An old prairie crop is showing new benefits to improve farmers’ bottom
lines and boost the health of their soil.
New studies at Agriculture Canada’s Lethbridge Research Centre show
that using sweet clover as a fallow replacement can increase soil
nitrogen and dramatically reduce weeds, resulting in higher subsequent
crop yields and lower fertilizer and herbicide requirements.
This adds to the fundamental benefit of cover crops to greatly reduce
the risk of soil erosion, ensuring healthier, more productive land over
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“Farmers are always looking for ways to get more out of their crop
rotations and we believe there’s a huge untapped scope for using cover
crops,” says Bob Blackshaw, a weed scientist who led the studies along
with Jim Moyer.
“Sweet clover is just one example. This crop has been around a long
time on the Canadian prairie, but its advantages as a fallow
replacement are only now fully coming to light.”
Fallow is traditionally used to conserve soil moisture, but the trade
off is greater soil erosion and reduced soil quality, Blackshaw said.
Finding ways to reduce fallow has been a major research thrust at
Agriculture Canada over the past 20 years.
“The shift to low till and direct seeding systems, along with improved
rotations, have reduced fallow by roughly 60 percent over the past two
decades,” he said.
“The next step is to look at systems that still have fallow, and find
better ways of managing that fallow year.”
Over three cycles of a three-year experiment, the researchers examined
the benefits of yellow sweet clover undersown in field peas, mustard
and flax. Sweet clover provided excellent groundcover throughout the
20-month fallow period and compared favourably to straight fallow in
several key categories. It boosted soil nitrogen by 14 to 50 pounds per
acre, which increased wheat yields in the third year. Sweet clover also
had a tremendous impact on weed suppression, reducing overall weed
densities by 75 to 97 percent.
“We were amazed at the weed results,” he said.
“We thought sweet clover would help suppress weeds, but I don’t think
we had any idea we could get this kind of powerful, long-lasting weed
control. The results suggest this is at least partly due to
allelopathic compounds released from decomposing sweet clover.”
Sweet clover’s potential to deplete moisture was a major concern
heading into the study, but the effects of the drought-tolerant crop
were minimal, he said.
“We were pleasantly surprised. We did have less soil moisture in the
first summer, but we ended up trapping more snow in the winter, which
tended to equalize things out. In years with reasonable moisture, the
difference will not be enough to significantly affect the following
crop.”
Sweet clover is likely to also work well as a companion with wheat or
barley, he said. The crop can slightly hinder weak competitors such as
flax, but this is unlikely with hardier cereals.
Sweet clover is seeded at the same time as the main crop and “continues
to grow into the fall after the main crop is harvested – so right away
you’ve got fantastic ground cover. It survives the winter and traps
snow, and resumes growth in early spring.”
Producers can terminate growth at the bloom stage, which is usually
around the end of June or early July.
Blackshaw said organic producers widely use sweet clover as a green
manure crop. The crop’s weed-suppressing ability will add to their
management flexibility. Sweet clover weevil is a potential pest, but it
is rare where sweet clover is grown periodically.
