Commentary for Avodah Zarah 48:12

The continuation of the verse refers to the young born of these cattle. But there is a tradition that animals cannot give birth under the age of three. This is seen from a baraita that discusses a cow or donkey bought from a non-Jew. The issue is whether the next birth of the cow or donkey should be considered its first born and therefore must be given to the priest. The baraita says that if the animal is over three years old then we can assume that it might have given birth and the subsequent birth might not belong to the priest. But if the animal is three years old (or just about) then the birth must be its first. This proves that animals under three years old do not give birth, and therefore we cannot solve the verse as referring to animals under that age. This is refutation of R. Yohanan.