The cell wall biomass of the forages and its utilization by the herbivous

Abstract

Cell-wall carbohydrates arising from photosynthesis produce large amounts of biomass. Some of this lignocellulose can only be effectively used by herbivores thank to the microorganisms living in the digestive tracts. The ruminant is the most studied herbivore. It was used in the studies as a model for the process of anaerobic degradation of lignocellulosic compounds and to study the specific role of the different micro-organisms in the rumen (bacteria, protozoa, fungi). The products of the microbial metabolism (volatile fatty acids - vitamins) are used by the host animal. Furthermore, the microbial biomass constitues the main supply of animo-acids to the ruminant. Some very lignified forages are not easy hydrolysed by the microorganisms. The main difficulty is the large amount of lignin they contain. It is possible to improve their degradation by three methods.

1°) Using of appropiate technology. Alkali treatments are economic and, especially ammonia, increasingly used, whereas grinding and steam treatment are only attractive in certain situations. Sometimes, white fungi treatments have produced interesting results but are still at laboratory and pilot stage and need further improvements. Other chemical (oxidants, SO₂), physical (irradiation) and biological (enzymes, bacterial protein enrichment) treatments are presently uneconomic and only partially tested on animals, but they have provided interesting information on the relationship between cellwall structure and degradability. In any event, upgrading of crop residues by best treatments presently available only increases digestibility to 0.5 - 0.6.

2°) Optimization of the bacterial activity in the rumen can be achieved by providing the organisms optimum quantities of the nutrients they need. In addition, genetical engineering opens up prospects for the improvement of bacterial muralytic enzymes activity, especially in unfavourable media (cellulolysis in low pH media).

3°) Optimization of morphologic and physiologic characteristics of the digestive tract can be attained examining the possibilities offered by different animals according to the position of the fermentation chamber (herbivores with fermentor at the beginning or at the end of the digestive tract) and by the characteristics of the fermentor (capacity, retention time of solid particles, turn over rate of liquid, distribution of liquid and solid phases...). This approach is particulary interesting for countries oriented towards the utilization of low quality crop residues.