Due to the rumen microbial synthesis of riboflavin, ruminants have no absolute dietary requirement. It has been assumed that riboflavin is synthesized in the rumen because of its relative concentrations in the diet and in rumen contents. The range of values reported for riboflavin range from 4 to 39 mg per kg (1.8 to 17.7 mg per lb) of rumen dry matter, a wide range (Mathison, 1984). Confirmation of a net synthesis in the rumen is illustrated from work of McElroy and Goss (1940a) in which the secretion of riboflavin in milk was equivalent to approximately 10 times the intake of the vitamin. Buziassy and Tribe (1960) estimated a net synthesis of 0.9 to 12.0 mg of riboflavin per kg (0.4 to 5.5 mg per lb) of dietary dry matter intake in sheep. Pearson et al. (1953) measured a net synthesis of riboflavin when semi-synthetic diets low in riboflavin were fed. More recently, Miller et al. (1986a) determined a net synthesis equivalent to 78% to 225% of the riboflavin intake (25 to 39 mg per day) in steers fed a diet containing 85% grain.
Diet composition influences total microbial synthesis of riboflavin. Data of Miller et al. (1986b) suggest a greater ruminal synthesis of riboflavin with increased dietary concentrate. Buziassy and Tribe (1960) measured increased riboflavin synthesis with increasing dietary protein level. These authors also noted that ruminal riboflavin synthesis was reduced with higher dietary intakes of the vitamin.
Rumen microorganisms can degrade riboflavin, forming hydroxyethylflavine, formylethylflavine and other metabolites (Owen and West, 1970). The importance of ruminal destruction of riboflavin, or of the metabolites produced, is unknown.
Riboflavin deficiencies have been demonstrated in young ruminant animals, and the estimated requirement for calves is 1 to 1.6 mg per kg (0.45 to 0.73 mg per lb) of dry matter (Roy, 1980). The majority of livestock species have requirements of 1 to 4 mg per kg (0.45 to 1.8 mg per lb) of dry matter. Expressed on a body weight basis, the riboflavin requirement of calves fed milk replacer reportedly ranges from 35 to 45 mg per kg (15.9 to 20.5 mg per lb) of body weight. The NRC (1989) suggests that calf milk replacers should contain 6.5 mg per kg (3.0 mg per lb) of dry matter.
