Fears for the agriculture boom sent Monsanto shares down 16.2%, despite the seeds-to-weedkiller group raising its earnings guidance for the seventh time this year.

Monsanto said it expected underlying earnings per share of $3.64 for the year to the end of August compared with a forecast of $3.58-3.60 given less than three weeks ago.

The group also added roughly $100m to gross profit forecasts from its Round-up herbicide for both the current and next financial years.

"Our business continues to be strong on all fronts, and that's reflected in both our performance and our confidence in our outlook," Hugh Grant, the Monsanto chairman, said.

Shares dip

However, the statement failed to stop Monsanto shares following other agriculture stocks sharply lower after Mosaic, the fertilizer maker, admitted it had missed sales guidance.

Monsanto stock closed down $15.83 at $82.01, albeit recovering some ground from an early lower of $78.91.

Donald Carson, a Merrill Lynch analyst, warned in a note that the surging cost of phosphate rock may hurt profits growth from Round-up.

Monsanto said phosphorus-related costs represented less that 10% of Round-up raw material costs.