Propionate uptake by rumen microorganisms:: the effect of ruminal infusion

We assessed the ability of rumen microbes to significantly incorporate propionate when they are subjected in vivo to no infusion, long-term infusion of minerals, and short- and long-term infusion of high amounts of propionate. Four ruminally cannulated sheep fed 1000 g hay (8 meals per d) were used in a 4 x 4 Latin square design. The treatments consisted of no infusion (C), ruminal infusion of propionate (86 g.d(-1)) for 1 (P1) and 7 d (P7), and of minerals for 7 d (M7). The infusion of propionate increased its ruminal molar percentage from 19 (C, M7) to 32% (P1, P7). Ruminal pH, osmolality, ammonia concentration, and protozoa counting were not or were poorly affected by the treatments. Rumen contents (100 mL liquid + 100 g solid) were incubated at 39 degreesC in anaerobic flasks containing artificial saliva, (NH4)(2)SO4, ground hay, and 0.45 muCi[2-C-14] propionate. After 6 and 16 h, clarified fermenter fluid, liquid-associated protozoa, and liquid-associated bacteria considered as representative of total bacteria were separated by fractional centrifugations. Microbial pellets were washed with saline before C-14 determination. In flasks, pH, osmolality, gas, volatile fatty acid (VFA) production, ammonia concentration, protozoal counting and the amount of C-14 in clarified fermenter fluid, liquid-associated protozoa and liquid-associated bacteria were similar among the treatments. Between 6 h and 16 h, the amount of C-14 decreased in the clarified fermenter fluid, and increased in liquid-associated protozoa and liquid-associated bacteria. The amount of estimated microbial DM was 11.0 g per flask. After 6 h and 16 h incubation, the amount of C-14 incorporated into microbial fractions averaged 9.1 and 12.2% of the total amount of C-14, and was estimated to account for 10.3% of propionate net production irrespective of the treatment and incubation time. It is concluded that the uptake of propionate by ruminal protozoa and bacteria is quantitatively significant, and may not be significantly affected when rumen microbes are submitted to minerals or propionate infusion. These results may explain the differences observed between the methods in the determination of the ruminal production rate of VFA.