An alternative approach that appears in Rashi suggests that according to Rava if the owner had placed only ten of the animals in the pen, in theory he could have sold or slaughtered the others. Nevertheless we require him to place all of them in the pen, at which point they all become obligated in this commandment so that the extra five animals will have to be combined with other animals born in the course of the year for the purpose of tithing.
The Mishnah on yesterday’s daf (=page) presents the technical details on how animal tithing should be done. According to the Mishnah the animals are brought into a pen and a small opening is made so that two animals cannot leave at the same time. The owner counts the animals one by one, marking every tenth animal with red paint announcing “this is the tithe.”
On today’s daf, Rava teaches: If he had fifteen lambs he cannot say: ‘I will select ten, bring them into the pen, take one as tithe from them and the rest will be exempt.’ Rather, he must bring them all into the pen, bring out ten lambs one by one, taking one from them as the tithe, while the rest he must combine with those born in a later tithing period.
Rashi explains that Rava is referring to the teaching in the earlier Mishnah (daf 57b) where we learned that there were three periods in the course of the year with regard to animal tithes, and that once one of those dates is reached it would be forbidden to sell or to slaughter any of the animals without tithing them first. Rava clarifies that even though this prohibition only applies to animals that can be tithed, and of the 15 animals there will be five that cannot be tithed at this time, nevertheless they cannot simply go free; they must be combined with other animals that were born in the course of the year. Furthermore, the owner cannot choose the weakest animals for current tithing, rather he must put all of them into the pen and allow whichever is tenth to be established as the tithe.