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The spread of herbicide resistance in grassy weeds

How does herbicide resistance spread to a producer’s field? Does it always arise locally due to overuse of single modes of action? Or, if it comes from elsewhere, how does genetic information carrying herbicide resistance gain a foothold in the field?

A recent review paper seeks to shed some light on these questions. The study, which was led by Amit Jhala, an associate professor in the Department of Agronomy and Horticulture at the University of Nebraska-Lincoln, set out to examine the potential for herbicide resistance transfer via pollen from the herbicide resistant (HR) grass weeds barnyardgrass, creeping bentgrass, Italian ryegrass, Johnsongrass, rigid ryegrass and wild oats–all economically important weeds in the U.S. and Canada.

Charles Geddes is a research scientist with Agriculture and Agri-Food Canada specializing in weed ecology and cropping systems, and a co-author on the paper.

He says the idea behind the study was to compile the state of knowledge in U.S. and Canadian contexts on the spread of herbicide resistance via pollen between grass weed populations. Prior to this work, no comprehensive review had been published on the topic, he says. A partner study, also led by Jhala, looked at the same topic in broadleaf weeds.

Herbicide resistance evolves in a weed population after repeated exposure to a single chemical mode of action over a period of years. After it evolves, it spreads through multiple means, explains Geddes. The spread of genetic information from one population to another is called “gene flow.” 

Seed-mediated gene flow happens when genetic information travels to new locations via seed, impacting local plant populations.

Pollen-mediated gene flow, on the other hand, refers to how genetic diversity spreads between plant populations through the exchange of alleles, or genetic variations of genes, in pollen. Geddes says this type of gene flow is a lot harder to manage or mitigate than seed-mediated gene flow. Pollen-mediated gene flow occurs for most flowering plants to a greater or lesser extent, the authors write in the paper. 

“If the pollen source population is large enough and has a high frequency of herbicide resistance alleles, pollen-mediated gene flow can cause effective migration of genes to distant population with a low plant density,” they write. 

Pollen-mediated gene flow is tougher to manage than seed-mediated gene flow partly because pollen can spread rapidly on the wind, and in theory, producers have greater ability to reduce the spread of weed seeds from field to field. 

While herbicides are an important tool, increasing seeding rates with narrower rows can improve weed management.
Photo courtesy of BRUCE BARKER.

Mitigating herbicide resistance
Understanding how herbicide resistance spreads can help the agricultural community take steps to mitigate that spread.

Geddes’ role in the paper was to bring a western Canadian perspective. Wild oats and barnyard grass, which are a major problem in the Canadian Prairies, are primarily self-pollinated, which means seed mediated gene flow has a larger impact than pollen-mediated gene flow on the spread of herbicide resistance.

The review paper cites research from Arkansas that found very low rates of pollen-mediated gene flow in fields at distances of 50 metres from the pollen source, which suggests seed dispersal is the main cause of HR spread in barnyardgrass.

Wild oat is highly self-pollinating, the researchers write, and while pollen-mediated gene flow can and does spread herbicide resistance, “it is of minor importance compared with seed-mediated gene flow.” 

“With these grass weeds of concern to Western Canada, for both of them anthropogenic spread is a big contributor to how resistance is spread,” says Geddes.

There’s a wide range of management tactics beyond chemical controls that can help limit the spread of herbicide resistance, he adds, primarily by boosting crop competition. 

These include selecting competitive cultivars, boosting seeding rates, reducing row widths, adjusting seeding timing, and importantly, using crop rotation.

“You can get an added benefit [from] integrating winter annuals into rotations that are focused on summer annuals. Or planting down annuals to a perennial for a few years. Those are tools that will be effective for many,” says Geddes. “The practices that matter the most will vary from one region to another.”

For weeds like wild oat, early seed shatter can make controlling weed seed a problem. Harvest weed seed control (HWSC)–which relies on a variety of methods to prevent weed seeds from returning to the seed bank–will become increasingly important for these weeds because it can also reduce the impact that combines have on the spread of weed patches. 

The Canola Council of Canada cites the following HWSC methods: “collecting weeds with chaff carts, direct-harvest crop residue baling, narrow-windrow burning and seed pulverization with integrated impact mills…[by which] 95 to 99 per cent of weeds entering the combine are destroyed or made non-viable from impacts.”

The message is that proactivity and ingenuity can make a big difference in controlling herbicide resistant weeds.

“These [weeds] are spread by humans, so humans can play a role in mitigating them,” says Geddes.