Shabbos, Mishnah 4:
The Mishnah permits clearing away the food from before an animal that is being fattened for slaughter. Such an animal is fed large quantities in order to fatten it, and one may clear out of the feeding trough whatever is not fit to be eaten, so that when the fresh food is placed before it, the animal will not find it repulsive because of that refuse.
Pushing the droppings to the sides:
Even in a place where it is not possible to rake up the droppings and remove them entirely, and we do not want them mixed into the food, one may push them to the side, so that they will not be lying in front of the animal while it eats.
The dispute of the Tannaim and the Gemara's explanation:
This is the opinion that permits these actions, while others forbid them. The Gemara clarifies that the discussion concerns a trough that is a vessel, and not a trough whose walls alone are constructed while its floor is the ground itself. For if its floor were the ground, then whether by raking or by pushing to the sides a person is liable to level out depressions in the ground, and in such a case even the lenient opinion agrees that it is forbidden.
It emerges that the dispute concerns only a trough that is a vessel, and the question is whether we impose a decree in such a case:
The first opinion: We decree even regarding a vessel, because the matter resembles a case in which the trough is not a vessel but its floor is literally the ground, where clearing it out involves a prohibition.
The second opinion: We do not impose this decree, and with a trough that is a vessel it is permitted.
Taking food from before one animal and placing it before another:
The Mishnah continues: "Notlin milifnei behemah zo v'nosnin lifnei behemah zo b'Shabbos" - we take food from before this animal and place it before that animal on Shabbos. The Gemara explains that this is not said of any two animals, but rather there is a distinction between them:
From before a donkey to before an ox - permitted: An ox is not repulsed by eating from food that a donkey has eaten from.
From before an ox to before a donkey - forbidden: When an ox eats, a certain amount of saliva remains in the food, and a donkey is repulsed by it and will not eat it. It emerges that such a transfer is nothing but exertion and movement with no purpose, since the donkey will not end up eating the ox's food, even though the ox does eat from the donkey's food.
In summary: We learned that one may clear the refuse in the trough away from before the animal being fattened, and push the droppings to the sides, and the Tannaim disagreed about this. All agree that the case is a trough that is a vessel, and the dispute is whether we decree in such a case on account of a trough whose floor is the ground. We further learned that taking food from before one animal and placing it before another was permitted from a donkey to an ox, but not from an ox to a donkey, since it amounts to nothing but needless exertion.