Farmers who want the benefits of sweet clover in a conservation tillage system will be encouraged by 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 chosen by many organic producers. 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. It also produces abundant biomass and fixes abundant nitrogen.
One limitation 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 percent bloom stage by one of four methods: incorporation with a mouldboard plow, incorporation with an offset disc, mowing and leaving the residue in place, and mowing and removing the top growth as hay.
They found that all methods were equally effective in killing the sweet clover.
One concern 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 residue on the surface helped trap snow or reduce evaporation.
The plowed treatment was drier in the spring after sweet clover in one of two trials.
One of the main reasons for green manure is to provide nitrogen benefits, and the Lethbridge study found that the greatest amount of available soil nitrogen in the spring after sweet clover was found in the plowed treatment.
The plow buried more than 95 percent of the sweet clover, which may have allowed more of the nitrogen to be retained in the soil. The offset disc buried 50 to 60 percent.
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 manure also reduces weed populations.
Sweet clover offers strong competition with weeds during its growth and continues to have an effect as residue after termination.
In both trials, mowing and plowing suppressed 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 spring.
In mowed plots, some of the suppression effect probably comes from the residue forming a mulch layer, which reduces the light that reaches the soil.
When residue was removed from the plots as hay, weed levels skyrocketed in the fall and were high again in the spring.
Unfortunately, the sweet clover mulch also suppressed the wheat crop that was seeded the following year. It did not emerge as well in mowed plots as it did in the disced, plowed or hay plots.
The experiment was not able to determine if the residues interfered with seed placement or with seed germination and emergence.
Wheat yields following sweet clover were similar when sweet clover was plowed and disced. In one trial, the plots that were mowed yielded equally well with the plowed 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 plowed plots in one trial and similar in the plowed, disced and mowed plots in the other trial. Protein levels were lowest in the plots where sweet clover hay was removed.
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.
Most sweet clover green manures are killed by discing, but 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.
Brenda Frick is affiliated with the Organic Agriculture Centre of Canada and co-ordinator of organic research and extension at the University of Saskatchewan. Contact: firstname.lastname@example.org.