In the novel Animal Farm, why does Napoleon order the animals to plow the grazing fields?

The answer to this question can be found in Chapter Eight of Animal Farm. With the Battle of the Windmill over, and the attack by Frederick driven off at great cost, the pigs celebrate by getting into a case of whiskey they find in Jones' cellar. They get very drunk, with Napoleon careening around the yard wearing Jones' old hat, and they even sing "Beasts of England," which the other animals had been forbidden from singing. Afterwards, Napoleon has such a hangover that the pigs fear he is dying. But when he recovers, he begins to take an interest in whiskey-making, sending Whymper to buy books on the subject. This is why the grazing ground was plowed up. Napoleon plans to plant barley there for making whiskey. This series of events also leads to another alteration in the Seven Commandments, which once ordered that no animal should drink alcohol. The Fifth Commandment now read "No animal shall drink alcohol to excess" (109).