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Field vegetable production: Using cover crops for weed management

Introduction

Weed management is an important crop protection component in vegetable production. Non-chemical weed control options are needed to help growers reduce reliance on herbicides and risk of resistance development. Cover crops (CC) have been identified as an approach to sustainable weed management.

Cover crops serve important functions such as enriching soil organic matter, cycling nutrients, and protecting soil from water and wind erosion. Cover crops have also been used as part of an integrated weed management approach. However, information on CC approaches, applications and benefits pertinent to vegetable crops grown in Canada is not readily available. Therefore, a literature review was conducted to determine the feasibility of using CC in field vegetable production systems as part of integrated weed management practices to minimize the use of herbicides.

Methodology

Published scientific literature and extension articles on CC research for key vegetable crops (potato, sweet corn, field tomato, carrot, onion, Brassica crops, peas, cucurbit crops, green and wax beans, and lettuce) in North America and other regions with similar climates in Europe were reviewed. From this review, approaches which can be adopted for weed management in field vegetable production in Canada were identified based on:

  1. Economics
  2. Potential to suppress weeds by allelopathy (inhibition of growth of a plant by a toxin released from a nearby plant of the same or another species)
  3. Amount of research that has been conducted for the system in temperate regions
  4. Environmental impact

Economics included establishment costs, impact on crop yield, and potential for the CC to add value via control of weeds (for example reduced herbicide input cost), via control of other pests (for example increased yield or reduced input cost of other pesticides) or as a product such as forage.

Results

Potential to adopt CC to reduce herbicide use in vegetables has been demonstrated mostly by studies conducted in the United States (US). There are few scientific studies on CC for weed control in vegetables from Europe or Canada. Full season weed control by CC was rare in the literature. Some additional weed control is usually required later in the season.

Cover crops can lessen herbicide use by:

Switching to POST usually involves herbicides that are less persistant in the environment than PRE. Savings in herbicide cost compensates the CC cost in some studies but not others. There is wide variability between studies and systems in degree of weed control, crop response and costs. Some systems add value beyond weed control, thereby increasing profitability.

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.

Recommended approaches

Table 1. Four cover crop systems are recommended for sustainable weed management that can be adopted by Canadian vegetable growers
Cover Crop Approach Comments
Fall-seeded cereal rye + hairy vetch mixture, chemically killed before no-till tomato Rye was selected for these systems because of allelopathy to weeds, low seed cost (Table 2), high availability (in many areas), and compatibility with existing equipment (combine, drill) facilitating home-grown, inexpensive seed. Both rye and vetch grow at low temperature and mixtures provide a number of advantages over monoculture cover crops. Hairy vetch adds nitrogen value and has a track record of increasing tomato yield and profit.
Fall-seeded rye chemically killed before zone-till cucurbits Zone tillage was selected for this system to avoid delay in crop maturity that can occur with mulches left on the surface. 
Aerial overseeded rye into late harvested crops such as potato or carrot This system may not increase profit in the short term (1 year), but may reduce the weed seed bank over the long term, and provides important off-site environmental benefits such as improved water quality.
Summer seeded smother crop of sorghum or sudangrass before or after a short season vegetable such as fresh market cole crops or pea Sorghum was selected for this system 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 or from control of other pests in the subsequent vegetable such as root rot.

Conclusions

Growers are encouraged to trial rye/vetch mixture prior to tomato or rye before cucurbits with minimum or no till planting, or cereal rye overseeded into late harvested crops, or sorghum before late planted or after early harvested vegetable crops. Adoption of these recommended approaches will likely lead to reduced use of herbicides, hence reduced risk from pesticides, better resistance risk management and other economical and environmental benefits.

If using any of these approaches, note:

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Figure 1. No-till seeding into a chemically killed rye cover crop

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Figure 2. Cereal rye in April that was overseeded by aircraft into a standing crop the previous August

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Figure 3. Measuring biomass of forage sorghum

Table 2. Seeding rates and seeding costs for recommended cover crop species
Species Seed Rate (kg/ha) Seed Rate (Source) Seed Cost ($/kg) Seed Cost (Source) Planting Cost* ($/ha) Planting Cost* (Source)
Rye - drill 125 Ball Coelho and others, 2003 0.36 Ontario, (retail price) 2011 45.00
Rye - drill 120 New Brunswick, Quebec 0.79 New Brunswick, Quebec New Brunswick MAAF 2008 (online)
Rye - drill 62-94 Hoffman and Regnier, 2006 52.00 Wilson, 2005
Rye - aerial 125 Manitoba Agriculture Food and Rural Initiatives (AFRI) online (in potato)
Rye - aerial 188 Ball Coelho and others, 2005 (in corn) 0.13 Ontario, (bulk price) 2005 24.44
Hairy vetch 20-30 2.75 68.75 VerHallen and others, 2003
Hairy vetch 28-45 Hoffman and Regnier, 2006; Abdul-Baki and Teasdale, 2007 148.00 Wilson, 2005
Hairy vetch 30 4.76 New Brunswick MAAF 2008 (online) 143.00 New Brunswick MAAF 2008 (online)
Rye + vetch 95-125 (rye),
28-45 (vetch)
Snapp and Mutch, 2003 105.00 Snapp and Mutch, 2003
Rye + vetch 35 (rye),
28 (vetch)
Groff (online)
Rye + vetch 45 (rye),
45 (vetch)
Abdul-Baki and Teasdale, 2007
Rye + vetch 45-123 (rye),
19-28 (vetch)
Burgos and others, 2006; Masiunas, 2006
Sorghum sudangrass 15 1.68 New Brunswick MAAF 2008 (online) 25.00 New Brunswick MAAF 2008 (online)
Forage sorghum 15 Wheeler and McKinlay, 2007 4.84 Ontario 72.60
*Seed only, except for Wilson 2005 which also includes seeding cost
Table 3. Example costs of some relevant field operations used in establishing and killing cover crops based on custom rates
Operation Cost $/ha Source
Grain Drill $42 Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) online
Air Seed $50 Manitoba AFRI online
NT drill $56 OMAFRA online
Mow $42 OMAFRA online
Incorporation $30 OMAFRA online
Spray $22 2009 retail, Ontario
Secondary tillage $30 OMAFRA online
Table 4. Example input costs from Wallace and Bellinder (1992) study in New York with tomato strip tilled into different cover crop mulches and metribuzin/sethoxydim applied as needed
Cover crop species Cover crop kill method Cost $/ha (seed & kill) Cost $/ha (herbicide)
Grain rye glyphosate 1.1 kg ai/ha $84 $121
Hairy vetch mow kill $193 $230
Annual ryegrass winter killed, glyphosate 2, 4-D for emerged perennials $111 $158 (two applications)
Conventional till plow, disc $57 $94

References

Acknowledgements

The main author, Bonnie Ball Coelho thanks Kristen Callow, OMAFRA (Ridgetown, Ontario), Robert Nurse, Agriculture and Agri-Food Canada (AAFC) (Harrow, Ontario), Diane Lyse Benoit, AAFC (Saint-Jean-sur-Richelieu, Quebec) and Danielle Bernier, Ministère de l'Agriculture, des Pêcheries et de l'Alimentation du Québec (Québec City, Quebec) for their collaboration, and recognizes research contributions from former AAFC specialists Robert Roy and Alex More. Photo credits: Bonnie Ball Coelho

This literature review and this factsheet were developed with funding provided by the Pesticide Risk Reduction Program of Agriculture and Agri-Food Canada's Pest Management Centre.

About the Pesticide Risk Reduction Program at Agriculture and Agri-Food Canada

The Pesticide Risk Reduction Program delivers viable solutions for Canadian growers to reduce pesticide risks in the agricultural and agri-food industry. In partnership with the Pest Management Regulatory Agency of Health Canada (PMRA), the Program achieves this goal by coordinating and funding integrated pest management strategies developed through consultation with stakeholders and pest management experts.

The Pesticide Risk Reduction Program is actively pursuing the development and implementation of strategies which are key to reducing pesticide risks in the agricultural environment. To view Program's current priorities and the issues being addressed, visit: www.agr.gc.ca/prrmup. To consult other factsheets in this series, visit: www.agr.gc.ca/sustainable-crop-protection.

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