Here is what the CSU Pomona’s “Dog Owners’ Guide to California Foxtails” has to say about them:
A serious hazard for field dogs, or any dogs in the field, are the hard seed-bearing structures of some kinds of grasses, often called “foxtails”. These structures have sharp points at one end, and microscopic barbs, so that they easily move in the direction of the point, but not the other way. They “work in”, but they don’t “work out”. They can become imbedded in the hair, especially the paws and ears, and in nostrils and even eyes. As they work their way in, they cause infection, and if not treated can sometimes be fatal. Southern California has a number of different grasses with this nasty feature, most of them originally from Mediterranean Europe, and most of them common.