The unexpected strength of the US corn harvest caught out Syngenta by raising the prospect of a surplus of corn seed, prompting the agrichemicals giant to take a writedown on inventories and cut full-year profit hopes.
Mike Mack, the Syngenta chief executive, said that "it has become clear that US corn seed production yields are significantly ahead of expectations".
Broader US corn yields have, with some 30-40% of the harvest completed, come in well ahead of expectations earlier in the summer of a figure in the low 150s a bushel.
Broker CHS Hedging said: "Many in the trade are leaning toward an average U.S. yield of 160 bushels per acre," above the current US Department of Agriculture forecast of 155.3 bushels per acre.
"Even the areas that were dry for most of August and September have had better than expected yields."
For Syngenta, the world's top agrichemicals group and a major player in seeds, the strong yield has raised the prospect of it being landed with more corn seed than it believes it is likely to sell next year.
"We will recognise this in 2013 by writing down the value of seeds in excess of estimated sales for the coming season," Mr Mack said, with the provision pegged at $170m.
With earnings also facing a drag from a "lower-than-expected currency benefit", it now appeared that earnings would come in "close to last year's underlying level", which was $19.70 per share.
Syngenta had said it expected "to achieve growth" in underlying earnings, with analysts factoring in a figure of some $22 a share.
The issue contrasts with that Syngenta and its competitors faced a year ago, when a drop in US corn yields to a 17-year low, thanks to a once-in-a-generation drought, presented the threat of a shortage of seed.
The problem was overcome in part by some farmers accepting their second choice in seed, but also by imports from South America.
Syngenta cautioned that its problem of excess seed this year appeared too likely to prove a sector-wide phenomenon, with John Ramsey, the group's chief financial officer warning it was "something that you'll see through the whole industry".
The comments came as Syngenta unveiled sales up 8% at $2.92bn for the July-to-September quarter, led by growth in the Americas.
In Brazil, the group highlighted a boost in soybean seed sales from ongoing plantings which are expected to reached a record high, as growers react to prices which remain high by historic standards.
"A robust commodity price and the depreciation of the real are boosting grower profitability," the group said.
In North America, sales were underpinned by "increasing concern" over weeds resistant to generalist glyphosate weedkillers, which are becoming increasingly prevalent – a factor many attribute to the widespread spraying of these herbicides encouraged by the development of GM glyphosate-resistant corn and soybean seed.
Sales of Callisto, a selective herbicide for corn growers, and Flex, its equivalent for soybean farmers, "both grew strongly in response to demand for effective weed resistance management", Syngenta said.
The group's statement initially received a cool welcome on markets, sending Syngenta shares down 2.7% to SFr354.40, their lowest in nearly a year.
However, the shares recovered in mid-morning deals to stand at SFr260.00, up 1.6%.