The high level of cobalt (Co) in soil relative to herbage means that soil accidentally ingested with herbage may constitute the larger source of the element for vitamin B12 synthesis by rumen microbes.

A continuous culture simulation of rumen fermentation (Rusitec: Czerkawski and Breckenridge. 1977) was used to determine the extent to which soil Co was used for microbial vitamin B12 synthesis. A low Co hay (0.044 mg/kg DM), 7.25 g DM/day, was “fed” to each of four vessels for a period of 8 days to provide resting values for vitamin production. A chalk, a sand, a clay or a loam, with Co concentrations ranging from 0.70 to 64.5 mg/kg DM (see Table 6.15.1), were then added at 0.70 g/day, to the hay in separate vessels each morning for 14 days. For a further period of 10 days, the hay in two vessels was supplemented with either chalk or loam at 1.4 g/day. Daily vitamin B12 production was measured by analysis of each vessel's effluent for cobalamin (cbl.), using a modification of the method of Green (1980). Vitamin B12 analogue production was assessed from the Lactobacillus leichmannii microbial assay of the effluent. All vessels produced a “roughage” fermentation (acetate c. 50% and propionate c. 20% of the total volatile fatty acids) with the soil supplements.