AMSTERDAM, June 13 (Reuters) - Farmers in the United States planting Monsanto's genetically modified (GM) soy reduced their use of herbicides by an average of about 10 percent, Dutch researchers said on Wednesday.

This was less than the 30 percent reduction originally hoped for by planting Monsanto's Roundup Ready soy, said a statement by the Centre for Agriculture and Environment.

The Roundup glyphosate herbicide used in conjuntion with Roundup Ready soy also had a lower environmental burden than conventional herbicides, consultant Piet Schenkelaars, co-author of the study, told Reuters.

Glyphosate is not an ideal herbicide, however, since run-off might contaminate surface water and could have negative effects on water organisms, the report added.

The study -- carried out on behalf of three government-linked product boards for vegetable oils, grains and animal feed -- analysed field data on the effects of GM soy cultivation in the United States.

It found changes in herbicide use ranging from a fall of 40 percent to a rise of seven percent depending on region, year and farm size.

Harvests of Roundup Ready soy were comparable to conventional soy, but limited data meant no conclusions could be made about the impact of GM soy on use of energy, any changes in biodiversity and degree of resistance to the herbicide.

Schenkelaars said the data reviewed included a study cited by environmental group Greenpeace showing that farmers planting GM soy used more herbicide, but that study was rejected because the statistical methods were not sound.

He said data did not show significant economic benefits to farmers planting GM soy, who probably did so because it made weed control much easier.

Roundup Ready soy is resistant to the Roundup herbicide so farmers can spray it to eliminate weeds without harming crops.

The U.S. Agriculture Department has estimated that 63 percent of soybeans planted this year will be genetically modified, versus 54 percent last year.

© 2000 Reuters Limited. All rights reserved.