Selling a forward means you agree to sell the wheat at a specific price in the future. Unlike the option, there is no question of whether the contract will be exercised. A payoff graph is here: Image: http://upload.wikimedia.org/wikipedia/commons/thumb/0/05/Short_forward_payoff.png/657px-Short_forward_payoff.png
Buying a put means you have the right (but not the obligation) to sell the wheat at the strike price at some time in the future. The payoff graph is here: Image: http://upload.wikimedia.org/wikipedia/commons/e/e5/Long_put_option.svg
Notice that when you buy the put, you do not have to actually sell at the negotiated price if the market moves against you. Of course, you pay for that privilege.
Another significant difference is that forward contracts tend to be specifically negotiated, whereas options are often standard contracts.