Crop Rotation for Weed Management
Crop rotation is a planned sequence of crops growing in the same field year after year. Rotating crops adds diversity to the cropping system, increasing the sustainability of the system. Although crop rotations are sometimes planned years in advance, they are also dynamic; farmers adapt their crop rotations in response to environmental, pest and market forces.
Crop rotation provides the foundation for long-term weed management. Planting a wide variety of crops with varied characteristics reduces the likelihood that specific weed species will become adapted to the system and become problematic. According to Liebman and Dyck (1993), “the success of rotation systems for weed suppression appears to be based on the use of crop sequences that employ varying patterns of resource competition, allelopathic interference, soil disturbance, and mechanical damage to provide an unstable and frequently inhospitable environment that prevents the proliferation of a particular weed species.”
Rotating crops allows farmers to rotate other aspects of the crop management system including timing and type of tillage operation, seeding date, timing and type of herbicide application, and type and amount of fertilizer application.
Some of the crop characteristics to consider when planning a diverse rotation include:
- grassy vs. broadleaved
- annual, biennial, or perennial
- cool-season vs. warm-season
- seeding date – fall, early spring, late spring
- growth habit
- competitive ability
- fertility requirements
Crop rotation can be used to set the stage for eliminating herbicides from one growing season. Farmers participating in an on-farm research project on Pesticide Free Production reported crop rotation as one of the most important tools for preparing for a pesticide-free crop (Nazarko et al., 2003).
Research Results – Annual cropping systems
Many studies have shown that in monoculture systems (i.e. continuous cropping of one crop year after year), a specific weed or set of weeds becomes very problematic. For example, in continuous winter wheat production in Alberta, downy brome became a major weed problem after several years. Including spring canola in the rotation prevented downy brome populations from increasing (Blackshaw et al., 2002).
Crop rotation can also slow the development of herbicide resistant weeds. An on-farm study in Alberta found that herbicide resistant wild oats were more likely to be found in fields that did not have forages or fall-seeded crops in the rotation (Beckie et al., 2004).
See also Forages in the Crop Rotation for more on including forage crops in the cropping system.
Continuous cropping systems are most likely to become infested with weeds, as weeds become adapted to the system.
A diverse crop rotation puts varying selection pressure on weeds, preventing any one weed species from becoming problematic, and slowing the development of herbicide resistance in weeds.
Beckie, H.J., L.M. Hall, S. Meers, J.J. Laslo and F.C. Stevenson. 2004. Management practices influencing herbicide resistance in wild oat. Weed Technol. 18:853-859.
Blackshaw, R.E., J.T. O’Donovan, K.N. Harker and X. Li. 2002. Beyond herbicides: New approaches to managing weeds. ICESA.
Liebman, M. and E. Dyck. 1993. Crop rotation and intercropping strategies for weed management. Ecological Applications 3:92-122.
Nazarko, O.M., R.C. Van Acker, M.H. Entz, A. Schoofs and G. Martens. 2003. Pesticide free production of field crops: Results of an on-farm pilot project. Agron. J. 95:1262-1273.