Environmental factors had the most impact on organic crop performance during a fifteen year study into the sustainability of organic stockless arable rotations.

But establishing fertility-building crops and controlling perennial weeds emerged as major management challenges during the study, ADAS’s Bill Cormack told an Association of Applied Biologists organic farming conference.

And poorer prices for organic crops since 1999 had made stockless organic arable farming less attractive.

Funded by DEFRA, the study took place at ADAS Terrington in Norfolk, on a fertile deep, stoneless silty clay loam. The rotation started with fertility-building white clover, followed by potatoes, winter wheat, spring beans and spring cereals undersown with clover.

Fertility-building crops were mown between one and three times a year in response to growth rate and the mowings were left as a mulch, and aluminium calcium phosphate (14% P) “Redslaag” was applied once per rotation from 1995 at 625kg/ha.

Crop yields were good, particularly for cereals, Dr Cormack said. “There was no evidence of either a post-conversion adjustment period or a fall in yield due to declining fertility.”

Hereward winter wheat yielded an average of 7t/ha with potato yields around 35t/ha – saleable yields averaging 23t/ha.

The latter were very variable, observed Dr Cormack, reflecting the effect of rainfall both directly on growth and yield and indirectly on the activity of slugs and blight. In the wet year of 1993 half of the yield was lost due to slug damage. Following clover with potatoes encouraged slugs, he said.

Beans generally established and grew well with yields of over 3t/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.

On average the spring cereal yielded considerably less than the winter wheat. This, said Dr Cormack, was expected as it was at the end of the crop sequence when nitrogen availability would be least. Foliar disease levels were low, and as for winter wheat, were unlikely to have been limiting to yield.

Fertility-building clover crops were the most difficult to establish and failed completely in some years despite one or two re-sowings. Over the 15 years of the project clover failed to establish in four of the eleven years from 1995-2005. Vetch was sown in spring as a replacement in three of the years but it proved slow to establish, competed poorly with weeds and had considerably lower accumulated nitrogen in the mulched foliage.

The results suggested new designs of stockless rotation were needed with better integration of fertility-building, he said.

Even so, the performance of the following crops was not clearly related to the poor performance of fertility-building crops. Environmental conditions proved to have a much greater influence on organic crop performance, particularly on fertile soils, he said.

“Because of the soil texture it was often too wet to travel when crops and weeds were suitable for mechanical weeding.”

Spring-sown crops and beans hosted the most weeds with the winter wheat and clover having the least. The biggest problem was the increase in the perennial species creeping thistle and docks.  Creeping thistle was particularly problematic and increased progressively despite mechanical and hand weeding.

The stockless rotation was substantially more profitable than comparable conventional farms during the 1990s but from 1999 relative profitability declined principally due to the decline in organic crop prices.

“In 1999 winter wheat was fetching £200/t but since then price has drifted downwards and conventional price has increased so the differential is less,” he said.

Lower organic crop prices in the new century had made conversion to stockless organic a less attractive option for farmers, he said.

“The introduction of the Single Farm Payment in 2005 has further changed the economic picture making the inclusion of high proportions of dedicated fertility-building crops look even less attractive,” said Dr Cormack.