The flow of c13 from feed to microbes in an in vitro experiment

The flow of 13C from feed to ruminal microbes was studied. The variations of the 13C value of bacteria and protozoa were measured in vitro (batch culture: 2, 4 and 8 hours of fermentation) with three diets based respectively on cracked (CMD), ground (GMD) and flaked (FMD) maize (40%), ground lucerne hay (50%), and soybean meal (10%). The pattern of the 13C relative enrichment of bacteria and protozoa was affected by the physical form of maize. With FMD the 13C value of bacteria increased linearly from -25.6 ‰ at 0 hours to -22.6‰ at 8 hours; with GMD and CMD the 13C enrichment of bacteria showed a lag phase of about 2 and 4 h respectively, and then increased up to values of -23.5 and -24.5‰ after 8 hours of incubation. The 13C enrichment of protozoa receiving FMD and GMD reached, respectively, a plateau at -23.6 and -24.1‰ after 2 h of incubation, while that of CMD increased continuously reaching a final value of -24.0‰. High correlations were observed between 13C enrichment of bacteria, starch degradability (r = 0.86, P<0.01) and bacterial yield (r = 0.96; P<0.01). The relative differences in the 13C enrichment of the microbes can be useful to study their feeding behavior and to compare the effects of different factors (feed additives, feed processing etc.) on the C flow from feed to the microbes.