Clover:cereal bi-cropping for organic farms (OF0173)

Summary

This is the final report of Defra project OF0173. The attached main report starts with a more detailed summary, from which this text is extracted.
IGER, IACR and others developed a system for growing cereals, especially for whole crop silage, that enables either greatly reduced or zero levels of N fertilizer and agrochemicals to be used. The system is simple and straightforward and relies on direct drilling of the cereal into an established understorey of white clover. The white clover understorey becomes permanent and perennial and successive cereal crops are drilled into it, harvested and re-drilled each year. The clover provides N for the cereal crop and the gross changes in crop architecture that occur (a) encourage large populations of predatory invertebrates that devour pest species and hence obviate the need for insecticide use (b) frustrate the spread of fungal foliar diseases and remove the need to use fungicides and (c) suppress most weeds.
Some of the advantages of the clover:cereal bi-cropping system that are especially relevant to organic farms are
a) simultaneous cropping and fertility building rather than separate seasons for each
b) effective nutrient cycling
c) weeds replaced by clover; limitation of crop area requiring intensive weed control
d) confusion of insect pests and habitat for beneficial insects, spiders etc.
e) improved field access relative to bare soil
The work investigated the agronomic viability and sustainability of bi-cropping for both silage and grain production in organic farming systems as specified under UKROFS. Experiments were carried out on three UKROF approved sites with contrasting soil types and environmental conditions. Randomised block, small-plot experiments evaluated the most satisfactory ways of modifying the IGER/IACR system for organic farming.
The main point to emerge is that although the bi-crop system is well proven and developed for use in a non-organic system, grass weeds remain a problem in translating the system to organic farming as herbicides can not be used to control them. There are a number of strategies that emerged during the course of the present work that indicated ways in which the grass weed problem may be overcome in future. Firstly, oats seem to suppress grass weeds and growing this crop instead of wheat appeared to show considerable promise. Secondly strip drilling alternate 20 cm strips of cereal with 30 cm strips of clover allows separate management of the two crop components, facilitating weed control. The work confirmed that spring-sown cereals are not an option for bi-cropping as they are easily outcompeted by the clover.
An important positive feature of all of the trials was the consistent absence or low levels of diseases and pests despite high levels of airborne pathogen inoculum in the trial area. It is difficult to gauge the relative contributions of the organic system as a whole and the bi-crop system in particular to this feature. There are certainly important theoretical reasons as to why the bi-crop system should help in this direction, including the restriction on spread of splash-borne diseases because of the presence of clover around the cereal plants, green background confusion of insect pests and the probable lack of surplus soluble nitrogen in the cereal plants.
The more positive results in the absence of grass weeds were limited to a single year for cereals (and two seasons for vegetables in another related trial). However, in both cases, there were clear indications that, with a relatively small amount of fine tuning of the system, it should be possible to obtain highly acceptable results from both types of inter-cropping and, indeed, from rotational integration of the two. Further progress would certainly be worthwhile for the organic producer because it seems clear that a modest further adjustment of the competitive balance between crop and clover will lead to a clearer expression of all of the potential advantages of such systems, outlined above. Despite the difficulties encountered, progress was made and the list of advantages and potential advantages of bi-cropping for organic agriculture is so large and significant that further work should be done to capitalise on that completed to date.