Explain "so the animals trooped down to the hayfield...that the milk had disappeared" from Ch.2 of "Animal Farm."

In Ch.2, the animals revolt and to their surprise successfully take over the farm from Mr.Jones:

"Now, as it turned out, the Rebellion was achieved much earlier and more easily than anyone had expected."

After the initial euphoria the animals organise themselves in order to manage and administer the farm themselves and especially to harvest the ripe grain:

"Now, comrades," cried Snowball, throwing down the paint-brush, "to the  hayfield! Let us make it a point of honour to get in the harvest more quickly than Jones and his men could do."

Just then the cows whose udders had been full and not been milked for the last twenty four hours demanded that some one milk them and relieve them of their discomfort. Soon, the pigs milk the cows and five pails of milk were got. Everyone was curious as to what to do with the milk. The hens said that Mr.Jones would sometimes mix some of the milk with their food.

At this juncture Napoleon becomes impatient and tells the other animals  that he would look after the pails of milk and asks them  to go quickly to the field and harvest the grain:

"Never mind the milk, comrades!" cried Napoleon, placing himself in front of the buckets. "That will be attended to. The harvest is more important. Comrade Snowball will lead the way. I shall follow in a few minutes. Forward, comrades! The hay is waiting."

However, when the animals return in the evening after a hard day's work harvesting the grain, they find all the milk pails empty. The obvious reason being Napoleon who was supposed to guard the milk had actually drunk all the five pails of milk!

This incident is symbolic of Napoleon's greed and paves the way for all his future megalomaniac actions which result in his becoming a complete dictator who takes over the entire farm and rules over it with an iron hand.