Animal Farm's third chapter goes into detail on the particularities of the first harvest. Generally speaking, almost all of the animals contribute at least to some degree (the cat is a major exception, always avoiding any work whatsoever), although the most labor intensive tasks tend to be given to the horses. As Orwell writes:
Boxer and Clover would harness themselves to the cutter or the horse-rake (no bits or reigns were needed in these days, of course) and tramp steadily round and round the field with a pig walking behind and calling out, "Gee up, comrade!" or "Whoa back, comrade!" as the case might be. (Orwell, Animal Farm, chapter 3)
Of all the animals, Boxer is described as the most laborious in his physical contributions to the farm, to the point that his efforts were, Orwell says, worth the equivalent of three other horses put together. Meanwhile, the pigs (in keeping with their role as the planners and organizers of the farm) supervise the work of the other animals, while also applying their intelligence towards problem-solving, in the case of any unexpected difficulties which might arise.
Even as the horses take the most labor-intensive tasks, the ducks and hens are described picking up any disparate grains or stalks which would otherwise go to waste. As Orwell puts it, "everyone worked according to his capacity" (chapter 3). That being said, there are exceptions. As has already been mentioned, there is the cat, who would always excuses to avoid doing any work, but there is also Mollie, who shows a tendency to begin work late and leave early. Meanwhile, Benjamin is described working "in the same slow obstinate way as he had done it in Jones's time, never shirking and never volunteering for extra work either" (chapter 3).
This first harvest after the Revolution emerges as the high-water mark for the animals on the farm. As the story continues, a far more crueler and more brutal picture begins to set in, with the growing dictatorship of the pigs (and ultimately the dictatorship of Napoleon himself) exploiting the other animals on the farm.