an important gap in summer annual grazing. Its hardiness, productivity and palatability make it an option worth considering for farmers looking to build their soil and grow their stock.

Sunn hemp, a tropical plant primarily grown as a cover crop or green manure, has increased dramatically in popularity over the last decade.