Shares of seed company Monsanto Co. (NYSE: MON) have been tumbling this morning, despite the firm posting better-than-expected second quarter earnings per share. Hurting the stock this morning is the company's full-year guidance, which was unchanged from a week ago.

Monsanto reported this morning that its profit during the second-quarter more than doubled to $1.13 billion, or $2.02 per share, boosted by strong corn seed and herbicide sales. These numbers are up from $543 million, or 98 cents per share reported in the same period a year ago. Analysts' forecast was for earnings of $1.72 per share in the quarter, according to Thomson Financial.

The company's quarterly revenue surged by a respectable 45% to $3.8 billion, compared with $2.6 billion a year ago. For this period, the producer of genetically modified seeds counted strong corn seed sales, which climbed to $1.7 billion. The company benefited from strong demand from the U.S., North America and Europe-Africa regions that helped to increase revenue. The increasing use of the grain to make ethanol, a gasoline substitute, also came as a catalyst for the company's quarterly sales.

Despite posting surprising earnings numbers, Monsanto's shares have been plunging in morning's trading on traders' disappointment that the company did not lift its full-year earnings outlook. Monsanto confirmed the fiscal 2008 outlook it lifted a week ago to $3.15 to $3.25 a share in earnings. Analysts, on average, were waiting for a higher profit of $3.20 per share.

Eliza Popescu is a financial writer for the online investment advisory service Investor's Observer.