Rice straw degradation and biomass synthesis by rumen micro-organisms in continuous culture in response to ammonia treatment and legume extract supplementation

The effects of ammonia treatment and fresh legume extract supplementation on the metabolism of rumen micro-organisms maintained on rice straw (Oryza sativa, variety Thaibonnet) in dual outflow fermenteurs were investigated. Alfalfa (Medicago sativa L.) was chosen as a model legume, similar in biochemical composition to cowpea (Vigna unguiculata). Diets provided comparable amounts of readily available N. The interaction between ammonia treatment and legume extract supplementation was generally negligible. The relative productions of VFA (in moles/100 moles of hexoses fermented) were oriented to an increased proportion of C4 to the detriment of C2 by ammonia pre-treatment (+100% versus −28% at the intermediate extract input level), which also lowered the relative production of CH4 by 36%. The true degradability of organic matter (OM) was 27% higher with treated rice straw (TRS) than with untreated rice straw (URS). The degradabilities of ash-free neutral detergent fibre (NDF) and acid detergent fibre (ADF) ranged from 30 to 50% and from 26 to 48%, respectively. At low levels of extract supplementation, the degradabilities of ash-free NDF and ADF tended to be lower with TRS than with URS (30% versus 46% in the case of ADF), while at high levels of the alfalfa extract they were not modified by straw pre-treatment. The outflow of non-ammonia nitrogen (NAN) was higher with TRS (+41% at the intermediate legume input rate). This increased NAN outflow was essentially of microbial origin. Alfalfa extract supplementation also increased net production of microbial protein. The efficiency of microbial protein synthesis was strongly increased by ammonia pre-treatment and by extract supplementation, in a linear manner. Both treatments were beneficial to rumen microbial metabolism in vitro, while acting independently on microbial functions. Ammonia treatment induced the most significant changes, especially in favouring extensive biomass synthesis.