Three experiments were conducted in order to develop and validate principal component (PC) regressions for predicting rumen volatile fatty acid (VFA) proportions, based on a combination of milk odd and branched chain fatty acids (MOBCFA). Grass- or legume silage and concentrate-based diets were fed to rumen fistulated Holstein-Friesian dairy cows. The diets differed in silage type (Expt. 1), amount of protected starch (Expt. 2) or varied in the forage to concentrate ratio (F/C). Each experimental period lasted for 28 days. Rumen fluid and milk samples were collected at regular intervals for the determination of VFA content. It was shown that equations based on OBCFA explained 30-75% of the variation in VFA. Compared to predictions based on MOBCFA, only predictions of propionic (HPR) and butyric (BUT) acids based on milk fatty acids showed higher coefficients of determination. Prediction of BUT was the least accurate, while there were relatively accurate predictions for HPR and acetic acid (HAC). Lower values for mean square prediction error (MSPE) suggest better HAC and HPR predictions by regressions based on MOBCFA than on MFA. These results confirm the potential of MFA, particularly MOBCFA, to predict rumen VFA proportions in dairy cows
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