Assessing the sustainability of a stockless arable rotation (OF0318)

Summary

The majority of organic conversions have been on upland grass-based livestock farms resulting in a shortage of UK-grown cereals and pulses. Across northern Europe, conventional agriculture has developed a specialised structure with many areas having no livestock, or knowledge or facilities to support livestock. Sustainable organic production on these farms has particular challenges for nutrient supply, for the management of weeds, pests and diseases and for economically viability.
This report covers a three and a half year period (four crop harvests) of a longer-term programme of work, which began in 1990. The overall objective was to identify and overcome factors limiting the sustainability of a stockless arable rotation. Specific objectives were to 1) quantify crop yield and quality, 2) quantify the financial performance of the rotation and contrast with Farm Business Survey data from comparable non-organic arable farms, 3) to monitor and evaluate indicators of sustainability; particularly for nutrients, weeds, pests and diseases and 4) to communicate results, and their implications, to Defra and other stakeholders. These were all achieved.
The study was located on the ADAS Research Centre at Terrington St Clement, Norfolk. The soil is a deep stoneless silty clay loam of the Wisbech series. It is derived from marine alluvium, has a naturally high pH of around 7.5 and is retentive of water and nutrients. The project was an unreplicated study with field-scale plots. There were five plots, each of 2 ha, each in a different phase of a five-year rotation. This design, with no replication within-year, whilst allowing more meaningful crop husbandry and economic evaluation than a conventional small-plot replicated experiment, did limit statistical analysis. This was a deliberate choice as economic evaluation and demonstration were the principal initial objectives of the project. Despite this limitation, it has provided a long-term data-set gathered under realistic conditions that gave useful indicators of crop performance in a stockless rotation, and provided data for other Defra-funded projects, such as economic modelling.
The project was managed by a steering group chaired by the Defra project officer. Other members included Roger Unwin, policy adviser on soil protection and organic farming in the Defra Rural Development Service, three organic farmers, and representatives of the Soil Association and Elm Farm Research Centre (EFRC). Meetings were held twice per year. Detailed written reports were submitted to each meeting for discussion. All changes to cropping and management were agreed by the group.
The crop sequence from 2002 to 2004 was: potatoes and calabrese (split ⅔ potatoes and ⅓ calabrese)  winter wheat  spring beans  spring wheat (undersown)  white clover (fertility building crop). The clover was mown three to four times per year and left as a mulch. Organic registration was with the Soil Association. All crop management operations were done using normal farm machinery; the aim was to simulate typical commercial practice. No fertilisers were applied apart from aluminium calcium phosphate (“Redslaag”) once per rotation, at 625 kg/ha. No irrigation was applied.
Undersown white clover thrived only in one year (2002) out of five. Establishment was affected variously by slug grazing, insufficient rain, and excessive rain. It was successfully re-sown in 2003 but had to be replaced by spring sown vetch in 2004 and 2005. Of all the crops grown post-conversion, fertility-building crops have been the most difficult to establish. Over the life of the programme, clover failed to successfully establish in four of the eleven years from 1995 to 2005, even when re-sown in some years. Vetch was sown in spring as a (reputedly) rapidly-growing replacement in three of the years but it was slow to establish, competed poorly with weeds and had a considerably lower accumulated nitrogen in the mulched foliage. The mean accumulated nitrogen in vetch was 102 kg/ha (range 90 to 121), whereas in clover it was 175 kg/ha (range 0 to 274). Despite the poor performance of some of the fertility-building crops, this was not clearly reflected in the performance of following cash crops. This conclusion was supported by results from replicated experiments comparing legume species over different seasons, done at Terrington in the 1990s as part of this programme (and published in a peer reviewed journal), and by results from a stockless arable study on fertile soil in Germany.
Yield of winter wheat was good with an average of 7.0 t/ha; this compares with a typical organic yield of 4.0 t/ha. Yield was relatively consistent, and reflected environmental conditions, with lowest yields in the very dry 1995 and the wet and dull 1997 and 2001. The highest yield of 9.8 t/ha was in 1996 when a dry and sunny early summer was followed by rain in July and August ensuring good grain fill. Grain nitrogen content ranged from 1.7% (1995 and 1997) to 2.2% (2003). This was generally below the 2.2 to 3.3% typically required for bread making and as a result it was sold for organic livestock feed. Potato saleable yield was very variable (from 7 to 40 t/ha) depending on the impact of rainfall pattern, slugs and blight. Beans generally established and grew well with yields of over 3 t/ha in all but two years. Yield was reduced in 1995 by drought, in 1997 by poor pollination and pod set in a very dull June, and in 2004 and 2005 by weeds. The spring cereal yielded considerably less than the winter wheat. This was expected as it was at the end of the crop sequence when nitrogen availability would be least.
Crop prices declined through the programme, particularly from 1999 (e.g. winter wheat price fell from £205/t in 1996 to £129/t in 2005). Non-organic prices also declined, but at a slower rate. The stockless rotation had a substantially greater gross margin than comparable non-organic farms until 2000 (e.g. £1,881/ha vs. £601/ha in 1997). From 2000, the advantage decreased as prices fell. From 2003 to 2005, gross margin was less than for non-organic farms (£561/ha vs. £601/ha in 2005). High margin crops such as potatoes and calabrese made a large contribution to the rotation gross margin and were necessary to balance the low income from the 20% of fertility-building crops in the stockless rotation. The introduction of the Single Payment Scheme in 2005 changed the economic picture making the inclusion of 20% fertility-building crops look even less attractive. New designs of stockless rotation are needed with better integration of fertility-building, and ideally with all crops earning revenue from sales.
Relatively stable crop yields and nitrogen contents suggested that the rotation was in balance for nitrogen. However, crop offtake of nitrogen was substantially greater than the estimate of nitrogen supply from the fertility-building crop mulched foliage, suggesting that this measure was underestimating nitrogen supply. Soil available potassium was consistently in Index 2 despite no additions, showing that the clay minerals were releasing potassium to replace offtake. Soil available phosphorus declined sharply initially and calcium ammonium phosphate was applied annually from 1995. It continued to decline but more slowly, and by 2005 was just in Index 1. Soil carbon showed a slow upward trend. In the longer term, sustainable additions of plant-available phosphorus and potassium will be needed in a stockless rotation, even on nutrient retentive and potassium rich soils as at ADAS Terrington.
Diseases and pests had little impact on cereals and beans. Significant pest issues were slugs on potatoes, calabrese and clover, and stem nematodes and Sitona spp. weevils on clover. The lack of an organic control for slugs makes production of root and vegetable crops on such slug-prone soils unreliable. Strategies to avoid stem nematode, such as alternating clover species, should be adopted in stockless systems with a high frequency of clover crops.
Total numbers of weeds did not increase, but the dominant species of annual weeds changed from those of autumn-sown arable crops to those of spring-sown crops. Perennial weeds, particularly creeping thistles, progressively increased from an initial sparse and patchy distribution to cover the whole study area at a dense population. By 2005, it was clear that the creeping thistle population could not be managed without substantial change to the rotation, such as the introduction of a cultivated fallow or a longer fertility building period, both of which would be costly. Strategies to avoid domination by perennial weeds should be put in place at the start of conversion. That would be more likely to succeed in the long term rather that trying to contain a well-established population at a manageable level. Any new rotations designed in response to the introduction of the Single Payment Scheme would also have to be more competitive against perennial weeds.
Two papers were published in peer-reviewed journals, and papers were presented at two Colloquium of Organic Researchers conferences. A web site dedicated to the project was established and maintained.
Further research is needed on:
1) An integrated strategy for the management of perennial weeds, particularly to prevent infestations spreading. This is likely to require research to clarify aspects of weed biology and spread mechanisms, and the interaction of those with crop husbandry practices;
2) Reliable and affordable controls for slugs, particularly to protect product quality in root and vegetable crops;
3) New designs of rotation with fertility building better integrated with cash crops to a) eliminate unproductive and failure-prone dedicated fertility-building crops, and b) offer better competitive ability against perennial weeds.