This week, Monsanto's (NYSE: MON) Chairman and CEO Hugh Grant and its CFO Carl Casale both filed SEC Form 4s to record purchases of stock. Grant bought 37,500 shares and Casale bought 30,000. With shares north of $50 a piece, the combined investment of more than $3.5 million is pretty substantial even for well-paid executives. Money doesn't grow on corn stalks, you know.

Why now? And should you follow their example?

The answer to the first one seems clear enough. Grant and Casale must think Monsanto has hit bottom. The stock has fallen 60% from its high in 2008, much farther than the S&P 500 that's off by just 20% over that timeframe.

Part of that fall is due to slipping earnings, which are expected to fall this year for the first time in years. Roundup, the company's cash cow, was hit with massive generic competition that caused sales to collapse.

But the herbicide is only a part of Monsanto's business. Its future growth lies squarely in its biotech seed trait business. The company has been stacking traits on top of one another to create super seeds capable of offering farmers better yields -- at higher costs of course. The company thinks earnings can grow in the mid-teens annually from here thanks to the growth of its seeds.

Clearly Grant and Casale are putting their money where their mouths are. Of course you don't have to follow their lead if you aren't as bullish on Monsanto. You could invest in direct competitors like DuPont (NYSE: DD), Dow Chemical (NYSE: DOW), or Syngenta (NYSE: SYT). If you want to broaden your exposure to the sector, other agricultural stocks like fertilizer producers Mosaic (NYSE: MOS) and PotashCorp (NYSE: POT), or even equipment manufacture Deere (NYSE: DE), are worth a look. All are down at least 20% over the same time period as Monsanto, with Mosaic having an incredible 72% shaved off its value.

Is Monsanto a worthy investment? Yes, but in moderation, considering the risk in a turnaround candidate. Grant and Casale's buying shouldn't be a reason in and of itself to buy shares. But if you were on the fence about making a purchase, executive insider purchasing should give you a little more added confidence that management is aligned with shareholders.

A tip of the Fool's cap to the Motley Fool Inside Value team for alerting me to the insider buying. The stock has been recommended by the newsletter, and Motley Fool Options has recommended a synthetic long position on Monsanto. Syngenta is a Motley Fool Global Gains selection. Try any of our Foolish newsletters today, free for 30 days

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