15 juin 1998
auteur
Les techniques de laboratoire surestiment généralement la valeur réelle des dicotylédones non légumineuses et des fourrages riches en dicotylédones. Des nombreuses espèces prairiales inhibent l'activité des enzymes cellulolytiques et de la population microbienne du rumen. Une nouvelle méthode de laboratoire permet d'évaluer l'importance de cette action.
Le contenu cellulaire peut contenir certaines substances défensives du métabolisme secondaire dont l'action sur la population microbienne du jus de rumen a été étudiée pour 51 espèces des prairies permanentes. L'extrait cellulaire a été obtenu par macération de la matière végétale dans de la salive artificielle. Cet extrait est mélangé à du jus de rumen et placé dans un appareil (fermenteur) en anoxie. L'appareil permet de mesurer le volume de gaz produit par la fermentation microbienne. La mesure est effectuée après 150 minutes de fermentation ; la répétabilité est bonne. Les graminées, généralement pauvres en métabolites secondaires, exercent une faible inhibition de l'activité fermentaire. Certaines dicotylédones riches en substances phénoliques, terpéniques et alcaloïdes, ainsi que certains acides organiques toxiques, peuvent fortement diminuer l'activité microbienne du rumen.
Due to the general acceptance of Van Soest's concept based on the assumption that the plant cellular content (or soluble fraction) is 98% digestible, the importance of this fraction is considered only quantitatively. Its qualitative aspect is rather neglected. The plant cellular content may contain some of the plant defensive compounds (secondary plant metabolites) of which the inhibitory potential on the microbial population of the rumen fluid was investigated. One hundred and twenty samples corresponding to 51 natural
grassland species were collected separately. The choice of species was based on both their agronomic interest and their secondary metabolite composition. The rumen fluid was obtained from rumen fistulated lactating cows, immediately frozen in liquid nitrogen and stored in a freezer at a temperature of -80°C. The plant extract was obtained by maceration of plant material in McDougall's artificial saliva (20h at 45°C) under anaerobic conditions. In order to prevent an important dilution of potentially active plant principles in artificial saliva, the sample/solvent ratio (2,5 g in 35 ml of saliva) was close to the rumen solid/fluid phase ratio. The plant extract mixed (ratio : 4 + 1) with strained rumen fluid, rapidly thawed in a 39°C water bath, was carried out in an anaerobic fermentation apparatus and incubated at 39 °C in a ventilated incubator. A simple fermentation apparatus which may be assembled in any laboratory was developed. It works on the principle that the gas produced during microbial fermentation is accumulated in the upper part of a graduated tube. The reading of the gas production volume was done after 150 minutes of incubation. The natural grassland species known for their poor palatability appear at the end of the observed species classification. These species are rich in secondary metabolites with anti microbial properties (phenolic and terpenoid compounds, toxic organic acids, alkaloids, cardiac glycosides and others).
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