Using multiple methods key to weed control success – Organic Matters

In weed control, combining different strategies can bring excellent results.

Organic producers rely on a variety of control techniques, such as varied rotations, heavy seeding rates and competitive crops. Research suggests that combining these techniques can pack an extra wallop.

Bob Blackshaw from Agriculture Canada’s Lethbridge Research Centre explained the benefits of stacking control techniques at the International Weed Science Society meeting this year in Vancouver. He claimed that in effect, it’s like finding that 1+1+1=5. If these combinations of techniques are continued over several years, it is even more successful, as if 1+1+1=7.

Neil Harker, from Agriculture Canada’s Lacombe Research Centre and a co-author on the paper said combining optimal agronomic practices can dramatically reduce weed infestations.

“In ongoing studies at Lacombe, we are investigating cultivar, seeding rate and crop rotation effects on barley health, productivity and weed management,” he said.

“Individually, these factors had considerable effects on wild oat, but when combined, the effects are dramatic …. Combining higher seeding rates with the taller barley cultivar decreased wild oat biomass and seed numbers approximately eight-fold. Growing barley in rotation with canola and pea rather than (seeding barley) continuously, and combining the rotation effect with higher seeding rates and the taller barley cultivar decreased wild oat biomass and seed numbers approximately 70-fold.”

The Alberta scientists found that crop rotation was the most important technique for weed management. Weeds respond in varying ways to the opportunities presented by different crops. For instance, the timing of tillage and competition differs among early seeded, late seeded and fall seeded crops. The relative timing of weed and crop emergence is especially important. Weeds that emerge with or before the crop can reduce yield far more than weeds that emerge after the crop is established. Rotating crop types prevents a weed that is favoured by one system from gaining a consistent advantage.

Extended rotations increase the diversity of the cropping system, which can improve insect and disease management as well. Adding cover crops, green manures, silage crops and intercrops can also diversify the system. Cover crops and mulches can reduce erosion; green manures can improve soil fertility.

Studies show benefits of diversifying rotations include improved crop yield, weed and pest management and ecosystem health.

Steve Shirtliffe from the University of Saskatchewan has found that increasing seeding rate at least 1.5 times to double, has consistently improved crop competition with weeds and improved crop yields. He has also done cultivar comparisons, looking for strong varieties for organic production. One of the stars of his research trials is 4010 forage pea.

“You could spot the 4010 plots while driving by at 120 km/h. They were that much better. 4010 suppressed weeds three times greater than all the other varieties combined …. Given what we have found from previous research comparing field pea, lentil and chickling vetch as annual green manures (pea was the clear winner) and this study I would recommend 4010 field pea as an annual green manure ahead of all other crop types or pea varieties (tested in the Saskatoon area).”

This year Shirtliffe started a system trial to look at multiple weed control techniques. The results, as in Alberta, are promising. So far, he has found that heavy seeding, competitive cultivars, narrow row spacing and in-crop mechanical weed control combine to give excellent weed control.

Eric Johnson, of Agriculture Canada’s Scott Research Station, has also found that multiple techniques provide benefits. He has considered a variety of mechanical controls with rodweeder, rotary hoe and harrow. Because weeds emerge at various times and young weeds are easiest to control, multiple weeding passes may be necessary to give the crop a head start. A combination of rodweeding before emergence and harrowing after has been particularly successful.

Weed management using multiple techniques requires more planning, but offers significant results. By using many effective tools, organic farmers can do more than just replace herbicides with tillage. They can work toward a more diverse system, a greater sustainability and a greater profitability.

All these techniques are useful to non-organic farmers as well, allowing them to reduce or even eliminate herbicides.

Harker, Shirtliffe and Johnson will be speaking on ecological methods of weed control at the Organic Connections conference in Saskatoon in November. For more information, visit or phone 306-956-3110.

Brenda Frick, senior research and extension associate for Organic Agriculture Centre of Canada at the University of Saskatchewan, welcomes comments at 306-966-4975 or



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