Hi, I am the proud adoptive mom of 4 lovely little ladies (an easter egger, rhode island red, redstar, and buff orpington. I joined thissite a couple of weeks ago. The girls are about 2 months old and geting bigger by the day. Sometimes I just love to sit out at the coop and "chicken watch". Recently, I have been letting them out of the pen and lead them to my fenced in garden. They are not always cooperative! I am realy enjoying this site. I am getting lots of useful info. Have so many questions!!
and Welcome To BYC! Sounds like you have a cute little flock! I've never used that sort of recipe, not sure what road dust is, the main thing I would worry about is if they are in a run they often will eat things they normally would not because they are bored etc, so even if a little won't hurt them, they may go overboard. Maybe do a search and ask on some of the dust bathing threads how people manage it? You might like to check out the BYC Learning Center, lots of good articles on all aspects of chicken keeping. 
