How to manage living mulch growth

Why is it important to suppress a living mulch?

A permanent living mulch comprised of clover amongst other plant species could provide many benefits; Soil cover year-round, as well as living roots, nitrogen fixing, and weed suppression. No-till and reduced till drilling are also possible directly into the mulch saving time, fuel, and labour.

However, with the living mulch being permanently established, it has the capacity to outcompete the cereal cash crop in early spring. So measures must be taken to ‘hold back’ spring growth in order to allow the cereal crop to get ahead and then shade the living mulch.

This balancing act presents one of the main challenges when working with a living mulch system. So, how can you suppress living mulches in order to enhance the benefits for the cash crop? Options range from grazing and machinery to varietal and development considerations.

The no-till with living mulches field lab has been building practical knowledge in this area.

When should I knock back the living mulch?

This depends on the drilling date, and how advanced the mulch is going into winter a decision much be made on pre or post drilling. A pre-drilling mowing might work better than post drilling and see the crop into the winter. The deep-rooted living mulch could start to grow mid-February, so a close mowing pre-drilling is important.

In the no-till with living mulches field lab, the triallists used their preferred method for mulch management. Of the successfully established mulches, two sites grazed the clover with sheep and one mowed. One of the triallists has adapted a CTM Weedsurfer to inter-row mow in the living mulch this year.

Inter-row mowing and tined weeders for living mulches

The main mechanical cultivation options are:

• Inter-row tined weeders can be used to disrupt the growth of the mulch. This would mean some soil disturbance, potentially leading to weed germination. Tines would need to be so designed not to gather plant debris. Machines currently available could be easily adapted.
• Inter-row mowers:
  • Rotary style mower units would be complex if hydraulically driven and have high power consumption
  • Electric drive is also a possibility. A tractor driven generator providing power to each mower unit could be cheaper to build, be more reliable and cheaper to maintain than hydraulic power.
  • Finger bar inter-row mowers are less complex to drive but could need regular maintenance to be effective in wet, matted winter growth of clover
  • Inter-row crimper rolls could be a way forwards as a way of retarding growth. But row widths are restrictive.
  • One of the living mulches field lab triallists is trying out an adapted Weed Surfer fitted with inverted plastic guttering, set to line up with the rows of cereal crop and sweep it down before the rotary mower blades mow everything including the standing living mulch cover crop. They reported that it works with multiple passes, but early passes are essential for the cereal crop to recover well.

The main challenges of interrow mowing a living mulch are row widths; the mower needs to be at least 6 meters wide due to ‘broad acre’ nature of cereal cropping. If we assume a 25cm row width for cereals then this allows a 20cm band for mowing. Many other drills have 16cm row spacing, driving a inter-row mower width of just 10cm, requiring 36 mower units across a 6meter working width.

Nitrogen release from a mown/tined clover crop could be an issue either by poor timing and/or lost nutrient.

Choosing the right variety of living mulch

Two stand out alternatives to mowing are grazing and varietal choice/development.

• Legume variety is important to ensure that the cash crop is not outcompeted. Low growing varieties of clover with more prostrate growth habits would provide less competition for cereal cash crops.
  • Trials at Cotswold Seeds’ Honeydale Farm found that trefoil was too strong and smothered the oat cash crop so added in some less aggressive small white clover. On their recommendation, the current field lab triallists are using a mix of wild white and small-medium white clover in a 70:30 ratio.
  • One of the field lab triallists found a few years ago that red clover outperformed white clover as a living mulch in wheat; soil cultivation was much easier and the wheat cash crop more green and lush when sown into a red clover stand than white clover, which may be due to better tilth. However the red clover competed more with the cash crop and caused problems at harvest, and was less competitive against weeds due to its upright rather than prostrate growth habit. Whilst logic would suggest that the red clover roots would cause more competition with the wheat crop than the shallower white clover roots, his was not observed. Their theory is that the white clover's greater, shallower root mass and large ground cover might be intercepting more moisture, preventing access to moisture for the wheat.  
• There is debate over whether the living mulch should be permanent or destroyed and re-established along with the cash crop every year. Long-established clover would always out-compete a newly established cereal crop, however the costs of annual re-seeding should be considered.

Grazing living mulches to manage growth

• Late autumn/early winter and early spring grazing would be beneficial to soil, crops and farmer gross margin if livestock are integrated into the rotation.
• As long as sheep are removed in good time, no yield penalty would be seen in the cereal cash crop. Don’t be afraid to graze hard or even top if the mulch isn’t knocked back enough; the key is to ensure it doesn’t outcompete the cereal crop.
• Some fields and farms would need a lot of work to keep the sheep grazing in the target areas.
• A separate field lab is exploring how grazing sheep on winder cover crops impacts the uptake of nutrients by sheep, the overall impact on their performance and health, and key soil health metrics. Find out more here.

Chemical control

Clover fixes its own nitrogen so doesn’t do well under added inorganic nitrogen, which prevents the natural production of nitrogen via leguminous plants. Conventional farmers could apply nitrogen fertiliser and herbicides to selectively weaken the living mulch, however this option is not available for organic farmers or those looking to living mulches to reduce chemical inputs.

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