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 of 25 different maize samples were determined in an in vitro assay using rumen fluid. The stalk OMD and CWD varied between 62-80% and 44-62%, respectively. The free phenolic acids formed in the liquid phase during fermentation in in vitro incubations were assessed by HPLC and 1H NMR. The following nine phenolic acids could be demonstrated to be present: 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) acids. Large variation was observed among the maize samples for all phenolic 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 μM, respectively, and were present as free monomeric molecules. HBA concentrations ranged from 6·9 to 21·1 μM 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 μM). Correlation studies revealed a negative relation between digestibility and NaOH labile esters in the medium (ie PCA, FA, VA and SYRA esters) and a positive relationship between digestibility and PPA and PAA. When path analysis was applied, an inverse effect of PCA esters in the medium on both OMD and CWD (r = -0·71 and -0·56, respectively) was observed and a high positive effect of PPA acid on CWD (r = +0·67). Principal component analysis revealed close associations between released phenolic acid esters, neutral detergent fibre, and cell wall esters of PCA and FA.