Features Bioenergy Biomass
Mulch-based weed control

Nobody is more familiar with the fight against weed pressure than organic farmers, but one weed control strategy that works in organic settings might be just as beneficial for conventional growers, according to a Laval University researcher. The secret is mulch.

Caroline Halde, a professor in the department of plant science at Laval University in Quebec, says cover cropping for weed control is a proven strategy in organic studies. But she’s also had plenty of interest from conventional no-till growers in the use of cover cropping.

“I’ve had no-till farmers come to me who are working with cover crops more and more, and now they are ‘almost organic’ because they use very little inputs in their cropping systems,” she says. “And now they want to make the switch because they’re almost organic but don’t get the premium.”

But mulch-based weed control takes cover cropping one step further. In year one, a cover crop is planted as green manure. In year two, a cash crop is planted directly into the mulch, with the mulch serving as the grower’s only form of weed control.

Halde, working under the supervision of Martin Entz, a professor of plant sciences at the University of Manitoba, completed a study investigating the use of mulches in an organic high-residue reduced tillage system near Carman, Man., in 2013. In the study, barley, hairy vetch, oilseed radish, sunflower and pea were used as cover crops, then planted with wheat.

The best cover crop for weed control and cash crop yield was hairy vetch or a barley-hairy vetch mixture.

“Green manure mulches with hairy vetch were effective at reducing weed biomass by 50 per cent to 90 per cent in the no-till spring wheat in 2011 and 2012, compared to other mulches,” Halde concluded.

The method is not a magic bullet. Halde says high cover crop biomass is key to achieving good mulch that will effectively choke out weeds the following year.

“First, you have to have a good establishment of your cover crop – that’s rule number one,” she says. Poor or excessively wet weather in the spring might hamper cover crop growth. “And another thing is to choose fields that have low weed seed banks, or at least for some particular weeds, particularly wild oats.” In Halde’s study, wild oats and perennial weeds, such as dandelion and Canada thistle, made for challenging conditions.

Halde’s study relied on removing a field from production for one full year each cycle, but she says the payoffs can be rewarding. In Western Canada, the benefits of such a system involve water conservation as well as weed control. In Eastern Canada, removing herbicides from a field for a year would also be a major boon for growers nervous about herbicide resistance. “That would be a great advantage, because we see more and more herbicide-resistant weeds in Eastern Canada,” she says.

But Halde is currently seeking funding for a study in Eastern Canada on the use of fall cover crops used as mulch in the spring and planted with short-season cash crops – a system which would keep fields in production, so growers do not have to lose a year each cycle.

Biomass is key
Carolyn Marshall, a PhD student at Dalhousie University, is currently studying the impacts of no-till green manure management on soil health in organic grain rotations on two sites – at Carman, Man., under the supervision of Martin Entz, and at the Dalhousie Agricultural Campus in Truro, N.S., under the supervision of Derek Lynch. The project, which is funded by the Organic Science Cluster through Agriculture and Agri-Food Canada (AAFC), began in 2013 and will conclude this year.

She says cover cropping shows enormous promise for weed control in both organic and conventional systems. “I would love to see more use of cover crops in all systems. I think they can solve all kinds of problems,” she says.

Marshall’s project is focused on determining how green manure termination method affects soil health in organic grain rotations, with three tillage intensities applied on all plots: no-till, minimum tillage and spring and fall tillage.

At Carman, Marshall’s team is employing a four-year rotation of hairy vetch-wheat-fall rye-soybean plus a red clover-red clover-wheat-soybean rotation. At Truro, the experiment is testing two green manures – pea/oat, and hairy vetch/barley, each followed by a wheat-fall rye-soybean rotation.

In the first round at Truro, Marshall says, “We had really good growth of the green manure. Some plots got up to 10 tonnes per hectare of biomass, and it was really effective at stamping out the weeds.”

When the experiment was repeated in 2014, a dry spring resulted in limited growth and very thin mulch. “The weeds went berserk in the no-till plots,” Marshall says. “Weed control seems to really depend on getting enough biomass to get a thick enough mulch, and that really depends on the weather.”

Termination methods matter, too: when mulches were mowed in the fall at Truro, they decomposed, leaving too little mulch on the soil surface in the spring. When a roller crimper was used instead, the cover crops continued to grow until winterkilled, resulting in heavy mulch cover in the spring.

“Researchers in North Dakota, Georgia and New England are also finding that if you don’t get enough biomass to suppress the weeds, they’ll take over your cash crop and cause a lot of problems in a very short time,” she says.

It’s early days for this research, but both Halde and Marshall are enthusiastic about the potential for mulch-based weed control in organic and conventional systems alike. “In conventional systems you can use different crops to get more consistent mulch levels, which has a lot of potential to help with long-term control,” says Marshall.

March 29, 2016  By Julienne Isaacs