Common Resistant Weeds in Canadian Field Peas

Field peas are an important pulse crop across the Canadian Prairies, valued both for their role in crop rotations and their contribution to global markets. However, weed management in field peas is becoming increasingly difficult due to the spread of herbicide-resistant species. These resistant weeds not only compete aggressively for nutrients, moisture, and sunlight but also reduce the effectiveness of traditional chemical tools, forcing growers to rethink their strategies.

Why Resistance Matters in Field Peas

Field peas are sensitive to early-season weed competition. Research shows that uncontrolled weeds during the first few weeks after emergence can significantly reduce yield potential. Unfortunately, herbicide resistance has limited the number of effective products available, especially in post-emergent weed control.

In many regions, Group 1 and Group 2 resistance has become widespread, leaving growers with fewer options to manage grassy and broadleaf weeds. This makes integrated weed management essential, combining herbicide programs with crop rotation, competitive seeding rates, and cultural practices to maintain long-term control.

Kochia: A Spreading Menace

Kochia has become one of the most troublesome weeds across Western Canada, particularly in pulse rotations. Its rapid growth, ability to thrive in dry conditions, and prolific seed production make it a persistent challenge.

• Resistance profile: Group 2 resistance is now widespread in kochia populations across the Prairies, with increasing reports of resistance to other herbicide groups.
  
• Impact in field peas: Kochia emerges early and grows quickly, outcompeting young pea seedlings for resources. Because of its Group 2 resistance, many commonly used broadleaf herbicides provide little control.
  
• Management strategies: Pre-emergent herbicides with multiple effective modes of action remain a key strategy. Cultural practices such as narrow row spacing and competitive varieties can also reduce kochia pressure.

Wild Mustard: Another Group 2 Challenge

Wild mustard has long been a problem in prairie cropping systems, and its resistance to Group 2 herbicides has made control in peas particularly challenging.

• Resistance profile: Group 2-resistant wild mustard is common across much of Western Canada.
  
• Impact in field peas: This weed competes aggressively with peas and is especially difficult to manage because post-emergent control options are limited.
  
• Management strategies: Reliance on pre-seed and pre-emergent herbicides is often necessary. Crop rotation and ensuring diverse herbicide modes of action across the rotation can help reduce wild mustard seed banks.

Wild Oats: A Grass Weed with Multiple Resistances

Wild oats remain one of the most economically damaging weeds across Canadian cropping systems. In peas, they are particularly problematic because selective grass herbicide options are limited, and resistance has spread widely.

• Resistance profile: Many populations now show resistance to Group 1 herbicides, with Group 2 resistance also reported. Some populations carry multiple resistance, making them extremely difficult to control.
  
• Impact in field peas: Wild oats germinate over a long period, making them tough to target with a single application. Resistant populations can survive both Group 1 and Group 2 treatments, leaving few post-emergent solutions.
  
• Management strategies: Use of pre-seed herbicides with residual activity, crop rotation with cereals where more options exist, and delayed seeding to allow for early flushes and burnoff are important tactics.

The Role of Herbicides in Pea Weed Management

Although resistance limits flexibility, herbicides remain a critical part of weed management in peas. Pre-emergent herbicides with multiple effective modes of action are particularly valuable, helping to reduce early-season weed competition while minimizing selection pressure for resistance.

Farmers selecting a herbicide for field peas, crop need to consider both spectrum of activity and risk of resistance. No single product provides a complete solution, so combining residual herbicides with strong agronomic practices is essential.

Integrated Weed Management: Beyond the Sprayer

Because resistant weeds are now entrenched across much of the Prairies, growers must adopt a multi-layered strategy:

1. Diverse crop rotations – including cereals and oilseeds that allow use of different herbicide groups.
   
2. Pre-seed burnoff – critical for starting clean, particularly in fields with known resistant populations.
   
3. Residual herbicides – provide suppression of early flushes and help extend the clean window.
   
4. Cultural practices – higher seeding rates, competitive varieties, and narrow rows can give peas an advantage.
   
5. Harvest weed seed control – an emerging practice that targets weed seeds at harvest to reduce seed return.

Looking Ahead: Protecting the Toolbox

Herbicide resistance is not going away, and in field peas, the challenge is particularly acute due to the limited number of post-emergent herbicide options. Continued reliance on a narrow range of products increases the risk of resistance spreading further.

The solution lies in stewardship and diversity—rotating modes of action, layering cultural practices, and exploring new technologies such as harvest weed seed management. Companies developing crop protection products are also working to bring new solutions to the market, but it is critical for growers to preserve current tools as long as possible.

Kochia, wild mustard, and wild oats represent the most common herbicide-resistant weeds in Canadian field peas, each presenting unique challenges tied to Group 1 and Group 2 resistance. For growers, success lies in recognizing these threats early and adopting a diversified management strategy that combines herbicides with cultural and agronomic practices.

By taking a proactive approach, Canadian farmers can reduce the impact of resistant weeds, protect their field pea yields, and ensure that their weed management toolbox remains effective for years to come.