I don't know about grass seed but I've seen enough alfalfa and clover seed coming up in cowpies to think at least some of it exits the cow as viable seed. I know I have never intentionally planted one seed of sweet clover but as I started planting crop land into grass it started showing up. It had to of lain in the soil for a long time. I've also had pastures with very little alfalfa suddenly develop a decent stand. If I let alfalfa mature and develop seed before grazing I'm assured it will eventually come back.