No abstract

Phenylpropanoids limit the degradation of cell walls of roughages in herbivores but at the same time undergo transformations in the digestive tract. This review outlines the main transformations that occur in the rumen. All the monomeric aromatics tested are fully degraded under anaerobic conditions which favour electron transfer. Six main strains of bacteria degrade monomeric phenols in the rumen by several mechanisms. In addition, some fungi and bacteria are able to release, and possibly to metabolise the esterified hydroxycinnamic acids found in forage cell walls. The first step in the degradation of these acids is their reduction to non‐toxic compounds, which are often growth factors. However, total degradation of monoaromatics is difficult to achieve in vivo because of the small population of organisms able to metabolise them abd the limited transit time of the substrates in the rumen. Oligolignols are also degraded to different extents depending on their size and molecular structure. Lignins are partly solubilised during cell wall degradation. They may also undergo other transformations such as demethylation and dehydroxylation. The amount of lignin that seems to be degraded in rumen fluid is low but probably higher than under other anaerobic environments over the same period of time. It is generally accepted that the digestibility of forage lignins is low. However, the wide range of values measured (from minus 0.46 to 0.64) is related either to the measurement method or to the transformations that the lignins may undergo in the digestive tract, or to both. An indigestible fraction of lignins could serve as a reliable cell wall marker but none of the fractions used to date has proved entirely satisfactory for this purpose. Future research in this field would involve better knowledge about the transformation of phenylpropanoids and the development of microbial activity on these compounds. This would improve phenolics degradation and consequently carbohydrate utilization.

Apical internodes of tall fescue (Festuca arundinacea Schreb. var. Clarine) harvested at flowering were sectioned into 5 or 10 equal parts to study in situ degradability and cell wall composition, respectively. The basal (youngest) section had the greatest primary wall content. Cell walls in the upper (older) sections had the highest xylose/arabinose ratio and lignin content and a lignin rich in syringyl units, all typical of extensive secondary wall development. Almost all of the p-coumaric (p-CA) and about half of the ferulic acid (FA) were released by 1 M NaOH and presumed to be ester-linked. The total FA content was approximately double that of p-CA in all sections other than the youngest with a distribution similar to that of total p-CA. However, the ratio of esterified to ether and ether plus ester linked (Et & Et+Es) FA differed with age. Whereas the esterified form remained essentially constant ( approximately 4.5 g/kg of cell wall), Et & Et+Es ferulate increased with increasing age of the tissue and was significantly related to lignin deposition (r = 0.79, P < 0.01). The extent of cell wall degradation after 48 h of incubation in the rumen was inversely related to maturity, falling from 835 g/kg of dry matter in the youngest section to 396 g/kg in the oldest. Both the rate and extent of cell wall degradation were significantly negatively related to the ratio of xylose to arabinose, lignin content, proportion of syringyl units present in lignin, and concentration of Et & Et+Es FA present. A positive relationship between Et & Et+Es FA was also found, with the rate (P < 0.01) being better correlated than the extent (P < 0.05) of cell wall degradation. Application of the newly extended internode model to fescue produced results consistent with the view that both the lignin content and the extent to which lignin was covalently bound to the other wall polymers crucially influenced the rate and extent of degradation.

Phenylpropanoids limit the degradation of cell wails of roughages in herbivores but at the same time undergo transformations in the digestive tract. This review outlines the main transformations that occur in the rumen. All the monomeric aromatics tested are fully degraded under anaerobis conditions which favour electron transfer. Six main strains of bacteria degrade monomeric phenols in the rumen by several mechanisms. In addition, some fungi and bacteria are able to release, and possibly to metabolise the esterified hydroxycinnamic acids found in forage cell walls. The first step in the degradation of these acids is their reduction to non-toxic compounds, which are often growth factors. However, total degradation of monoaromatics is diicult to achieve in vivo because of the small population of organisms able to metabolise them and the limited transit time of the substrates in the rumen. Oligolignols are also degraded to different extents depending on their size and molecular structure. Lignins are partly solubihsed during cell wall degradation. They may also undergo other transformations such as demethylation and dehydroxylation. The amount of lignin that seems to be degraded in rumen fluid is low but probably higher than under other anaerobic environments over the same period of time. It is generally accepted that the digestibility of forage lignins is low. However, the wide range of values measured (from minus 0.46 to 0.64) is related either to the measurement method or to the transformations that the lignins may undergo in the digestive tract, or to both. An indigestible fraction of lignins could serve as a reliable cell wall marker but none of the fractions used to date has proved entirely satisfactory for this purpose. Future research in this field would involve better knowledge about the transformation of phenylpropanoids and the development of microbial activity on these compounds. This would improve phenol& degradation and consequently carbohydrate utilization.

No abstract