Monsanto has hit back at claims that genetically-modified sugar beet is no better than conventional varieties, saying that it farmers have confirmed productivity benefits.

The seeds giant said that its Roundup Ready sugarbeet variety had been "successfully planted in North America for the past four years", with more than 1m acres sown in 10 US states and two Canadian provinces.

Indeed, the variety - which is resistant to generalist weedkiller, allowing farmers to spray off weeds without damaging the crop – accounted for 95% of the region's beet planting last year.

"Sugarbeet growers have confirmed that Roundup Ready sugar beets reduce impact on the environment, and make their operations more efficient and productive," the company said in a statement to Agrimoney.com.

'Increased cost of production'

The comments followed claims from US beet sugar figures at a conference in London that genetically modified crop had bought no improvements to yields, besides costing more than conventional crop.

"We have not seen an increase in production yield," Nicholas Sinner, executive director of the Red River Valley Sugarbeet Growers Association told the World Association of Beet and Cane Growers conference.

Indeed, some farmers were worse off, with yields flat, yet seed costs raised by 22% last year.

"There are some producers that have seen an increased cost of production," Mr Sinner said.

'Brutal' price rise 

Mr Sinner, in comments to Agrimoney.com at the edge of the conference, said that he was "not disparaging" genetically modified crops, noting the potential for breeding in resistance to rhizomania.

Rhizomania is a viral beet disease that, according to the UK's Biotechnology and Biological Sciences Research Council, can cause yield losses of up to 80%.

However, current varieties had not proved the "yield pump" expected by many farmers, who had often adopted them after enjoying improved productivity in other biotech crops, such as corn.

"They have not seen a pick up in yields yet,"

His comments were echoed by William Baldwin, a director at the American Crystal Sugar Company, which refines Red River beet, who termed as "brutal" that last year's rise in seed prices.

US Department of Agriculture data show America's average beet yield at 25.8 tonnes per acre last year, below the 26.8 tonnes per acre the year before, but above 2007's 25.5 tonnes per acre.