Microbial synthesis of biotin occurs in the intestinal tract of most species, making it difficult to determine minimum requirements for practical diets. Ruminal vitamin synthesis confers an advantage over monogastric species because the contents of the rumen pass through the abomasum and small intestine, which are ideally suited for digestion and absorption of microbial nutrients, including biotin. More important than the minimum requirement is the optimal level of biotin supplementation of livestock diets to prevent marginal deficiency and enhance productivity.
Biotin is a required nutrient in the rumen (Baldwin and Allison, 1983). Thus, Bentley et al. (1954) found that biotin stimulated cellulose digestion by washed rumen bacteria. Milligan et al. (1967) reported that omitting biotin from in vitro rumen fermentation significantly reduced production of propionic acid. Biotin is a required cofactor for methylmalonyl-CoA-carboxytransferase, an enzyme that participates directly in bacterial synthesis of propionic acid.
Biotin synthesis in the rumen can be affected by diet. For example, feeding urea, a readily available nitrogen source, to cattle increased ruminal biotin content (Briggs et al., 1964). Rumen synthesis of biotin is greatest when dietary biotin is lowest, and as a result, the diet type often has little effect on overall biotin concentration in ruminal contents (Hayes et al., 1966; Kon and Porter, 1953). Antibiotics appear to have little appreciable effect on biotin levels in rumen contents (Kon and Porter, 1953, 1954; Miller et al., 1986b).
A reduction in the forage-to-grain ratio in the diet has been shown to significantly reduce rumen biotin synthesis (Da Costa Gomez et al., 1998) (Figure 1). Reducing dietary forage content from 83% to 17% produced a linear decrease in biotin synthesis in rumen continuous culture. Biotin synthesis was reduced by 50% when the forage-to-grain ratio decreased from 83:17 to 50:50 (Da Costa Gomez et al., 1998). Miller et al. (1983) reported that grain source affected biotin synthesis in steers fed diets of 85% grain and 15% alfalfa hay. Absolute values of biotin production were small, ranging from 0.1 mg per day with oats to 1.3 mg per day for corn. The ranking of grain sources (low to high) for rumen biotin synthesis was oats, sorghum, barley, wheat and corn. Total rumen biotin synthesis in steers fed high-concentrate rations averaged 1.0 mg per day. Intestinal synthesis exceeded rumen synthesis in these trials (Miller et al., 1986a, b). Zinn et al. (1987), using a diet with 34.5% forage from alfalfa hay, reported apparent rumen synthesis of 2.4 mg per day in steers. Poe et al. (1972) found that rumen biotin concentrations of lambs tended to decrease from birth to 21 days of age whether nursing alone or with creep feed or pasture. Biotin synthesis in the rumen is more fully established after weaning (Kon and Porter, 1954).