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Gaining Ground - Excerpt from Part 2: Managing the Transition in the Fields

8 Managing Weeds


3.2 Designing a diverse and competitive crop rotation

Diversity is the key to disrupting weed cycles. It prevents the niche conditions where weeds can thrive. Rotation changes the growing conditions from year to year, creating an unstable environment for weed establishment. The more diverse the rotation, the more effective the disruption to weed-growing cycles. Some organic farmers have found that including frequent legume green manures on an organic farm will help prevent weed problems.

“I grow much wider varieties for my crop rotation now and use it for weed control, in addition to cultivation.” – Elmer Laird, SK

“Change your rotation. If things don’t work, change it again.” – Lorne Jamieson, ON

“For a new grower, use a legume green manure plowdown one out of three years as a minimum, ideally alternating every other year for optimum results.” – Steven Snider, AB

- Use smother crops in the rotation. Smother crops are rapidly-growing cover crops (e.g. buckwheat, rye, hairy vetch, oilradish and mustard) grown to suppress weeds. They must be carefully monitored and cut or tilled in before they set seed.

- Intercrop and underseed where possible. Intercropping two or more crops with different growth habits and nutrient needs increases diversity, and helps crops compete with weeds. The crops fill the gaps that weeds would otherwise occupy. Studies have shown that intercropped systems such as barley/canola or barley/pea often have fewer weeds than one crop alone. In Québec, growing various combinations of wheat, oats, barley and peas is a popular technique among organic livestock operators. In the prairies, oats or wheat can be intercropped with peas or other legumes. In fields of flax, wheat can be intercropped (at low seeding rates) to control weeds.

“Sixty bushels per acre of barley intercropped with clover was a great success.” – Gerry DeRuyck, MB

“We broadcast ryegrass [right before] seeding grain. Someone is a field ahead, or a couple of acres ahead, broadcasting the ryegrass.” – Ronald Boyle, NS

“Last year, I seeded Eston lentils and durum wheat together. I had no intention of harvesting the lentils. They were there to compete with the weeds. I seeded late, both the durum and the lentils at the same time, with my air seeder, one in each tank. I had a beautiful crop and no weeds.” – Cal Cowan, SK

“I tried seeding Selkirk wheat with flax at various rates – about 20 pounds of wheat per acre worked best not to affect the flax.” - Gerry DeRuyck, MB

- Include a silage or greenfeed crop in the rotation. Thiscan be an effective weed control strategy during transition, particularly when intercropped with peas, underseeded with a forage crop, or amended with compost.

“If you are going into transition and there’s a market for it close by, grow greenfeed, especially silage, to clean up the land. I would grow feed barley or feed oats, or a fall crop like fall triticale. This will help to get ahead of the weeds. In my area, I would stay away from fall rye. It can take over and become a weed.” – Morley Forsyth, SK

Research in Alberta has shown that cutting barley for silage, particularly when it was cut earlier, was as effective as herbicides in reducing wild oats. But early cutting reduced the dry matter yield by 5 – 10%. In Manitoba, cereal fields had fewer weeds (wild oats, Canada thistle, wild mustard and false cleavers) when they followed alfalfa in the rotation, than when they followed another cereal crop. Placing a forage crop in the rotation, then, is beneficial not only for soil building but also for weed management.

- Use competitive crops in the rotation.

Select competitive crops and appropriate varieties to outcompete weeds and minimize problems in a weedy field. Choose varieties that are adapted to your specific site conditions (soil and climate), and are quick to emerge, cover the soil quickly and grow tall in later growth stages. The most competitive crops have prostrate leaves in the early growth stages, fill out quickly, and shade the soil well.

“We need crops that provide the maximum competition for weeds, and most of the newer, dwarf varieties don’t do that.” – Ray Bauml, SK

Keep in mind that growing conditions will affect the competitiveness of these crops. For example, oats may be less competitive than barley under dry conditions. Buckwheat is competitive under warm and moist conditions, but not when the weather is dry.

Many competitive crops are allelopathic – this means that they release substances from their roots that can suppress the growth of other types of plants. Both the living plants and even the dead mulch from these can inhibit germination of weed (and crop) seeds. The most commonly known allelopathic crops are rye and sweetclover, but oats, barley, wheat, red clover, white clover, Berseem clover, alsike clover, hairy vetch, buckwheat, mustard species, canola, creeping red fescue, tall fescue, sunflowers and perennial ryegrass also have allelopathic properties to varying degrees.

A study showed that sweetclover strongly suppressed several weed species during the first fall after underseeding, and during the spring of the fallow year. The residue remaining after cutting in the fallow year continued to suppress dandelion, perennial sow thistle, kochia, flixweed, Russian thistle and downy brome. The weed-suppressive effect of the clover in the fallow year was similar whether it was harvested as hay (cut 30 cm/12 in high), mowed and left on the surface as mulch, or incorporated into the soil. The following spring, before seeding wheat, weed densities in clover plots were 75 to 97% lower than in weed fallow. But only hayed and mulched clover still provided some weed suppression in the following wheat crop.

- Seed quality

The more vigorous the seed, the faster the crop jumps out of the ground. Brian and Bernie Ehnes of Etzicom, Alberta, operate a large certified organic farm and seed plant. They became registered seed growers in 2000 in order to market registered certified organic seed. They advise, “Seed quality is very important. You need to look for vigour and good seed quality. You should request tests for germination, vigour and Fusarium. Vigour is especially important in late springs when the soil is slow to warm up. Grain quality such as protein also declines if the same seed is used year after year.”

“I always get a better catch with certified registered seed.” – Ian Miller, SK

“We save all our own seed. Some people say you should only save your seed for a year or two, but we save winter wheat seed from the very best field, year after year. We also keep some back after planting in the fall in case of winterkill.” – Murray Bunnett, NB

Blackshaw, 2003, Presentation to Organic Growers, AAFC Research Station Lethbridge Nov 19/03 & Hauggaard-Nielsen et al., 2001, Interspecific Competition, N use and Interference with Weeds in Pea-Barley Intercropping, Organic Eprints - Orgprint 48,

O’Donovan et al., 2002, Integrated Weed Management: Reducing Herbicide Costs and Managing Resistance, Farm Tech: Global Perspectives, Local Knowledge, p.68-76

Ibid, 2002.

Frick and Johnson, 2002, Using Allelopathic & Cover Crops to Suppress Seeds, Organic Agricultural Centre of Canada,

Blackshaw, 2003, Presentation to Organic Growers, AAFC Research Station Lethbridge Nov 19/03 & Hauggaard-Nielsen et al., 2001, Interspecific Competition, N use and Interference with Weeds in Pea-Barley Intercropping, Organic Eprints - Orgprint 48,

For more from Part 2: Managing the Tranistion in the Fields and more technical information about farming during transition, order Gaining Ground: Making a Successful Transition to Organic Farming from the publications page.