The current definition of subacute ruminal acidosis (SARA) is based on the pH of the ruminal fluid in dairy cattle and ration physically effective fiber could be used as a non-invasive method for prediction of ruminal pH. OBJECTIVES: The purpose of this study was to evaluate the correlation between ration physically effective fiber, measured using old and modified Penn-state particle separator and ruminal pH. METHODES: The physically effective fiber measured in the ration of 17 rational group (7 fresh and 10 mid lactation cow groups). Ruminal fluid obtiained by rumenocentesis and SARA affected cows were detected. RESULTS: The physically effective fiber measured by modifed Penn-state particle separator had a correlation with mean ruminal pH of cows (r= 0.595, p= 0.012) and the ratio of subacute ruminal acidosis affected cows in rational groups (r= -0.533, p= 0.027). The ration of SARAaffected mid lactation groups had less physically effective fiber than non affected groups (p= 0.041). CONCLUSIONS:According to our results, the physically effective fiber measured by modifed Penn-state particle separator had more correlation with ruminal pH in dairy cattle and can be used as a predictive index for ruminal pH.

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