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

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Effect of silica content on rice straw ruminal degradation

International audienceThe present study investigated how increasing silica content modifies the chemical and morphological characteristics and microbial degradation of rice straw. Two rice straws grown on culture media with different silica contents were collected and analysed for chemical and morphological composition and ultrastructure. Silica content (g/kg DM) of the two straws was high (98.0, straw H) and low (0.0, not detected, straw L). The ground (2 mm) straws were incubated in the rumen of three ruminally cannulated cows for 4, 8, 16, 24, 48 and 96 h for in sacco degradability measurements. The main fractions (stem, leaf sheath and blade) of each straw were incubated for 24 It. Kinetics were fitted to an exponential model with lag time. Scanning electron microscopy was used to examine intact fragments of stems and fragments incubated for 6, 24, 72 and 96 h in cow rumen. X-ray analysis was carried out on non-incubated fragments for silica mapping. A reduction of stem content from 320 to 270 g/kg straw, and an increase in leaf sheath content from 360 to 400 g/kg straw, were found for an increase in silica content from 0 to 100 g/kg DM. Chemical composition differed little between straws H and L, except for ash and silica contents. The ash originated mainly from the leaves and increased with silica content from 70 to 140 g/kg DM. The DM degradability was lower in the H than in the L straw. This reduction came mainly from a lower non-NDF component degradation in straw H. Stems were more degraded than leaf fractions for DM, NDF and non-NDF components; their degradation was depressed in the H straw. X-ray analysis showed that the deposition of silica on the stem epidermis was different, with a thick layer on H stems and no deposition on L stem surfaces. As a consequence, microbial colonization and degradation were faster on the epidermis of straw L. Epidermal silica did not hinder degradation from the internal cavity

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

International audienceIn 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$\cdot$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$\cdot$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$\cdot$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.Les effets quantitatifs de l'apport d'extrait de luzerne sur la dégradation de la paille de riz, la fermentation et la synthèse de biomasse par les micro-organismes du rumen in vitro. En Asie et en Afrique, l'affouragement des ruminants en paille de riz est parfois associé à des déchets de maraîchage ou à des légumineuses (Niébé). Les bénéfices éventuels de cette pratique doivent être vérifiés, en portant une attention particulière à l'action des composés cytoplasmiques autres que les protéines et les minéraux majeurs sur le métabolisme des micro-organismes du rumen. Le contenu cytoplasmique du modèle choisi est un extrait par pressage d'une luzerne (Medicago sativa) récoltée en début de floraison (EL). Nous avons étudié l'effet de l'EL sur une communauté de micro-organismes du rumen maintenue en fermenteurs à double effluent sur une ration paille de riz + azote inorganique (MAT : 73.3 g$\cdot$kg$^{-1}$ MS). Trois niveaux d'apport ont été appliqués pour détecter d'éventuels effets curvilinéaires : 0 ; 0,227 ; 0,455 ml$\cdot$g$^{-1}$ MS paille. Le pH et le potentiel redox ont varié de 6,72 à 6,84, et de -335 à -370 mv respectivement. L'EL n'a pas significativement modifié les variables fermentaires, ni la dégradabilité réelle de la MO, ou les dégradabilités du NDF et ADF déminéralisés. L'extrait de luzerne a diminué le flux de MO fermentée (MOF) de 14 % tandis qu'il a accru le flux de MO microbienne (+ 33 %) et l'efficacité de la synthèse de protéines microbiennes (de 15 à 26 g N$\cdot$kg$^{-1}$ MOF). Cet effet a été curvilinéaire et paraît négligeable au delà de 45-50 % du niveau d'apport maximal. Dans notre essai, l'EL n'a pas significativement modifié l'apport d'énergie et d'azote aux micro-organismes. Son action catalytique sur le métabolisme microbien justifie l'apport de matière végétale fraîche dans la ration même en faible quantité