Federal and state government agencies, technology developers, universities and other stakeholders should collaborate to document weed resistance problems and develop cost-effective ways to control weeds in current GM crops and new types of herbicide-resistant GM plants that are now under development.

Impacts< p>

Several NRC reports have addressed the effects of GM crops on the environment and human health, but the latest, The Impact of Genetically Engineered Crops on Farm Sustainability in the United States, is the first comprehensive assessment of the environmental, economic and social impacts of GM crops on US farms.

Ervin noted: "As more GM traits are developed and incorporated into a larger variety of crops, it's increasingly essential that we gain a better understanding of how genetic engineering technology will affect US agriculture and the environment now and in the future. Such gaps in our knowledge are preventing a full assessment of the environmental, economic and other impacts of GM crops on farm sustainability."

Improvement s in water quality could prove to be the greatest benefit of GM crops, the report says. However, Wolfenbarger noted that no infrastructure exists to track and analyse the effects GM crops may have on water quality.

The US Geological Survey, along with other federal and state environmental agencies, should be provided with financial resources to document the effects of GM crops on US watersheds, the NRC report notes.

In many cases, farmers who adopted GM crops have either lower production costs or higher yields - or both - due to more cost-effective weed and insect control and fewer losses from insect damage, the report says. Although the farmers have gained such economic benefits, more research is needed on the extent to which these advantages will change as pests adapt to GM crops, as other countries adopt biotechnology and as more GM traits are incorporated into existing and new crops.

The economic effects of GM crops on farmers who grow organic and conventional crops also warrant further study, the report says. For instance, organic farmers profit by marketing their crops as non-GM, but their crops' value could be jeopardised if genes from GM crops taint non-GM varieties through cross-pollination or seed mingling.

Some farmers have expressed concern that consolidation of the US seed market will make it harder to purchase conventional seeds or those with only specific GM traits.

With the exception of seed industry consolidation, the effects GM crops have on other social factors of farming such as labor dynamics, farm structure or community viability have largely been overlooked, the report says. More research is needed on the range of effects GM crops have on all farmers, including those who don't grow GM crops or those with less access to credit.

Studies also should examine impacts on industries that rely on GM products, such as the livestock industry, the report suggests.

Sharon Bomer Lauritsen, Biotechnology Industry Organization (BIO) executive vice president for food and agriculture, said BIO supports the NRC recommendation for continued study of potential weed resistance problems and further development of resistance management practices.

"While this is a matter that warrants monitoring, it is not one exclusive to GM crops but a common problem facing all farmers who use herbicides," she said.

"BIO supports public-sector researchers having the opportunity to conduct studies on commercially available, patent-protected seed products to understand the technology, education, extension and the safe and effective use of these products," Bomer Lauritsen said.