With 74% of Utah being Public Lands and Certified Weed Free is required on all public lands. It is very common for us to have buy a weed free product. Most feed stores sell certified hay for $1.00 to $2.00 more per bale than non-certified hay.
Places like Yellowstone Park are even more restrictive. They are not only concerned about invasive weeds. But want to prevent the spread of ANY non-native plant. So they pretty much don't want any baled hay brought into the park. If you have it in your trailer, It needs to be in unbroken bales, usually wrapped or bagged to prevent the possibility of seeds blowing off the bales. And if you have fed your horses in mangers, They usually want your trailer windows shut. So the safest bet for feed in the park is to haul in some kind of processed feed. They consider the process of grinding and pelletizing of hay to be sufficient to destroy seeds.