I have not had a lawn of my own ever, just lived with my parents who did all that stuff; so I don't know about these things. What makes crab grass evil? Is it poisonous? Dangerous? To whom-- dogs? Birds? Does it look different from other grass in a way that is ugly? Is the problem that it is non-native to the US? If so, what makes it different from other non-native grass, or are you supposed to keep only native grass on your lawn?

I have the idea that it is unhealthy for cows and horses to eat, but I don't really understand why it is bad if one does not have cows and horses.