The pigs’ taking the milk is foreshadowed when Napoleon tells the animals to forget the milk and it disappears.
So the animals trooped down to the hayfield to begin the harvest, and when they came back in the evening it was noticed that the milk had disappeared. (Ch. 2)
Milk has a special significance for the animals on Animal Farm. Early on when Old Major is describing the weakness of me, milk is one of the things he mentions.
`Man is the only creature that consumes without producing. He does not give milk, he does not lay eggs, he is too weak to pull the plough, he cannot run fast enough to catch rabbits. (Ch. 1)
Milk is one of the things of value the animals have. It is a luxury, and something that men prized. By taking it for themselves, the pigs place themselves on level with men. When the men neglect the animals, one of the worst things they do is not milk the cows.
When the animals revolt, the pigs milk the cows. The animals instantly want to know what is going to happen to the milk, because Jones used to mix it into their mash.
`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.' (Ch. 2)
The milk disappears. When the pigs are finally forced to account for it, they say they deserve it because they are the braintrust. The other animals rely on them for leadership, so they get the milk and apples. This is one of the first examples of the abuses of the pigs, as they take everything for themselves and act just like the humans.