The effects of anti-nutritne compounds in tropical legumes on ruminant nutrient utilization, excretion and decomposition of manure in the soil
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Many smallholder farmers in sub-Saharan Africa are using herbaceous and shrub legumes for livestock feeding and as green manures. Besides their beneficial high nitrogen contents, these legumes also contain a variety of antinutritive compounds, such as tannins, alkaloids and saponins. The objectives of this study were: i) to investigate the roles of anti-nutritive plant compounds in tropical forage legumes on nutrient utilization by ruminants and the consequences of their interactions, ii) to explore the effects of tannins and alkaloids and their interactions on decomposition and mineralization patterns of animal and compost manures in acidic tropical soils, and iii) to investigate the role of micro- and meso-fauna on decomposition of soil amendments containing tannins and alkaloids. In vitro, purified condensed tannins (CT) and sparteine (quinolizidine alkaloid) significantly (P < 0.05) depressed neutral detergent fiber degradability. However, only CT significantly reduced cumulative gas volume and the rate of gas production. A significant negative interaction (P < 0.01) between tannins and alkaloids on rate of gas production was observed. Simulations with the Cornell Net Carbohydrate and Protein System (CNCPS) showed that reduction of the degradation rate of the B2 carbohydrate fraction (digestible fiber) by CT reduced energy allowable milk by 1.0 kg! d when the rate changed from 0.067 h-l to 0.051 h-l. Microbial protein synthesis from fermentation of forages with alkaloids and tannins was responsive to the level and quality of available energy in a feed and the inhibitory effects of the secondary compounds were most evident under conditions of limited nutrient availability. Tannins, but not alkaloids, reduced N digestibility by animals, but there were no significant effects on intake and dry matter digestibility. Feces from animals fed legumes containing tannins and alkaloids and compost from these same plants decomposed significantly faster (P < 0.05) than feeds when incubated in the soil. Decomposition and release of nutrients from soil amendments placed in 2000 11mpore size litterbags were higher (P < 0.001) than those placed in 53 Jlffi mesh size bags. These studies indicate that presence of anti-nutritive compounds in ruminant feeds, especially tannins, can reduce nutrient availability but may not necessarily affect animal performance. These studies also suggest that presence of meso-fauna is critical in litter and manure decomposition in the soil.