Weeds on organic farms

B. Frick, E. Johnson - Scott Research Farm

Problem
Are the weeds on organic farms different from those on neighboring farms?


Background
Weeds, like all plants, live in the context of their environment, the history of the land and the random workings of chance. Some of the most important factors that make up the environment are beyond a farmer’s control, such as climate. Some factors important to weed development, like soil temperature and moisture, reflect both the local climate and specific farm management. Farm management on organic farms is often different from neighboring farms, and thus weed communities might be expected to be different.

One study in Saskatchewan indicated that organic systems had more weed species and more individual weed plants than conventional systems. Wild mustard, lamb’s quarters and Canada thistle especially were more abundant in organic systems. Wild oats were relatively less of a problem. In that study, differences among years were greater than differences between organic and conventional systems.

A second study compared larger numbers of Saskatchewan organic and non-organic farms. It concluded that cropping system, especially if the farm had a history of perennial forage crops, had a greater influence on the weed community than whether the farm was managed organically or not. Perennial forages tended to encourage perennial and biennial weeds, while annual cropping systems encouraged annual weeds. Organic fields had greater numbers of weed species most susceptible to herbicides -- wild mustard, lamb’s quarters and bluebur (Table 1). Organic fields had more individual weeds, on average, and greater numbers of weed species than neighboring fields.

In a Manitoba study, organic producers reported problems with wild mustard, Canada thistle, red root pigweed, green foxtail and wild oats. Wild mustard was much more of a problem for organic producers than their conventional counterparts (Table 2).In experimental comparisons of organic and conventional systems in South Dakota, annual broad-leaved weeds were not consistently different in organic systems than in conventional systems. Grassy weed numbers, mostly green and yellow foxtail, were substantially higher after six years in organic cereals than they were in conventional cereals. They were not consistently higher in organic soybeans.


Conclusions
Weed communities vary because of many factors. Factors such as cropping system, year and region can be more important than whether farms are managed organically or not. Organic farms will likely have more wild mustard, lamb’s quarters, Canada thistle, bluebur and redroot pigweed than will conventional farms.

Funding
Provided by the Canada-Saskatchewan Agri-Food Innovation Fund


Contact information
Brenda Frick, Ph.D., P.Ag.
Prairie Coordinator
Organic Agriculture Centre of Canada
c/o Department of Plant Sciences
University of Saskatchewan
51 Campus Drive, Saskatoon
Saskatchewan, Canada S7N 5A8
Tel: (306) 966-4975
Fax: (306) 966-5015
Email: brenda.frick@usask.ca


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