RELATIONSHIP BETWEEN PHENOLIC-ACIDS FORMED DURING RUMEN DEGRADATION OF MAIZE SAMPLES AND IN-VITRO DIGESTIBILITY

The organic matter (OMD) and cell wall (CWD) digestibility of stalks o f 25 different maize samples were determined in an in vitro assay usin g rumen fluid. The stalk OMD and CWD varied between 62-80% and 44-62%, respectively. The free phenolic acids formed in the liquid phase duri ng fermentation in in vitro incubations were assessed by HPLC and H-1 NMR. The following nine phenolic acids could be demonstrated to be pre sent: p-hydroxybenzoic (HBA), p-hydroxyphenylacetic (HPAA), vanillic ( VA), syringic (SYRA), 3-(4-hydroxyphenyl)propionic (HPPA), p-coumaric (PCA), ferulic (FA), phenylacetic (PAA) and 3-phenylpropionic (PPA) ac ids. Large variation was observed among the maize samples for all phen olic acids identified. The most abundant phenolic acids, PPA, PAA and HPAA were present in concentrations ranging from 0.57 to 1.41 mM, 0.32 to 0.46 mM, and 71 to 105 mu M, respectively, and were present as fre e monomeric molecules. HBA concentrations ranged from 6.9 to 21.1 mu M and this acid was also present as free molecules. The other phenolic acids could only be observed after NaOH treatment of the liquid phase and were detected in minor amounts (0 to 12 mu M). Correlation studies revealed a negative relation between digestibility and NaOH labile es ters in the medium (ie PCA, FA, VA and SYRA esters) and a positive rel ationship between digestibility and PPA and PAA. When path analysis wa s applied, an inverse effect of PCA esters in the medium on both OMD a nd CWD (r = -0.71 and -0.56, respectively) was observed and a high pos itive effect of PPA acid on CWD (r = +0.67). Principal component analy sis revealed close associations between released phenolic acid esters, neutral detergent fibre, and cell wall esters of PCA and FA.