Shares in Monsanto slumped by 10% on Tuesday amid talk that some of the group's top-of-the-range corn seed, genetically modified to protect against insect pests, was delivering disappointing yields.

Laurence Alexander, an analyst at Jeffries & Co in New York, said that SmartStax corn seed, which Monsanto claimed was its "the highest yield corn available", was yielding less than cheaper seeds with fewer added genes, to judge by harvest reports from Iowa.

"While it is still early to assess the field data... the initial data is weaker than we expected," Mr Alexander, who rates Monsanto shares a "hold", said.

The concerns disappointed investors who have already this year seen the company admit setbacks from high pricing of seed, which has allowed rivals, notably DuPont, to gain market share, and struggle to shore up takings from its Roundup weedkiller, which faces bitter competition from generic alternatives.

Monsanto last month warned that profits would fall short of Wall Street expectations, and extended to about 2,500 job cuts needed in a cost-cutting drive prompted by Roundup difficulties.

Shares tumble 

With only 27% of the US corn crop harvested, according to US Department of Agriculture data released on Monday, there is a distance to go before a verdict on SmartStax, which Monsanto says "offers the industry's broadest spectrum of insect control".

"It's still very early in the harvest for SmartStax," company spokesman Kelli Powers told Agrimoney.com.

However, the group admitted on September 20 that the brand was, on some farms, showing "yield variability below" one of the group's cheaper alternative.

This data was drawn from about 10% of the 3m acres of SmartStax sown, and of longer-maturity hybrids, rather than the shorter-term varieties sown further north which comprised the majority of sales.

Monsanto shares closed 8.1% lower at $48.75 in New York, having touched $47.50 earlier.

The losses added to a decline on Monday, taking the total fall this week, at Tuesday's low, to 14%, and wiping more than $4bn from the company's stock market value.