Producers may be able to manage weeds with sweet clover in a reduced tillage system, says a new study from Agriculture Canada’s research centre in Lethbridge.
Scientists Jim Moyer, Bob Blackshaw and Henry Huang found that killing sweet clover with a wide blade cultivator and leaving the residue on the surface could suppress weeds and in some cases virtually eliminate them for the rest of the season.
Legume-based green manure is a vital tool for improving soil fertility and suppressing weeds. Traditionally, organic producers have disced or cultivated to terminate green manure and incorporate it into the soil. The Lethbridge study asked if green manure could fit into a system with less soil disturbance.
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Sweet clover was terminated with a wide blade cultivator that minimizes soil disturbance and leaves the killed crop standing. Residues were either removed or left on the soil surface.
Five types of sweet clover were used in the study, including yellow and white and both high and low coumarin levels. It was seeded with wheat in its first year. It was terminated in the second year at either the bud to 20 percent bloom stage in early June or at the 70 percent to full bloom stage in late June or early July.
In the establishment year, yellow sweet clover produced greater biomass than white. Common sweet clover was the only variety that significantly suppressed weeds in its establishment year. Weed biomass with common sweet clover was less than half of what it was without.
White sweet clover did not suppress wheat yields in its first year, but yellow suppressed wheat yields 12 to 20 percent.
Weeds in the Lethbridge test plots included flixweed, stinkweed, prickly lettuce and wild oats. Most were suppressed by the growth of most types of sweet clover, but some combinations were more effective than others. For instance, Norgold sweet clover suppressed prickly lettuce but not flixweed or stinkweed.
Sweet clover residues left on the soil surface after termination at the 70 percent bloom stage suppressed the growth of fall emerging winter annuals and new annual weeds, especially lamb’s-quarters. Weed suppression was best when there was either a dense growth of live sweet clover or a thick layer of residue.
Sweet clover in this test did not increase available soil nitrogen above the amount available in black cultivated fallow plots. Generally, more nitrogen is available after sweet clover is killed by cultivation, which suggests residues laying on the surface do not provide as much available nitrogen to the soil as those incorporated into the soil by tillage.
In the Lethbridge study, sweet clover did not increase the availability of soil phosphorus. The results may have been different if a low phosphorus soil had been used for the test.
Sweet clover’s use of moisture differed from year to year. In some cases it used up to 10 millimetres of soil moisture compared to checks.
Wheat yields following sweet clover were not reduced by this moisture loss, even in a drought year. The moisture used by the growing sweet clover was regained by reduced moisture loss from the soil because of the residue layer.
Other studies suggest that green manures should be terminated before mid-June to avoid excess moisture loss in low moisture years.
Weeds in the wheat crop that followed green manure were generally fewer if the sweet clover had been terminated at the 70 percent bloom stage, with residues left on the soil surface. Yukon, a high coumarin yellow variety produced large biomass and consistently suppressed weeds both in the sweet clover years and the following year.
An ideal green manure would suppress weeds, provide sufficient nitrogen for the following crop, provide cover to protect soil from erosion and recapture any moisture used during its growth.
Traditionally, sweet clover has been the green manure most commonly used by organic producers on the Prairies. The Lethbridge study suggests that organic producers may find new ways to incorporate sweet clover into their reduced tillage systems.
Frick is the prairie co-ordinator for the Organic Agriculture Centre of Canada located at the University of Saskatchewan. She can be reached at 306-966-4975, at brenda.frick@usask.ca, or www.organicagcentre.ca.
