Back in the old days farmers would run pigs behind the cows to finish getting the undigestted food value out of what the cows didn't use. So I would not count on weed seeds being killed, by passing through the digestive tract. Of course very few cows are allowed the luxury to graze any more. They may never have a chance to eat a live seed. Weed or otherwise. Compostted is really the only safe way to go for lawns, other than your own.