The Game & Wildlife Conservation Trust, a substantial research charity part-funded by shooting interests, says most of the stubbles encouraged by Entry Level Stewardship – the most basic grant for environmental effort by farmers – do not have enough seed in them to be much help.

Planting specifically for wild birds, at field margins, would be much better, says the trust.

Defra is under heavy pressure from wildlife experts to come up with 'son of set-aside' – a replacement for the European Commission's abandoned requirement to leave a percentage of all farms uncultivated.

The new report offers some support for the campaign but might also help the farmers' case.

The Game & Wildlife Conservation Trust employs dozens of researchers, including about 14 postgraduates at a time, and was hired by the Pesticides Safety Directorate to look into the knock-on effect of pesticide use on winter stubbles. A report on five years of investigation is now published in the journal Weed Research.

It says weed control using herbicides is usually so effective that few weeds survive to produce seed and modern harvesting machinery leaves very little spilt grain, so the stubble provides slim pickings for birds and mammals.

However, Higher Level Stewardship schemes, which restrict the use of weedkillers in the run-up to harvest, do leave enough weeds to make a difference.

John Holland, head of the Game & Wildlife Trust's farmland ecology unit, said this week: "Many agronomists might view these weedy stubbles as a threat, but our study shows that in fact weed control and conservation might not be mutually exclusive because seed predators, such as farmland birds and many small mammals, consume these seeds in winter, thereby carrying out the important function of natural weed control.

"However, we believe the most effective solution is to target wildlife management outside the crop and to make space for wildlife in areas specifically left for them. If you want to help birds, grow wild bird seed mixes and if you want to help rare arable flowers, then manage land specifically for that purpose by leaving it herbicide-free."

Gareth Morgan, the RSPB's top expert on agricultural impacts on birds, said: "Wherever its money comes from, the Game & Wildlife Conservation Trust does sound science on this sort of thing and this is a good study."

He said the findings could help the farmers as well as the RSPB. Set-aside rules had encouraged farmers to leave up to 10 per cent of their harvest stubble over winter, when they might have wanted to plough it up and replant.

He commented: "Most of that over-wintered stubble was not doing much good. The benefits came from the sheer scale of the area involved. So any new scheme need not involve anything like as much land for similar benefits for wildlife."