Since the unloading stations are competing with each other, it can happen that a station detects a sudden need for a certain commodity. In that case a temporary great demand starts.
During a great demand the unloading station in question offers a significantly higher price (between x1.5 and x2 the current market price of demanded product) than its competition. You receive a message as soon as a great demand starts and you can look up all currently running demands on your PDA. Great demands last 6 to 48 hours.
To maximize profits and if you can afford to wait to sell, keeping your harvested grain until a great demand happpens will allow the best profits.
The in-game selling prices are updated at the beginning of every in-game hour and they will rapidly drop down (but they will come back up after a couple of in-game days) as you deliver more and more crop to selling station. This means that you only have one in-game hour to sell your harvest with maximum profit until the next price update happens.