These are Vitamin C and the eight B Group vitamins which include: Thiamin B1, Riboflavin B2, Niacin B3, Pantothenic Acid B5, Pyridoxine B6, Biotin B7, Folate or Folic Acid B9 and Cyanocobalamin B12. This classification is derived from human requirements where all the above are required in minute amounts in the daily diet. In horses however, most of these vitamins are synthesised by the hind gut microbiota and are not essential dietary components for maintenance needs. Under some conditions, such as hard work, disease or stress, intestinal production may not keep up with demand which is when dietary intake becomes beneficial.
See also Fat Soluble Vitamins