English: “To keep seed alive” — to exclude the aged and the sterile (who cannot produce offspring).
On the daf / in context: Even though a previous verse already excluded a treifah (which cannot live twelve months), the additional phrase “lechayot zera” further excludes animals that are alive but incapable of reproduction.
Type of Learning
Custom (exclusion of non-reproductive animals from the purpose of preserving seed).
What Is Learned
Only animals capable of reproducing were taken; aged and sterile animals were excluded so that seed could truly be kept alive.