Review of published literature on applications of cover crops as part of integrated weed management systems in field vegetable crop production in Canada

Project Code PRR10-010

Project Lead

Bonnie Ball Coelho - Agriculture and Agri-Food Canada

Objective

To identify field vegetable production systems where cover crops can be used as part of integrated weed management practices to minimize or eliminate the use of herbicides

Summary of Results

Background

Weed management is an important crop protection component in vegetable production in Canada and responsible for considerable use of chemical herbicides. Non-chemical weed control options are needed to help growers transition away from full reliance on herbicides and achieve reduction in pesticide use, risk to resistance development and cost of production.

Cover crops (CC) have been identified as a key approach to sustainable soil and crop management. Such crops usually are not grown for harvest, but used to serve many important functions such as to enrich soil with organic matter, cycle nutrients, and protect soil from water and wind erosion. Cover crops have also been used, with some success, as part of an integrated weed management approach with reduced use of herbicides. However, information on CC approaches, applications and benefits pertinent to weed management in vegetable crops grown in Canada is not readily available.

This project aimed to review the current published literature on CC applications and make recommendations for approaches which can be adopted for weed management in field vegetable production in Canada.

Approach

The review included over 165 published scientific literature and extension articles focussing on results from CC research and development studies conducted on key vegetable crops (potato, sweet corn, field tomato, carrot, onion, Brassica crops, peas, cucurbit crops, green and wax beans, and lettuce) in North America (Canada and the U.S.) and other regions with similar climates in Europe in the last 15 years (1994-2009).

The main criterion for selecting CC approaches suitable for weed management was economics, the primary driver for adoption. Economic factors considered included establishment costs, impact on crop yield, potential for the CC to add value via control of weeds (e.g. reduced herbicide input cost) and other pests (e.g. increased yield or reduced input cost of other pesticides), and as a product. Other selection criteria were potential to suppress weeds by allelopathy, the amount of research that has been conducted for the system, and environmental impact.

Results

Cover crop systems are unlikely to replace the use of herbicides in any vegetable production system, as full season control has rarely been documented in the literature. The general consensus is that some additional weed control is required later in the season. The potential to adopt CC to reduce herbicide use in vegetables has been demonstrated mostly by studies conducted in the US. There are few scientific studies on CC use for weed control in vegetables from Europe, and nearly none from Canada. Cover crops can lessen herbicide use by either reducing the number of pre-plant or pre-emerge (PRE) applications, switching from broadcast to band application or switching from PRE to post-emerge (POST) applications as needed. Switching to POST usually involves herbicides that are less persistent in the environment than PRE. The savings in herbicide cost compensates the CC cost in some studies but not others, with wide variability between studies and systems in degree of weed control, crop response and costs. With respect to profitability, it is concluded that adding value beyond weed control may be necessary to promote adoption of CC.

Species of weeds controlled varies widely between and within systems. In general, annuals, and not biennials or perennials are suppressed by CC. Allelopathy is a promising mechanism of control, and is likely to work best where weeds are small seeded, and the crop is not. Rye residues are allelopathic with better efficacy against annual dicots than grasses and have consistently controlled lambsquarters, nightshade, plantain, goosegrass and barnyardgrass. Brassica residues are also allelopathic, depending on stage, and notably provide control of crabgrass and pigweed. Smother crops such as sorghum or sudangrass can provide control of perennials such as quackgrass during growth, but at the expense of about half of the growing season. Sorghum residues also have allelopathic effects, controlling pigweed, barnyard grass and others.

Approaches suited to sustainable weed management that can be adopted by Canadian vegetable growers were identified based on experiments conducted in temperate regions. These selected approaches include:

  1. fall seeded cereal rye + hairy vetch CC mixture, chemically killed before no-till tomato;
  2. fall seeded rye chemically killed before zone-till cucurbits;
  3. aerial overseeded rye into late harvested crops such as potato or carrot; and
  4. summer seeded smother crop of sorghum or sudangrass before or after a short season vegetable such as fresh market cole crops or pea.

Rye was selected for approaches A, B and C, because of low seed cost, high availability (in many areas), compatibility with existing equipment (combine, drill) facilitating home-grown, inexpensive seed, and allelopathy to weeds. Hairy vetch (option A) adds Nitrogen value and has a track record of increasing tomato yield. Both rye and vetch grow at low temperature, and mixtures provide a number of advantages over monoculture CC. Zone tillage was selected for approach B to avoid delay in crop maturity that can occur with mulches left on the surface. Option C is not profitable in the short term, but will reduce the weed seed bank over the long term, and provides important off-site environmental benefits such as improved water quality making the system eligible for stewardship or land use services programs where available. Sorghum was selected for Option D because it is a smother crop, residues are allelopathic to weeds, it is drought tolerant and therefore suitable for summer planting (typically dry), and it has potential to add value as livestock feed or a biomass crop (e.g. if near a market for bioenergy), and from control of other pests such as root rot. Systems utilizing thick residue mulches from mechanically killed CC to control weeds by physical means were not recommended for Canada because the available growing degree days limits the quantity of mulch that can be produced and more research is needed on these systems prior to recommendation.

A system of fall-seeded brassica before potato, with high adoption in northwestern coastal US, was not recommended for Canada due to a number of uncertainties, but is proposed as an area for study, due to advantages such as allelopathy to weeds and potential to increase yield by controlling other pests or providing a biofumigation effect. This approach may be limited to areas where potato follows an early harvested crop and brassicas (such as rapeseed or arugula) can overwinter.

Canadian data are needed to verify all of the recommended approaches in terms of impact on weeds, herbicide use reduction, and economic impacts from relevant sites across the country. Other research needs identified include cultural practices to enhance weed control, improved understanding of allelopathy to optimize its use, and development of cultivars with enhanced traits such as increased concentration of allelopathic compounds, adaptation to climate, and self destruction at the appropriate time or stage.

Similar to the amount of Canadian research conducted on CC for weed control, current adoption of CC for weed control in Canada is nearly nonexistent, although CC are often planted for other purposes. The major barriers to adoption are profitability and information. Also, many benefits from CC are long term or off site and do not add value to Canadian vegetable production systems within the decision making time frame for adoption. Formation of focused, interdisplinary teams comprised of researchers, extension and industry is recommended to simultaneously develop economical, low risk systems, promote awareness, and provide demonstration and answers.

The new knowledge compiled and analysed through this literature review will be made avalible to growers, crop specialists, extension experts and scientists through publication of extension and journal articles. Promoting adoption of approaches recommended in this review will likely lead to reduced use of herbicides, hence reduced risk from pesticides, better resistance risk management and other economical and environmental benefits. For more information on this project and its outcomes, please contact: Dr. Bonnie Ball Coelho, Research Scientist, Agriculture and Agri-Food Canada.

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