No abstract

SU MMARYThe 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 h. 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.

The combined effects of lucerne (Medicago sativaL.) extract supplementation and ammonia treatment of rice straw (Oryza sativa, variety Thaibonnet) on the ruminal digestion of cell wall components were investigated in six continuous culture systems using a randomised complete block design. Data were fitted to second-order polynomial models. Untreated rice straw had higher contents of ash-free cell wall residues (CWR; 763v.687 g/kg dry matter (DM)) and non-cellulosic sugars (191v.166 g/kg DM) than treated rice straw. Ammoniation preferentially removed xylose, which resulted in a lower xylose-to-arabinose ratio (5.1v.5.8). In absence of lucerne supplementation and ammoniation, degradability coefficients were 0.54, 0.46, 0.58, 0.54, 0.42 and 0.60 for cellulose–glucose, xylose, arabinose, galactose, mannose and uronic acids, respectively. Both factors had significant effects on the microbial degradation of structural polysaccharides. With lucerne extract at an optimal level, ammonia treatment increased ash-free cell wall degradation by more than 10%. The degradability coefficients were increased by ammoniation without any significant interaction with lucerne extract, except for glucose, whose degradability was mostly influenced by lucerne extract in a curvilinear way. The comparison of regression coefficients in cell wall and CWR models suggested that ammoniation improved the degradabilities of xylose, galactose and mannose by partly solubilising the corresponding hemicelluloses and by improving the susceptibility of the remaining fraction to microbial attack, whereas it increased the degradability of arabinose only by favouring microbial attack.

-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.

The chemical composition, intake, digestibility, ultrastructure and microbial degradation of rice straw from Camargue were compared with barley straw. These variables were observed in two different herbivore digestive ecosystems: the sheep rumen and the donkey caecum. The two straws differed essentially in their ash content, which was three times higher in rice, owing to its silica content. Other chemical components were comparable, except a higher phenolic acids-to-lignin ratio in rice. Rice straw was better ingested than barley straw. Organic matter and neutral detergent fibre digestibilities were the same in both straws. Dry matter and cell wall disappearances could be adjusted to the exponential modelling equation with lag time, and differed between animals but not between straws. The sheep rumen had a higher extent of degradation, but the donkey caecum had a higher degradation rate. Statistical analysis revealed that cell-wall components degradation was similar in the two straws except for ferulic acid, which was more degraded in rice straw. Scanning electron microscopy showed important differences in parenchyma degradation, which was much more effective in rice.