By Alex Kirby BBC News Online environment correspondent

Researchers say attempts in the Netherlands to make farming more environmentally friendly have not worked.

They say the fields affected are no richer in plant and bird species than conventionally farmed land.

With European Union (EU) spending on environmentally friendly farming set to double before long, the researchers urge a scientifically sound evaluation of its usefulness. The EU scheme began in 1992, but the Dutch experience goes back to 1981.

The researchers, from the nature conservation and plant ecology group at Wageningen University in the Netherlands, report their findings in the magazine Nature.

They evaluated agri-environment schemes' achievements in protecting biodiversity in intensively used Dutch farming landscapes by surveying plants, birds, hover flies and bees on 78 pairs of fields.

Fewer birds found

One of each pair was managed with environmental protection written into a management agreement. The other was managed conventionally.

The authors write: "Management agreements were not effective in protecting the species richness of the investigated groups: no positive effects on plant and bird species diversity were found.

Bees did better than birds

"The four most common wader species were observed even less frequently on fields with management agreements.

"By contrast, however, hover flies and bees showed modest increases in species richness on fields with management agreements."

Dutch farmland is important for meadow birds in general, and especially for waders. Most management agreements pay farmers to delay mowing or grazing their fields until June or July, to allow the chicks to hatch safely.

Many also pay them to delay mowing and grazing, and to restrict fertiliser use, to encourage wild plants along field edges.

Positive effects

With these agreements, the researchers say, they found no significant differences between the two types of field.

They write: "Fields with agreements that aimed to support wader populations by postponing first farming activities had lower counts of the target species, lapwing, oystercatcher, common redshank and black-tailed godwit.

"However, management agreements do have a positive effect on reproductive success."

Species richness of hover flies was higher on fields with management agreements than on control fields, and the same applied to bees.

The authors conclude: "Most farmers lack the knowledge to judge in what way measures taken to improve the economic position of their farm may interfere with nature conservation measures.

"Our findings indicate that it is imperative to evaluate current agri-environment schemes, and to ensure that any new scheme is accompanied by a scientifically sound evaluation plan."

Valid concept

Dr David Gibbons, of the UK's Royal Society for the Protection of Birds, told BBC News Online: "It's an interesting study, and I'm not entirely surprised at the findings.

"With plants, you'd have to reduce the nitrogen input much more than the Dutch farmers did.

Wild flowers fared no better

"With birds, breeding success wasn't necessarily feeding through into higher population numbers.

"The adult birds may have been taking the chicks elsewhere. You really need to look on a much bigger scale where birds are concerned.

"The study's useful: as it says, you have to monitor agri-environment schemes closely and adapt the details to make sure they're working.

"But I don't think it devalues the concept. The UK schemes are now much better than they were 15 years ago."

About 4% of the EU's common agricultural policy spending (1.7bn euros, more than �1bn) goes on agri-environment schemes, and the amount is expected to rise to 10% before long.