Cover cropping has many advantages in cropping systems, and weed suppression is certainly an important one. Cover crops grown with the main crop are sometimes called smother crops or living mulches. Cover crops can also be grown before or after the crop season, providing weed suppression and other benefits during the off-season.
Off-season cover crops
Cover crops can provide many benefits to cropping systems whenever the main crops are not covering the ground. This may be before main crop seeding and after main crop harvest, or as a substitute for fallow in other years. Cover crops can include forage legumes and grasses, as well as fall and winter cereals.
As a fallow substitute, cover crops effectively suppress weeds while providing cover for the soil. In an Alberta study, cereal cover crops were evaluated for their effect on weeds in a wheat-fallow system. Under favourable growing conditions, a short-term fall rye cover crop controlled weeds as well as tillage operations in fall and in spring, or herbicides in spring. Winter wheat was also effective at suppressing weeds, whereas spring rye was not as successful. Wheat yield following winter cereal cover crops without other weed control measures was similar to wheat yield with tillage or herbicidal weed control (Moyer et al., 2000).
Fall rye and other cover crops can be used as a killed mulch in reduced input and organic pulse production systems. In preliminary research, University of Manitoba researchers found that a fall rye mulch significantly reduced weeds in beans, but also reduced bean yield (Flood and Entz, unpublished), while earlier research in a very wet year found that bean yield was increased in the presence of a fall rye mulch, especially in the absence of herbicides (read more at Rye Cover Crops for PFP and Organic Pulse Production).
Organic soybean growers in North-Western Minnesota are using the following system of killed rye mulch:
- Fall of year 1. Seed fall rye in September.
- Spring of year 2. Seed soybeans into living rye crop in late May (rye is about 3 feet tall).
- June of year 2. After rye has flowered, use a flail mower to mow rye. Mowing height should be just above the soybeans, which are about 8 inches tall at this stage.
- Do not use a batwing or rotary mower as it tends to windrow the mulch.
Limitations to this system include need for moisture to support rye and bean crops and limited options for in-crop tillage due to heavy mulch cover.
Cover crops grown in the same season as the main crop have the potential to provide similar weed suppression benefits as those in fallow years. Growing cover crops after main crop harvest is a fairly common practice in longer season parts of the world. The south-eastern parts of the Canadian prairies have enough heat and water resources available to grow a cover crop after harvest of early-maturing crops such as winter wheat (Thiessen Martens and Entz, 2001). In cooler, shorter-season parts of the prairies, late-season cover crops may not have enough heat available to grow and provide any benefit.
Living mulches or smother crops
Growing a legume or cereal cover crop together with the main crop can also provide weed suppression during the crop year. A good cover crop for use as a living mulch will have rapid establishment, tolerance to field traffic, tolerance to drought and low fertility, and low cost of maintenance (Paine and Harrison, 1993). In a review of literature on this topic, Liebman and Dyck (1993) found that the vast majority of studies reported that smother crops suppressed weeds more than the main crop alone.
A balance must be reached between weed suppression and crop suppression, so that crop yield is not reduced by competition from the cover crop. Seeding a main crop into an established cover crop is not generally successful, since the cover crop out-competes the main crop. On the other hand, establishing cover crops every year can be difficult and expensive. A potential solution to this set of problems is the development of self-seeding annual cover crops such as black medic, that can be controlled before seeding the main crop and then re-establish themselves from the seedbank each year.
Cover crops can be grown as a fallow substitute, suppressing weeds and providing other benefits.
Fall rye or other cereal cover crops can be killed by mowing and then used as a killed mulch for pulse crop production.
Short duration cover crops grown before or after the crop growing season can also provide weed suppression benefits.
Cover crops can be grown together with main crops as smother crops or living mulches. Self-reseeding annuals such as black medic may have potential for use as long-term cover crops in grain production systems.
Liebman, M. and E. Dyck. 1993. Crop rotation and intercropping strategies for weed management. Ecological App. 3: 92-122.
Moyer, J.R., R.E. Blackshaw, E.G. Smith and S.M McGinn. 2000. Cereal cover crops for weed suppression in a summer fallow-wheat cropping sequence. Can J. Plant Sci. 80: 441-449.
Paine, L.K. and H. Harrison. 1993. The historical roots of living mulch and related practices. HortTechnology 3:137-143.
Thiessen Martens, J.R. and M.H. Entz. 2001. Availability of late-season heat and water resources for relay and double cropping with winter wheat in prairie Canada. Can. J. Plant Sci. 81:273-276.