Hi All, I wanted to share with you a recent review paper on the use of living mulches that was shared with me by Mark Lea who has been very much one of the pioneers of the approach, running trials over the last three seasons to investigate how to make the system work on his farm.
The paper below outlines how to introduce living mulches into the system, evaluates the practice in terms of its advantages and disadvantages and explores the strategies for managing competition between mulch and crop. Although the review is focussed on non-organic farming and the need to reduce mineral nitrogen fertiliser and herbicide inputs to improve sustainability of crop production, there is still a lot of useful information to help apply the cropping practice to organic conditions. There are also one or two organic experiments that have been included in the review that show forage legumes can help suppress weeds and that realy intercropping the legume with winter wheat can be successful as an establishment technique (Amossé et al. 2013a) .
The review considers perennial legumes that are well established once a cereal is sown as a true living mulch system, and looks at winter cereals. Simultaneous sowing of perennial legumes and cereals or undersowing are considered to be relay intercropping. So for example, a clover undersown into spring barley is a relay intercrop, but after the barley is harvested and the clover is well established, sowing a winter cereal into this would be considered a living mulch.
The review explores the advantages of the system including N fixation and transfer to the crop, weed suppression, soil conservation and reduced nitrate leaching. It also examines the competition between the mulch and crop for light, water and nitrogen as a major challenge and disadvantage of the system. Of particular relevance over the last few seasons is the increasing water deficit that has occurred in spring and the potential for a living mulch with a well established root system to exacerbate this deficit, reducing moisture for the cereal crop. The advanced stage and rooting depth of the legume at the time the cereal is sown explains why the legumes compete for soil N in the system.
The review highlights the key levers to controlling the competition from the legume, and whilst herbicides are not an option, mechanical control and crop density and spatial distribution can help. Partial destruction of the living mulch at cereal sowing can really reduce early competition, particularly through the use of strip tillage. A more extreme approach would be to cultivate the whole living mulch surface using a rigid tine or disc cultivator just ahead of cereal sowing. Autumn grazing of the mulch can also help reduce competition during the cereal establishment phase. Experiments have looked at mechanical control within crop and shown them to be successful at reducing the yield penalty for the cereal but so far this has not been studied at farm system scale. Increasing uniformity of crop spatial distribution and crop density can increase its competitive ability, but the effect is minimal compared to mechanical (and chemical) control since the established legume has an initial biomass advantage over the cereal. Crop species and variety can also help offer improved competition, due to differences in their competitive ability. Winter rye for instance is considered highly competitive. Cereal varieties that are taller and have higher ground cover can be expected to be more competitive with the mulch. Cereals with planophile (horizontal) leaves appear to have a particular advantage through intercepting more light. Mulch species and cultivar also has a major impact on competition with less competitive species needed to help reduce the cereal yield penalty. As well as white clover, birdsfoot trefoil appears to be a potential option as a living mulch. Subtarranean clover also appears to offer another option as a self-relay cropping annual.
The review looks at some of the traits needed for living mulch ideotype, with short prostrate types that are winter dormant and start growth late in spring with excellent disease and pest resistance particularly against root pathogens.
Key practical Recommendations
Sow perennial legume in spring, undersown in a companion crop e.g. spring barley/wheat, alternatively relay intercrop with winter wheat
Can sow the legume at the end of summer as a sole crop but it may be slow to establish and damaged by winter crop sowing
Perennial legumes can be broadcast or sown in rows, shortly before or after the companion crop is sown. Crops can be sown between living mulch rows using GPS guided precision drills.
Cut or graze the the living mulch very short before of soon after sowing to reduce competition for light with the emerging crop
In Autumn sow winter cereals into established living mulches with a no-till seed drill
Another option is cultivation of the living mulch in narrow bands using a rotary cultivator followed by sowing of the cereal in these cultivated strips
If possible control the mulch mechanically within crop
Select competitive cereal crop species/cultivars and less competitive mulch species/cultivars
The review concludes that despite the advantages of the system, the competition for resources leads to high cereal yield losses, and the management of competition between the crop and living mulch is the "cornerstone" of the system. However, managing the competition using current techniques is difficult. Unfortunately the review hypothesises that a significant breeding effort is required to produce the right cereal varieties and living mulches to make the system work.
Many questions remain unanswered including the economic profitability of the system and whether there is a positive effect on biodiversity, especially in the context of land sharing versus sparing. Through ley arable cropping and mechanical weed control organic farming has already increased the sustainability of cereal crop production, though reduced tillage within the rotation could improve this further. Growing organic cereals on a living legume mulch is possible but there is still a long way to go to optimise the system.