Took some finding but here's a Versele-laga feeding chart http://www.versele-laga.com/Nutri/Nutrition/Library/Versele-Laga/PDF/Products/EN/Dvnschms2005.pdf
http://www.internationalmodenaclub.com/The%20Doctors%20Corner/diagnosis.htm#Salmonellosis%20(paratyphoid)
Worth a look. Check out Streptococcus Infection which is doing the rounds.
10 different people would more than likely give you 10 different answers on this. Myself I give them multivits after one meal a week during racing generally on a Monday or Tuesday. During the off season twice a week when going through the moult also twice a week during a cold damp spell, other than that once a week.
My club have been discussing same. We're working along the lines of paying £1 a week throughout the racing season. Two screens will be done, one prior to oldbird racing the other prior to youngbirds.