Lipase is a naturally occurring enzyme, not an animal product per se. Lipases perform essential roles in the digestion, transport and processing of dietary lipids (e.g. triglycerides, fats, oils) in most, if not all, living organisms.
So, Lipase from a cow should be exactly the same thing as Lipase from a plant. The problem is that plants produce very small quantities, so it's not practical to extract it. "Oats" may have a lot of Lipase, but not in a form that is readily available for making cheese.
Milk, even pasteurized, whether it's cow or goat milk has some Lipase depending on the diet of the animal, etc. In fact, raw milk has over 60 different enzymes. Some make it through pasteurization, but many are deactivated or damaged.
The aging and flavor of all cheeses comes mainly from enzyme activity and NOT from bacteria, so the Lipase is VERY important, especially for Italian type cheeses. If you want a great Parmesan, I would NOT trust that any milk is going to have enough natural Lipase to give that nice sharp flavor.
