Yes, except:
A.this is very very targeted manipulations we're talking about here. It's not going to be poisonous because they aren't putting the genes for poison in.
2.even if by some unknown by science at this time mechanism somehow makes hornless milk cows highly toxic or whatever (which is totally impossible, but I'm humoring you here) they'd notice it in the testing
It's not only highly toxic effects that need to be checked out. DES is a classic example of a drug that while not immediately poisonous had considerable side effects. Studies are showing that it may even be affecting the
grandchildren of the women who took it during pregnancy. What all having phytase inside the body of a pig does to a pig is something that definitely needs investigation in my opinion