Getting Stuck in the Weeds

The problem with herbicide resistance and pulse crops

by Jane Caulfield

The number of herbicide resistant weeds have been slowly increasing across the prairies for nearly four decades, with a large number of them posing frustrating situations and economically problematic growing seasons for pulse producers.

“The number of resistant weed species in Canada has grown by one to two new unique species per year from 1975 to 2015,” says Eric Johnson, a research assistant at the Department of Plant Sciences at the University of Saskatchewan. “However, there has been a levelling off of new cases in the past five years.”

Yet, despite this apparent slowing down of new cases, the amount of weeds found in fields across the country are still growing.

“The latest field surveys have indicated that 37 per cent of annually cropped land is affected by herbicide resistant weeds, totalling about 9.9 million hectares of land,” says Dr. Hugh Beckie, Research Scientist specializing in Herbicide-Resistant Plants, with Agriculture and Agri-Food Canada. “And we expect the next round of surveys to indicate that the number has increased to at least half.”

While the survey looks at all cropped land across the prairies, pulse crops are faced with a significant problem when it comes to herbicide resistance.

“Pulse crops are highly reliant on Group 2, which are an ALS inhibitor herbicides,” says Beckie. “But resistance to Group 2 is prevalent in broadleaf weeds such as kochia, wild mustard, and cleavers.”

Overall, there are 65 unique cases of weed resistance in Canada and many of the affected crops are found in the prairies. For Group 2 resistance, there are 22 confirmed cases in Canada, 11 of which are found in Saskatchewan.

“The 2014/15 Saskatchewan weed survey reports that five of the top 10 most abundant weed species in lentils have confirmed resistance to at least one herbicide group in Saskatchewan, and three others have been confirmed Group 2 resistant in Alberta,” says Johnson. “Thus, eight of the top 10 most abundant weed species in lentils are potentially Group 2 resistant.”

Similar challenges have been found in pea crops, where seven out of the top 10 most abundant weeds in those crops demonstrate Group 2 resistance as well.

Finding a Solution Is Not Going to be Easy

According to experts, the increase of herbicide resistant weeds is the result of a number of factors coming together to create an environment in which the weeds can flourish. For example, tighter crop rotations can help promote the growth of herbicide resistant weeds.

“As farm size has increased, time has become an important input. Cropping systems with low diversity are generally easier to time manage than diverse rotations,” says Johnson. “In addition, crops like canola and lentils have been more profitable than other crops, resulting in these crops being grown more frequently in a rotation.”

While a direct application of Group 2 herbicides is not going to be entirely effective, Johnson says there are some options available for producers which include a mixture of herbicide group modes of action to help reduce selection pressure for the post-emergence herbicide application.

“Farmers need to consider non-chemical methods of weed control and integrating them with herbicides to reduce selection pressure,” says Johnson. “These include cultural methods such as seeding rate, row spacing, and manipulating seeding dates, and in the future, may include weed seed
management through mechanical methods and insect predation, weed-wiping, and mechanical cultivation.”

The biggest issue in weed management, however, is due to the fact that there has been no new herbicides introduced to the market in more than 30 years. This means that producers are using the same chemicals on their fields more often, which increases selection for resistant strains.

“We have become quite dependant on the Group 2 herbicides,” says Beckie. “We need to be able to diversify, which is why we are looking to see how we can work with other herbicide groups, such as Group 15, and other modes of application.”

Research has shown that sequential applications of pre- and post-emergence herbicides can be effective in managing hard-to-control resistant weeds such as cleavers. In field peas, pre-seeding applications of soil active Group 3, 13, 14, or 15 herbicides, followed by a post-emergence applications of a Group 2 and 6 herbicide provided highly effective control of cleavers. This system will also reduce the risk of developing resistant weed species. Research is underway to investigate layering pre- and post-emergence herbicides in managing resistant weeds in lentils and faba beans.

“It takes a number of years to properly test, collect data, and find appropriate solutions,” says Beckie. “And it takes the financial support of industry to champion the work. Producers directly contribute to research through their pulse check-off levy.”

Beckie says that partnerships with industry has led to advancements and creates opportunities for future research, which can lead to stronger and more effective weed management solutions.