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Mulch-based weed control

Using mulch as a weed control strategy isn’t just for organic growers anymore

As conventional farmers search for a wider variety of tools to control hard-to-kill weeds, a Manitoba research has found that mulches may help.

Martin Entz, a researcher at the University of Manitoba’s Plant Sciences Department, has led or participated in several recent studies of weed control through the use of mulch.

“One of the first things farmers need to ask themselves is, ‘How am I going to achieve my mulch?’ Do you grow a plant specifically to create a suppressive mulch? Do you use straw? Do you use chaff? What are the possibilities there? It’s a new idea for farmers,” Entz says.

“The two main ways we’ve thought of is to grow a cover crop that suppresses the weeds after the main crop is harvested, or to use the economic crop residue to create the mulch, or to use a combination of the two.”

One study, led by PhD student Caroline Halde, looked at the use of hairy vetch and barley to create mulch in organic no-till systems. In the first year, hairy vetch and barley were grown as green manure. In the second year, flax or spring wheat was seeded directly into the mulch. “Cover crop mulches with hairy vetch were effective at reducing weeds biomass by 50 per cent to 90 per cent in the no-till spring wheat, in 2011 and 2012, compared to other mulches,” the study found.

Entz says the team is still perfecting the hairy vetch system, but many farmers have adopted this method with good results. “What we’ve established with the hairy vetch system is that it’s possible to grow one or two no-till crops without using herbicides, if you take a year off,” Entz says.

Another study has looked at fall-seeded cover cropping, or post-harvest cover cropping. “If you get cover crops planted after your barley, winter wheat or even canola crop, they’re very helpful for weed control,” Entz says. “Again, it doesn’t eliminate the need for herbicides, but it reduces weeds, which reduces reliance on herbicides.”

A research team led by University of Manitoba PhD candidate Harun Cicek looked at the productivity and nitrogen benefits of late-season legume cover crops in organic wheat. The crops were grown alongside or after the cash crop in the same growing season. “We conclude that late-season cover crops enhance the following wheat yield and facilitate reduced tillage in organic crop production,” write the authors.

But cover cropping is not a one-size-fits-all strategy for weed control, Entz warns.

One study he participated in calculated the availability of late season heat and water resources across 21 sites in Manitoba, Saskatchewan and Alberta for relay and double cropping with winter wheat.

“If you look at Lacombe in our data, there are some limitations there in terms of whether you’re going to get much late season harvest,” he says. “If you look at Swift Current, Lethbridge or the southern prairies — there you’ve got way more potential.”

In short growing season regions, growers need to carefully select the most effective cover crop.

Cereal straw residue mulch

Entz and his team have also looked at cereal straw residue mulch. “The idea with that project was to use a stripper header that strips the oats while leaving the straw standing,” Entz explains. “Our idea was to use the existing plant as the mulch, and seed some cover crops into that wheat to supplement the biomass.”

Stripper-header straw results in 30 to 80 per cent more straw biomass in late fall compared to chopped straw, according to the study.

Entz says this method does not allow growers to eliminate herbicide use, but “buys growers time” by reducing the growth of weeds in the fall and reducing new weed infestation over time.

Another benefit of the mulch is that it conserves water, he says, pointing to a body of research conducted at Agriculture Canada’s Semiarid Prairie Agricultural Research Centre in Swift Current, Sask. Herb Cutforth, an agrometerologist and soil physicist at the Centre, published a study in 2011 analyzing the effects of extra-tall stubble on crop yield.

“Generally, compared with cultivated stubble, extra-tall stubble increased yield by about 17 per cent,” write the authors. They conclude, “Crop yield and the overall average water use efficiency increased linearly as stubble height increased to 45 cm.”

One downside of using the stripper-header, Entz says, is that crop residues were pushed down when the researchers used a blade roller, which resulted in insulated soil temperatures to the point where crop development slowed the following year. Entz says recent research in Switzerland and France indicates that the mulch is just as effective when the material is left standing.

Residue managers are commercially available that can remove mulch where growers are seeding, so that the field warms in a more typical fashion and plant emergence is rapid.

About the author

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Julienne Isaacs is a Winnipeg-based freelance writer and editor. Contact her at [email protected]

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