Quantitative effects of alfalfa extract supply on rice straw degradation, fermentation and biomass synthesis by rumen microorganisms in vitro

Abstract

In Asia and Africa, rice straw enters into livestock feeds as the main constituent. It is sometimes associated with fresh legume material, from local garden farming or from legume-cereal rotations, but the possible benefits of this practice need to be estimated more precisely, with special attention to the influence of cytoplasmic contents other than nitrogen or macrominerals on rumen microbial metabolism. In the present study, fresh Medicago sativa extract (AE) was chosen as a model of cytoplasmic contents from tops of tropical legumes such as Vigna unguiculata. It was obtained from whole plants harvested at the beginning of flowering (primary growth). The effects of AE supplementation on a rumen microbial community maintained on a diet made of rice straw and inorganic nitrogen (CP: 73.3 g·kg-1 DM) were assessed in dual outflow continuous culture. Three input levels were applied in order to detect curvilinear effects: 0, 0.227 and 0.455 ml·g-1 straw DM. The pH and the redox potential varied from 6.72 to 6.84, and from -335 to -370 mV respectively. The addition of AE had minor effects on fermentation variates. The true degradability of dietary OM and the degradabilities of ash-free NDF and ADF were not modified by AE. Alfalfa extract lowered the outflow of fermented OM (FOM) by 14% while it enhanced the microbial OM outflow (+33%) and the efficiency of microbial protein synthesis (15 to 26 g N·kg-1 FOM). This effect was curvilinear, and appeared negligible above 45-50% of the maximal extract input rate. This legitimised the addition of fresh legume materials to the diet even in small amounts.