Rice straw will not seed.... Ask the supplier of the rice straw for the Ag certificate, they should have a copy to give you, or at least show you, but i guess it depends on how big the place is and how much they sell.
I had 70 bales of Rice Straw delivered a couple months ago for erosion control purposes.... And I'm going to get more to use as mulch for my plants once the season gets started for me.
2 years ago we got rice straw to put down so the dep tarps wouldn't get dust on the flowers, last year i went to same place and the lady goes "you know that stuff has been sprayed with chemicals right? that's why there's no seeds" (in bitchy voice). which kinda bummed me out so i got the regular straw and had a few sprout but if you let um get a foot tall they pull right out then you can tip your hat and just chew on em like the old school farmer you are.
then again i use it for walkways that stay pretty dry.