Cash cows sometimes die. It's a fact of life when patents expire. Pfizer (NYSE:PFE) will lose its blockbuster Lipitor in the coming years, and Monsanto (NYSE:MON) is facing the same situation with its herbicide, Roundup.

Sales of Roundup and other glyphosate-based herbicides fell 47% in its third fiscal quarter, from over $1.1 billion last year to just $614 million this year. Competition from Syngenta (NYSE:SYT), Dow Chemical (NYSE:DOW), and Chinese generics forced Monsanto to lower the price of Roundup, now that it's no longer patent protected. The company knew a drop in sales was coming, but didn't expect such a quick deterioration.

Considering the decline, Monsanto is going into triage mode. It'll move Roundup and its other herbicides into their own division so that it can be a little more agile as dynamics in the chemical industry change. It's also cutting 900 jobs -- less than 4% of the company -- which seems prudent, given the situation.

All is not lost for Monsanto; the company has just shifted from being a chemical company to being a biotech-seed company. Seed sales in the third quarter grew nearly 10% year over year, competing nicely with Syngenta, DuPont's (NYSE:DD) Pioneer, and other seed makers. The company has seeds stacked with multiple traits in the pipeline, which should drive sales in the future.

Right now increased seed sales aren't making up for the loss of revenue from Roundup, but, once year-over-year comparisons no longer include lofty Roundup sales, Monsanto should be able to get back to growing the top line. Investors just need to be patient.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is a Motley Fool Inside Value recommendation. Try any of our Foolish newsletters today, free for 30 days. The Fool has a disclosure policy.