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 texting
The FDA is there precisely to check and make sure that all those "totally impossible" things don't actually end up happening. Which, as we have seen, they sometimes do.