VANCOUVER — Organic crops have more weeds than conventional crops, and organic crops yield less than conventional.
As a result, many producers often assume that weeds cut into production, which might explain the yield gap between the two systems.
However, Dilshan Benaragama, a University of Saskatchewan PhD candidate in plant science, said the assumption is false.
“That’s a misconception. We think that when you have more weed density there’s going to be a big yield loss,” Benaragama said during the Weed Science Society of America’s annual conference in Vancouver Feb. 4.
Read Also
Message to provincial agriculture ministers: focus on international trade
International trade stakeholders said securing markets in the face of increasing protectionism should be the key priority for Canada’s agriculture ministers.
“Organic systems have lower yields (even when there are) no weeds.”
Benaragama, who participated in the graduate student oral presentation contest at the conference, conducted plot trials in 2011 and 2012 at Agriculture Canada’s research centre in Saskatoon to evaluate how weed competition and rotation influences the yield of organic and conventional crops.
He said studies suggest that organic systems yield five to 30 percent less than conventional systems.
Benaragama considered three crop rotations in his study: high diversity with annuals and perennials, low diversity with wheat-canola and a diverse annual system with cereals, canola and pulses.
He also layered four weed scenarios on top of those rotations: weed free (hand weed), no weed control, herbicides and seeding tame oats into plots to simulate a high level of weed pressure.
He determined that diverse rotations do reduce weed pressure, but organic yields remained below conventional levels.
“No matter what the rotation is, organic has low grain yield compared with conventional.”
As well, he said organic yields were lower than conventional regardless of weed treatment.
“Under weed free conditions, organic has lower yield. No matter (if) weeds are present or not, still, organic systems are having (lower) yield,” he said in a later interview.
“What this says is that crop weed competition is not a limiting factor on these (organic) systems.”
Benaragama said weeds do reduce crop yields in organic and conventional systems.
In certain treatments within his experiment, he found that weed pressure hindered conventional crops more than organic, reducing yields by 57 percent in conventional and 41 percent in organic.
He said the lower organic yields, whether weeds were controlled or not, suggest that other factors are responsible for the yield gap, possibly soil nutrients.
“In our cropping system in Saskatchewan, soil fertility is the main problem.”
Steve Shirtliffe, a U of S plant science professor and Benaragama’s adviser, agreed that inadequate soil fertility often restricts organic yields.
“You almost always see a yield penalty with organic rotations unless they’re really heavily manured … (but) in those situations they can yield even better.”
John Heard, a Manitoba Agriculture soil fertility specialist, said University of Manitoba research indicates that phosphorus becomes depleted in organic systems, but importing manure restores the system and enriches yields.
Benaragama said American organic farmers tend to add more inputs to their crops in the form of compost and manure.
