According to the Mishna (55b) if someone was watching an animal which slipped and fell into private property, the person responsible for the animal will have to pay the owner of the property the equivalent of whatever the animal benefited. If, however, the animal walked normally into the private area, then the person responsible for it will have to pay for the damage that was done.
The amount that the animal benefited is invariably less than the amount that was damaged. For example, if an animal ate a more valuable crop, still the amount of benefit that the animal derived is its normal feed, which is worth less.
In our Gemara, Rav adds another type of benefit for which the person responsible for the animal would have to pay. If the fall into the private property would have injured the animal, but because the animal fell on the growing produce its fall was broken and it was not injured (or it was injured less severely), the person responsible for the animal will have to pay the amount of the benefit. The Me’iri says that we gauge this based on what a person would be willing to pay to keep his animal from a more severe injury; Rabbeinu Yehonatan says that the payment will be the cost of straw that would have needed to be purchased to ensure a similarly soft landing for the animal.
One of the questions that is raised regarding this case is why the person who was instructed to watch the animal should not be required to make full restitution, since he should have been guarding the animal more carefully. The Ra’avad explains that we cannot expect a person to keep hold of the animal in the public thoroughfare at all times, a position taken by the Talmud Yerushalmi, as well.