Vitamin B12 requirements are exceedingly small, measured in units of micrograms per kg of feed. The vitamin B12 requirement increases with higher rates of animal production and metabolism. The requirement is also affected by the relative intake of other nutrients. Excess dietary protein increases the vitamin B12 requirement. Interrelationships exist with dietary cobalt, choline, methionine and folic acid. Vitamin B12 also appears to be affected by ascorbic acid metabolism (Scott et al., 1982).
Dietary cobalt and subsequent ruminal synthesis of vitamin B12 normally meet the requirement for vitamin B12 in ruminants. Under typical conditions, rumen synthesis of vitamin B12 would be functional by six to eight weeks of age, depending on intake of dry feed. Preruminant calves, lambs and kids require supplemental vitamin B12. Vitamin B12 requirements of the dairy calf are estimated between 0.34 and 0.68 µg per kg (0.15 to 0.31 µg per lb) body weight (NRC, 1989). On a dietary basis, the requirement of young dairy calves ranges from 20 to 40 µg per kg (9.1 to 18.2 µg per lb) of dry matter (Radostits and Bell, 1970).
The established dietary cobalt requirements for ruminants are 0.1 to 0.2 mg per kg (0.05 to 0.09 mg per lb) of diet. Precise estimates of minimum cobalt requirements are difficult because of the influence of variables such as seasonal changes in herbage cobalt concentrations, selective grazing and soil properties. Under grazing conditions, lambs are the most sensitive to cobalt deficiency, followed by mature sheep, calves, goats and mature cattle (Andrews, 1956).
Cobalt content of the diet is the primary factor affecting the synthesis of vitamin B12 by ruminal microflora. However, studies indicate that synthesis of vitamin B12 can be affected by other factors. High-concentrate diets can reduce the synthesis of vitamin B12 and also increase the production of vitamin B12 analogs, certain of which are antagonists of vitamin B12 (Sutton and Elliot, 1972). These natural analogs have little or no vitamin B12 activity.
Ruminants have higher vitamin B12 requirements per unit of body weight or diet than nonruminants, presumably because of the involvement of B12 in the metabolism of propionic acid. Experiments with sheep suggest an oral requirement for growing lambs of about 200 µg per day, about 10 times the reported oral requirement of other species per unit of food intake (Marston, 1970). The comparatively high cobalt requirement of ruminants arises partly from the low efficiency of production of vitamin B12 from cobalt by the rumen microflora and partly from the low efficiency of absorption of vitamin B12 (McDowell, 2000).
Grasslands containing 0.1 ppm cobalt will prevent deficiency symptoms, while levels of 0.05 to 0.07 ppm cobalt are deficient. Further evidence that 0.1 ppm cobalt in the diet will meet both the cobalt and the vitamin B12 requirements of sheep was provided by Mohammed (1983), who reported that ruminal concentrations of both vitamin B12 and propionic acid were maximized at 0.1 ppm dietary cobalt.
