Valorisation nutritionnelle d’Opuntia et Acacia spp. dans l’alimentation des ruminants

Abstract

Plant rich secondary compounds have antimicrobial effects by acting against different rumen microbial populations. The current thesis is based on the study of the effects of fodder shrubs (spineless cactus, Acacia nilotica and Acacia cyanophylla), rich in secondary metabolites, alone or in various mixtures on the metabolic activity of rumen microbiota of cattle. The qualitative and quantitative determination of in vitro fermentation gases is used as a marker of fermentation substrates. The Monitoring of quantitative fluctuations in the equilibrium of the major populations of rumen microbiota is performed by a molecular biology technique, the real-time PCR. Based on the chemical analysis of the studied substrates, Acacias have the advantage of being rich in crude protein but with the disadvantage of being too rich in lignin (indigestible cell wall fraction) and in polyphenols, mainly condensed tannins (TCT) which are antinutritional factors. Moreover, cactus cladodes are rich in energy, water and minerals. However, they seem to be poor in crude protein content and high in total oxalates, these compounds are anti-nutritional compounds that are able to bind calcium, making this anion unavailable for the animal. In general, the percentage of shrubs in the mixture negatively affects their in vitro digestibility and microbial fermentation (p˂0,001). However, the proportion of 30% incorporated cactus stands out positively from other mixtures by synergy in terms of gas production, digestibility and a remarkable fermentation pattern, indicating a balanced combination of energy and nitrogen content of the ration, nutritionally for the animal. The TCT contained in A.cyanophylla seem to influence significantly ruminal digestion and their inhibitory effects are more pronounced than those of A. nilotica. Analysis of the methanogenic activity of rumen microbiota shows that the spineless cactus and Acacias significantly reduce the production of CH4 (p˂0, 0001). The impact of TCT contained in the Acacias, on reducing CH4 production is more tangible, however with a higher effect for A.cyanophylla relatively to A.nilotica. CH4 production is decreased by up to de 62,5% for 60% of A.cynophylla, added to vetch-oat hay. This indicates the use of Acacia to reduce energy losses and, in turn, minimize atmospheric emissions of CH4 and the adverse effects on the environment (greenhouse effect and global warming). Molecular analysis reveals important changes in the microbial profile of the rumen: a significant imbalance among cellulolytic groups, including, Fibrobacter succinogenes population which increases relatively with all the substrates, unlike Ruminoccocus flavefaciens species which decrease significantly (p <0,05), suggesting a selective effect of tannins on the latter (from 60% of Acacia in the mixture). A significant anti protozoan effect (p <0,05) of shrub species is also observed in parallel to the inhibition of the production of ammonia. Furthermore, the tannins content of A.nilotica and A.cyanophylla have antimethanogenic activities but without, apparently, direct effects on relative abundance of methanogenic archaea populations, indicating a potential resistance of this population to tannins of the studied species. This study also showed an antimicrobial activity of oxalates content of spineless cactus cladodes but confirmation would require further investigations.