Plz help, Rice cereal confusion?

hi all, my daughter is 4 months and 3 weeks her pedi suggested that i feed her rice cereal. before i started her on the cereal she drinks about 24 oz a of formula a day, but now that she eats about 8 TBS of rice cereal daily she reduced her her formula to 18 oz...is that ok? it says on the can that it has iron and zinc and some minerals but am not sure if it gives her the same nutrition that the formula does.