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Investigating economic thresholds for Canada thistle in forages

Pick your best strategy against this weed.

March 5, 2008
By Donna Fleury


Perennial weed control in forages and pastures can be a challenging and complex management issue. In the past few years, Dr. Edward Bork, associate professor, range science and management, Department of Agricultural, Food and Nutritional Science at the University of Alberta, has been studying the impacts of Canada thistle on forages and the various management tools for weed control in the Aspen Parkland.

Producers would benefit from spraying out Canada thistle in a straight grass stand.

“Our research, so far, has confirmed that Canada thistle infestations cause significant forage losses and that the use of integrated approaches, including herbicides, fertilizer and rotational grazing, can work together to minimize the impact of this weed on forage production,” explains Bork. “However, this research has also led to many more questions relating to the role of species diversity in pasture mixes, including the impact of management actions on diversity and associated forage yields.” For example, managing weeds in pastures containing a mix of grasses and desirable legumes can be difficult, particularly if maintaining legumes is a priority.

To address these questions, Bork initiated a new four year research project in 2004 in conjunction with Dow AgroSciences and the National Sciences and Engineering Research Council of Canada (NSERC) to examine strategies to effectively manage pastures containing both legumes and weeds like Canada thistle. “We reviewed the literature, expecting to find information assessing the interaction among forage grasses, legumes and broadleaf weeds, but we could not find anything,” says Bork. “We also discovered that while ample research has shown the general benefit of including legumes in forage mixes, we were surprised to find that little work has been done identifying the specific amount of legumes needed in mixed pastures to maximize forage yield, quality and economic returns.”

Greenhouse studies are exploring the three-way interactions of two forages and Canada thistle. Photos Courtesy Of Danielle Gabruck.

The goal of the current project is to expand the understanding of the agronomic role legumes play within mixed forage stands, including those containing perennial weeds such as Canada thistle. Canada thistle is a noxious weed that producers are mandated to control by law. “We anticipate our study results will provide practical guidelines for producers attempting to maximize forage yield and quality in grass and legume mixes, as well as establish decision making rules for managing legume-based forage stands requiring weed control.”

To find answers, Bork and his research team have set up three complementary experiments, ranging from greenhouse investigations to field studies. The first experiment is a forage cross-seeding trial looking at various common mixes of grasses and legumes, and their ability to contribute to forage ‘over-yielding’.

Bork explains, “Over-yielding is the ability of a mixture of different plant species to produce more biomass than any single species alone in monoculture. Over-yielding occurs due to the ability of various plant species having different root and shoot systems to capture and utilize different resources such as light, water and nutrients when grown together. However, over-yielding can also occur through facilitation, where plants such as legumes add soil nitrogen and contribute to overall pasture fertility, benefitting adjacent grasses in the forage stand.”

The Ellerslie cross-seeding plots in June 2004, shortly after seeding.

The four forage mixes they are testing include alfalfa and meadow bromegrass, typical of newly established, highly productive power pastures. Another mix was designed to simulate old decadent pastures consisting of sodgrasses like smooth brome and Kentucky bluegrass, in a mixture with either alfalfa, or white and alsike clovers, common volunteer legumes in many Alberta pastures.

“We varied the proportion of legume in the initial seeding rate of 14lb/ac such that legumes ranged from zero to 100 percent of pure live seed,” explains Bork. “Over time, we expect to quantify the optimal legume abundance that will give us peak yields. We also expect these optimums to vary by forage mix, based on the complementarity of root systems, leaf architecture, water use, and nitrogen fixation and transfer within the forage stand.” After two years, Bork also sprayed half of each plot to quantify grass responses to the removal of a desirable competitor such as alfalfa or clover, and the variation in response among forage mixes and seeding rates.

Although it is about half way through the field trial, there are some preliminary observations that can be made. Bork notes that all forage stands are undergoing major changes in the amount of legume present. “Our experimental stands are rapidly evolving through time, with only the initial composition depending on how much legume was seeded with the various grasses. Additionally, evidence of over-yielding has only been evident in some of the forage mixtures tested, suggesting that the agronomic benefits of using forage mixes may not be universal or consistent.”

A 100 percent clover stand makes Canada thistle control difficult.

A second experiment, conducted in the greenhouse under simulated field conditions, is exploring the detailed three-way interactions of two desirable forage plants, Kentucky bluegrass and white clover, with Canada thistle grown from root cuttings. “This controlled approach allows us greater opportunity to tease apart complex interactions, both positive and negative, among these three plants. For example, we hope to determine how strong a competitor Canada thistle is to both clover and grass, how beneficial the legume is to grass growth and whether clover may contribute positively to thistle abundance.”

The third experiment, which is the most interesting of all but also the most difficult, is attempting to isolate these same three-way interactions among forage grasses, legumes and Canada thistle in existing pastures of central Alberta. One site is located north of Wabamun near Lake Isle, which is an old decadent stand with a lot of grass, volunteer clover and a major thistle infestation. The second site is situated at the Parkland Conservation Farm near Mundare, which is a newer forage stand containing alfalfa as the primary legume.

“In this study, we are trying to bring everything together under actual field conditions to quantify the agronomic benefit of legumes, the detrimental impact of Canada thistle on forage yield, and establish decision rules for producers on if and when they should consider weed control in pastures containing legumes,” explains Bork. “This approach is not without its challenges, as pasture yields are affected by many factors other than plant composition that we can’t control, such as drought, grasshoppers and ground squirrels.”

Overall, this study is designed to provide producers with two important pieces of information. The first is in selecting pasture mixes that take advantage of the complementary relationships among forages so that yields and quality can be maximized. This has implications for both the selection of forage mixes and in determining the amount of legume that should be seeded during initial establishment. Strategies that minimize unnecessary input costs associated with legume seed will improve profitability.

A pure legume (clover) plot side-by-side with a pure grass (meadow brome) plot.

Of equal importance, this study will identify agronomic and economic thresholds associated with managing legumes and weeds like Canada thistle in Alberta pastures. “Producers often have the concern that with legumes in their pastures, either seeded or volunteer, spraying for weed control will result in a net economic loss,” explains Bork. “Identifying those specific conditions likely to result in a loss is the most challenging to address, but it is also what the industry is most interested in.” What Bork wants to know is how big the response is when thistle, a major competitor, is removed, as compared to the response from the removal of legumes, a beneficial competitor and nitrogen fixer.

Although the study is not complete, Bork notes that some suggestions can be made. For example, if a stand does not have legumes but has a lot of thistle, then clearly spraying the thistle is a good option. “Based on the results of previous studies, we know producers will be losing far more than they would gain by not spraying in this situation,” says Bork.

“However, if the stand has a lot of legumes and a small amount of weeds, the best strategy may be to mow, hand pull or hand spray individual weed patches. It is in situations where we have a moderate to high amount of both legumes and thistles that producers are likely to hesitate over whether or not to spray for weed control.” Bork hopes to have answers for producers and industry at the end of this study in 2008. -end-