We did not run that risk.  We used alfalfa cubes and complete feeds.  And then paid $$$ for some certified hay there to give the horses something to munch on.

Suggest you ask the campground what the current regs are.  When we went, they could seize your whole rig over the hay (as I remember). 

Issie, have you all been back out there?

The Forest Service and BLM require that you feed the certified hay for 3 days prior to riding on public lands. But it is pretty hard to test for that. I've heard of one occassion where they took manure samples and tested to see if the owners had been feeding certified for 3 day prior. But I just don't know how they would prove or disprove that. They have done some studies and found that seeds in manure are not a big deal.  The seeds that do survive the pass through the gut, don't grow well, because they typically land on the compacted earth of the trail. Not a great place for a new seedling.

If you are camping on private land. It's really the land owners call if they will allow non-certified hay.  And maybe thats why certified hay isn't a big deal in the eastern part of the country since most camp grounds there are private. But here in the west, where public lands are the dominate recreation areas. It is a big deal.  Yellowstone park doesn't even allow any hay ( certified or not ) into the park.  You have to have your trailer swept clean. The ranger comes and looks in the trailer at the park entrance. Their concern is not just about weeds, but rather ANY non-native plant. (  Alfalfa or Timothy are both non-native to the park and would  pass as weed free hay but are not allowed in)