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A research has been carried out to optimize the rumen function and the rumen microbial protein synthesis
in Bali Cattle (Steer). Sixteen Bali cattle were used in this experiment with Completely Randomized Block
Designed four treatments and four blocks. The first treatment was ration based on agriculture by
products without supplementation (R0), while the other three treatments were supplemented with 0.075%
(R1), 0.150% (R2) and 0.225% (R3) multivitamins-minerals. The variables of rumen microbial protein
synthesis including urinary exretion of allantoin, purin derivatives absorbsion, rumen microbial protein
synthesis and efficiency of rumen microbial protein synthesis, Feed and nutrients degradation on the
rumen, pH, concentration of NH3-N, VFA and amount of protozoa in the rumen fluid were measured in
the experiment. The result showed that rumen microbial protein synthesis and amount of microbial
purines absorbed in suplementation of 0.150% (R2) and 0.225% (R3) multivitamins-minerals were
significant difference (P<0,05) compared with without supplementation (R0) (222.83 and 222.24
gram/day Vs 197.83 gram/day) and (49.45 and 49.35 Vs 44.76 mmol/day), but they were not significantly
different with without supplementation on rumen microbial protein synthesis (213.57 gram/day) and on
amount of microbial purines absorbed (48.67 mmol/day). Degradable organic matter and Sulfur
absorbed in the rumen, amount protozoa, and ruminal pH, concentration of NH3-N, totally VFA and
propionate acids of rumen fluid significantly (P<0,05) increased as well. Increasing supplementation
until 0.15% (R2) multivitamins-minerals were decreased concentration of acetate acid, butirate acid and
methane gas production compare with without supplementation, but increasing until 0.225%
supplementation of multivitamins-minerals (R3) increased concentration of butirate acid and methane gas
production. Moreover, regression analyzed showed that suplementation of multi vitamins-minerals of
0.188% coused maximum rumen microbial protein synthesis of 223,39 gram/day. It was concluded that
rumen microbial protein synthesis of Bali cattle given ration based on ammoniated rice straw could be
increased as maximum 12.92.%. The level optimal multivitamins-minerals supplementation used in this
experiment was 0,188%.
Key words: Agriculture by-products, Bali cattle, Multivitamins-minerals, Microbial Protein Synthesis,
Rumen Function.