For U.S. farmers, summer brought the worst drought since the 1950s. For Monsanto (MON) it brought plump profits and a rising stock price.

Shares of the world's largest seed company gained 24% so far this year, double the increase for the Standard & Poor's 500-stock index.

But Monsanto now looks pricey relative to the broad market and other farm stocks, suggesting value investors should move on.

Before the summer drought, Monsanto benefitted from the largest corn planting in 75 years, as farmers rushed to take advantage of already elevated prices. Now, a lack of rain during crucial growing months has left half the nation's corn crop in "very poor to poor" condition, according to the U.S. Department of Agriculture.

That sent the price of corn soaring from less than $6 a bushel in May to close to $8 now. Farmers can lock in more than $6 a bushel for the corn they plant next year. Accordingly, analysts expect another big planting in 2013.

Monsanto shares now sell for 27 times the company's forecasted earnings for its current fiscal year, which runs through August. That's about double the valuation for the S&P 500.

Wall Street raised its earnings estimate for Monsanto's next fiscal year from $4.07 in March to $4.31 today. That would mark a 16% increase year-over-year, at a time when earnings growth for most large U.S. companies is slowing.

But it still leaves Monsanto at 20 times forward profits. Deere (DE) and Agrium, which make tractors and fertilizer, sell for less than 10 times forward profits. Syngenta, a Swiss maker of seeds and products for controlling weeds and insects, trades at about 15 times forward earnings.

The high price for Monsanto may be related to its testing of a drought-resistant corn strain that produces plants with deeper roots. So far, the plants are doing better than those in a control group, says Michael Cox, who covers Monsanto stock for Piper Jaffray. But the drought-resistant corn isn't designed to hold up to the extreme conditions U.S. farmers saw this year, he says.

Monsanto managers spent Monday and Tuesday hosting analysts and investors for meetings in Monmouth, Illinois to discuss business conditions. The thrust of the message was that the world will need far more crop acreage in coming decades than it will likely have available, and the difference must be made up with higher crop yields per acre, which will require new seed technology.

That bodes well for Monsanto over the long term, and in the short term, it helps that the company has opened new seed factories in South America, giving it increased capacity for next year at a time when seed supplies are likely to run tight.

But the stock is prone to extreme movements in sympathy with corn prices. During the last corn price spike, in 2008, Monsanto shares soared to $140 apiece. Two years later, they were selling for less than $50.

At $86 and change today, the stock could head well higher or lower on investor sentiment. But it already seems to have priced in Monsanto's corn windfall, and then some.