Abstract: The method of invitro was used, the orthogonal design of L8 designed, to investigate effect of different combination of mixture isoacids that including isobutyric acid ,isovaleric acid and 2-methyl nail acid on rumen fermentation in cow and find out their appropriate ratio. The result showed that different combination of mixture isoacids had no significant influence on pH of rumen (P＞0.05); the NO.1 combination of mixture isoacids could significantly decrease the concentration of NH3-N of rumen to 18.36%(P＜0.05) as well as increase the concentration of BCP and digestion of the residue of organic dry matter by 13.43% and 10.97%(P＜0.05). In this case, the evaluation of MFAEI of this concentration was positive. Therefore, we could conclude that the combination of 45%∶33%∶22% of mixture isoacids could release the protein degradation and increase the concentration of BCP and the digestion of organic dry matter residue.

Key words: cow; rumen fermention; isoacids; in vitro