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Weed Wisdom Feb 2017


Weed Resistance

by Dr. Bob Blackshaw

Repeated herbicide use is undoubtedly the cause of worldwide development of herbicide resistance, but a major contributing factor is the lack of diversified crop rotations in many regions. For example, the US Corn Belt has a severe glyphosate-resistant weed problem that threatens its no-till production system.

Crop census records indicate that over 75% of this region has a two-year rotation of corn-soybean. In some states, 40% of the acreage is continuous corn. Western Australia is noteworthy for having the worst weed resistance in the world. 

Glyphosate resistant Kochia in southern Alberta
A pollinator visits flowering canola.

Once again limited rotations are a factor; 70% of its acreage is wheat and 20% is canola.

Monoculture and short-duration rotations facilitate a buildup of weeds (and other pests) adapted to that production system. This in turn leads to shifts in weed species and higher weed densities that increase selection pressure for resistance development. Bad things happen.

It is tempting to say we are doing better on the Canadian prairies. We grow a wide array of annual cereal, oilseed and pulse crops; winter annual crops such as wheat, rye and triticale; and perennial forages are present in many regions.

Many growers consistently implement a 4-year crop rotation and include a legume crop in at least one of those years. Yet we know that a two-year wheat-canola rotation has become common recently and even continuous canola occurs in some areas. A nine-year Agriculture and Agri-Food Canada study conducted at five sites across the prairies found that, compared with continuous canola, a two-year rotation with either wheat or barley increased canola yield by 9-14%. A 3-year rotation that included a cereal plus a pulse crop such as field pea or lentil increased canola yield by 15-27%.

Thus, short-duration rotations not only have negative consequences in terms of weed resistance development, but they may not be as profitable as one thinks. Long-term farm profitability and sustainability go hand-in-hand. You can’t have one without the other.