It's becoming increasingly harder to figure out whether Monsanto (NYSE:MON) is a bargain or a value trap. Yesterday, the agriculture giant announced less-than-stellar guidance for its 2010 fiscal year, which started at the beginning of the month.

It's really a tale of two product lines for Monsanto. The seed and trait business is growing and competing well against -- and sometimes with -- DuPont (NYSE:DD), Dow Chemical (NYSE:DOW), and Syngenta (NYSE:SYT).

Its Roundup product, on the other hand, is headed in the wrong direction. Once a cash cow, Roundup now faces generic competition, and a glut of chemical herbicides in the supply chain is pushing down prices. Unlike drug companies such as Pfizer (NYSE:PFE) and Merck (NYSE:MRK), which can pretty much kiss off most of their sales once generic competition starts, Monsanto does expect to bring in $650 million to $750 million in gross profits from Roundup in the coming year. Still that's a long drop from the nearly $2 billion in gross profits that the herbicide brought in during fiscal 2008.

In a couple of years, it's not going to matter much: By 2012, the company expects that seeds and licensed traits will make up 85% of the company's total gross profit. But in the meantime, the drop is hurting the bottom line.

Earnings per share, after adding back restructuring charges, are expected to come in between $3.10 to $3.30, a sharp decline from the $4.40 or so that's expected from the recently completed year. Trading at a forward price-to-earnings ratio of more than 24, Monsanto is a little cheaper than we've seen in the past, but it doesn't leave investors much breathing room, if Roundup sales continue to fall faster than expected.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.