As we learned on yesterday’s daf, as long as an animal is in the public domain, its owner will not be responsible for damage done when it eats normally. Nevertheless, even in the public domain the owner will be obligated to pay for fruits or vegetables that his animal ate, if the animal benefits from the food. In essence, this payment is not for damages, rather it is payment for the benefit derived by the animal who ate the food.
Our Gemara discusses how this payment will be determined. Rabba rules that the owner will only have to pay for the amount of straw that the animal ordinarily eats. The owner has benefited inasmuch as he will not need to feed his animal, but he can argue that his benefit was limited to what he would have chosen to feed the animal, which could be the cheapest food. Rava argues that he will have to pay for what the animal ate – if it ate barley, the owner must pay for barley – although he would pay for the barley be-zol – at the cheapest rate. Since he may not ordinarily feed his animal barley, we arrange for him to pay a cheaper rate, which, according to Rashi, is 2/3 the normal price. The Me’iri suggests that according to Rava the owner should pay full price for barley if that is what he ordinarily feeds his animal, since that is the benefit that he has received. The discussion of offering a cheaper rate to someone who does not ordinarily offer such high quality feed to his animals is based on the recognition that he is truly benefiting from the better quality, and presumably healthier, food.
The conclusion of the Gemara is that it is clear that if the animal had eaten wheat or something that was bad for it – things that the owner would never have given to his animal – the owner will not be obligated to pay anything, since he does not benefit and he is not responsible for damages of shen in the public domain.