Let me describe this not-so-hypothetical scenario. Here you have a year where suddenly, from early June onwards, weather is much hotter and drier than usual. At some point, the market starts seeing that such weather is not going away, and sensing that agricultural yields will suffer from the drought, it bids up corn (and wheat, and other ag commodities) up 40%+ in a short while.
Then, on July 11 - today - you get a USDA report (United States Department of Agriculture) showing that indeed, yields have suffered. Indeed, surprise, they suffered even more than what analysts seemed to have been expecting!
No. You don't. You'll do that only if you want to suffer an immediate loss. Look at the way corn futures traded today. Look at what happened to those traders who bought the news (that's the spike on the second day). The same thing would happen to you If you bought CORN or some other instrument tied to these futures.
The fact is that when faced with a persistent rumor which you know has no way of being false - namely, that the yields would suffer; and when you also know that the rumor led to significant price movement, you simply cannot expect to buy the news, and not suffer an immediate loss.
Sure, down the road corn, wheat, and God knows what else can turn around and take out the highs. The drought can finish off most harvests. It can persist, and whatnot. But on the day those widely anticipated news get reported, the likelihood of someone buying them getting creamed is simply too high.
And this is a lesson that is worth learning, because it replays time and time again on many different assets, with people buying widely anticipated news and then being surprised by the asset behaving exactly the opposite of what they expect, and handing them instant losses.
So before you act on any piece of news, check if the asset has already reflected them, and think about how likely it is that those news have been anticipated. In today's move on corn, it was rather obvious.
As an aside
Even with the drop in yields, USDA is still seeing production growth for corn and wheat from 2011 to 2012 due to larger plantations, which made it even less likely for the prices to hold. Still, this is somewhat beside the point, as what matters is that the news were widely anticipated and thus, sold.
Disclosure: I have no positions in any stocks mentioned, and no plans to initiate any positions within the next 72 hours.