Using Multiple Methods Key to Weed Control Success

By Brenda Frick, Ph.D., P.Ag.

In weed control, combining different strategies can bring excellent results. Organic producers rely on a variety of weed control techniques such as varied rotations, heavy seeding rates, and competitive crops. Research suggests that combining these techniques can have much more than an additive effect.

Bob Blackshaw from the Lethbridge Research Centre of Agriculture and AgriFood Canada explained the benefits of stacking weed control techniques to the International Weed Science Society at meetings 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 the Lacombe Research Centre of Agriculture and AgriFood Canada, and a co-author on the paper, explains: “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. 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 8-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 single most important technique for weed management. Weeds respond differently to the opportunities in 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 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. An overwhelming number of research 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 to 2 fold, has consistently improved crop competition with weeds, and improved yields. Steve 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 120km/hour – they were that much better” claimed Steve. “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 chuckling 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 Steve has begun 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 the Scott Research Station of Agriculture and AgriFood Canada, has also found that multiple techniques give 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 rod weeding before emergence and harrowing after has been particularly successful.

Weed management using multiple techniques requires greater levels of planning, but offers significant results. By using many effective tools, organic farmers can do more than just replace herbicides with tillage. They can work towards as a more diverse system, a greater sustainability, and a greater profitability. And, of course, all these techniques are useful to non-organic farmers as well, allowing them to reduce or even eliminate herbicides.

Neil Harker, Steve Shirtliffe and Eric Johnson will be speaking on ecological methods of weed control at the Organic Connections conference in Saskatoon in November. For more information, see www.organicconnections.ca or call 306-956-3110.

Brenda Frick, Ph.D., P.Ag., is the Organic Research and Extension Coordinator at the University of Saskatchewan and is an affiliate of the Organic Agriculture Centre of Canada. She welcomes your comments at 306-966-4975 or via email at organic@usask.ca.

 

Posted October 2008