No one does the harvest for the animals. They do it themselves. This can be seen in Chapter 3, right at the very beginning of the chapter. There, we see a fairly long description of how the harvest is conducted and why the animals are able to do it better (even with their physical limitations) than the humans had been able to. We see that the horses are able to pull the haying implements themselves with only a bit of supervision from the pigs. All the animals help with this and with harvesting the grain. They even thresh the grain by blowing the chaff away with their breaths.
In these ways, the animals are able to harvest for themselves and to do so more effectively than the humans had.