Bechoros 4:2-3

Bechoros 4:2

If a firstborn animal developed a blemish during its first year, it may be kept the whole 12 months. If it developed the blemish after its first year, it may only be kept for 30 days.

Bechoros 4:3

If a person slaughtered a firstborn animal and only subsequently discovered its blemish, Rabbi Yehuda permits the animal to be eaten. Rabbi Meir says that since the animal was slaughtered without the an expert clearing it, it is prohibited.