Growing up, we simply called them fox tails. I’m not sure what type of grass, plant, or weed they actually are (a friend guessed some sort of wild rye), but if the little barbs from the tips of these grasses get under your dogs fur (and they quickly and easily do), they are known to burrow under the skin and can cause real danger, requiring surgery to remove.
Here, Tucker and Maya are running through such a field. We removed no less than half-a-dozen of the barbs from each of them. We had to look, brush, and feel all over to find each one.