There are many benefits to growing sweet clover as a cover crop, but its place in the crop rotation must be planned because it can also harm some crops, say scientists at the Lethbridge, Alta., research centre.
Clover has been used to conserve soil moisture on the Prairies for years, but this is the first time scientists have collected data that measures the crop’s ability to control weeds.
In rotation
In this particular study, scientists grew sweet clover as a cover crop in a rotation with peas, flax, mustard and wheat. They found sweet clover controlled weeds, fixed nitrogen and conserved soil by providing an excellent plant residue cover.
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“We found that there’s real promise in terms of agronomics, but we also found that growing certain crops with sweet clover results in large reductions in yield,” said lead researcher Jim Moyer.
“For example, we found sweet clover was too competitive with peas and flax and actually reduced yields. On the positive side, the mustard yield was satisfactory and with wheat, we had no significant yield reduction at all.”
In the second year, sweet clover and its residues maintained excellent weed control without cultivation or herbicide application. On summerfallow without sweet clover there was a dense weed population.
In the beginning of the third year, after a fallow year, researchers measured the soil moisture after various sweet clover treatments. They also measured the nitrogen available in the spring and did weed counts.
Scientists noted a large population of weeds, such as kochia and dandelions, in plots without sweet clover the previous season. These weeds were significantly reduced in the plots where there was sweet clover residue from the year before.
They also observed that Canada thistle growth seemed to depend on which crop the sweet clover had started with. Moyer said the plots with sweet clover generally had less Canada thistle than the plots without sweet clover.
Weeds were significantly reduced in the sweet clover plots with wheat compared to wheat following summerfallow with no sweet clover.
In terms of soil moisture, overall, plots with sweet clover had a slight increase in moisture at the surface. The moisture decreased progressively at lower depths.
Researchers also recorded 50 percent more nitrogen in the soil when sweet clover was included in the rotation.
Sweet clover rotated with wheat led to a significant jump in wheat yield compared to when sweet clover was not added to the rotation.
“A small amount of nitrogen was added with the wheat seed at planting time,” Moyer said.
“But it wasn’t enough to grow crops. The performance of our wheat crops was heavily dependent on the nitrogen the crop was getting from sweet clover.”
Promising crop
Although researchers recorded reduced yields in flax and peas when those crops were grown with sweet clover, results generally indicated that sweet clover is a promising cover crop.
Scientists observed generally lower weed density over the entire three years of the study, increased surface soil moisture, more plant-available nitrogen and increased yield. They also found that plant residue in the plots with sweet clover was high enough to prevent soil erosion.
