There is always the possibility that chemicals/preservatives etc may have been added to the seed coating to prolong shelf life. But the buns don't nom the seed, just the grass that sprouts. Just make sure you grow the grass well before feeding it. If in trays it will need feeding as the nutrients in compost won't last long, but if you bottom feed (i.e. dunk the pot/tray, in a large tray of organic feed e.g. seeweed/nettle/manure feed for it to soak up) and always water with clean water from the top (fine rose watering can is best) then any residue will be in the soil, not on the leaves of the grass.
Good luck!