Reduced Till Termination of Sweet Clover

Organic Agriculture Centre of Canada

Farmers who wish to get the benefits of sweet clover in a conservation tillage system will find encouragement in a new study from the Lethbridge Research Centre. Bob Blackshaw and his colleagues show that mowing can be an effective alternative to tillage for sweet clover termination.

Sweet clover is an excellent green manure option that many organic producers choose. Seed costs are low, it competes well with weeds and, as a biennial, it can be underseeded with a cereal, eliminating separate seeding costs in the green manure year. Sweet clover produces abundant biomass and fixes abundant nitrogen. One limitation in its use has been the need for tillage to kill the plants and incorporate them into the soil. 

In their study, Blackshaw and colleagues terminated sweet clover plants at the 80% bloom stage by one of four methods: incorporation with a mouldboard plough, incorporation with an offset disc, mowing and leaving the residues in place, and mowing and removing the top growth as hay. They found that all of these methods were equally effective in killing the sweet clover.

One of the concerns in using sweet clover in the brown soil zones is its potential to dry out the soil. Mowing retained as much, or more, moisture than the tillage options. Perhaps the residues on the surface helped with snow trap, or reduced evaporation from the surface. The ploughed treatment was drier in the spring after sweet clover, in one of two trials.

Of course green manures are intended to give nitrogen benefits. In the Lethbridge study, the greatest amount of available soil nitrogen in the spring after sweet clover was found in the ploughed treatment. The plough buried more than 95% of the sweet clover, and this may have allowed more of the nitrogen to be retained in the soil. The offset disc buried 50 to 60% of the sweet clover. There were no differences in available nitrogen between mowed (which buried none) and disced treatments. When hay was removed, the available nitrogen was reduced in one of the two trials.

Green manures are also useful in their ability to reduce weed populations. Sweet clover offers strong competition with weeds during its growth. After termination, residues can continue to have an effect. In both trials, mowing and ploughing were successful at suppressing weeds in the fall after treatment and into the spring of the next year. Discing suppressed weeds in the fall, but the effect did not last until the spring.  In mowed plots, some of the suppression effect probably comes from the residues forming a mulch layer, reducing the light that reaches the soil. When residues were removed as hay from the plots, weed levels skyrocketed in the fall, and were high again in the spring.

Unfortunately, weeds were not the only thing suppressed by the sweet clover mulch. Wheat seeded the following year did not emerge as well in mowed plots as it did in the disced, ploughed or hay plots. The experiment was not able to determine if the residues interfered with seed placement or if they interfered with seed germination and emergence.  

Wheat yields following sweet clover were similar when sweet clover was ploughed and disced. In one trial the plots that were mowed yielded equally well with the ploughed and disced plots. In the other trial, yield was suppressed in the mowed plots. In both years, the plots where sweet clover hay was removed yielded less well.

Protein levels in the wheat grain were highest in the ploughed plots in one trial, and similar in the ploughed, disced and mowed plots in the other trial. Protein levels in the plots where sweetclover hay was removed were lowest.

The yield and protein levels show that mowing can be as effective as incorporation of residues, but that it carries higher risk.  Environmental conditions from year to year determine if the wheat in mowed plots can catch up, from its initial disadvantage at emergence, to yield as well and have as much protein as wheat in plots where the sweet clover residues were incorporated.

Currently most sweet clover green manures are killed by discing. This study suggests that mowing can be equally effective at preventing sweet clover regrowth, reducing weeds and retaining soil moisture. Mowing would be less expensive, faster and would reduce the risk of soil erosion.  Further research is required to find ways to retain these benefits while still reducing the risk of yield and quality reductions.

Written by Brenda Frick for OACC. For more information: 902-893-7256 or

Posted April 2010