Abstract:The detergent system of analysis partitions lignin into two fractions : acid detergent lignin (ADL), usually equated with forage lignin, and 'acid detergent dispersible lignin' (ADDL), the existence of which has scarcely been recognised, but can be higher than ADL in tropical grasses. For nine species ADL and ADDL as percentage of original dry matter were, respectively: Aristida calcyna, 8.9 and 9.8; Astrebla squarrosa, 8.6 and 14.7; Chloris gayana, 3.9 and 6.7; Eragrostis sp, 4 3 and 8.8; Heteropogon contortus, 3.9 and 10.4; Iseilema membranaceum, 7.2 and 8.2; Panicum maximum, 1.8 and 5.7; Themeda triandra, 7.3 and 8.2; Sorghum sp, 3.4 and 13.8. Tropical legumes (four species) had much lower levels of ADDL. Azo-stained fibre showed that most of the free phenolic functions and the alkali-soluble lignin were in the ADDL fraction. The difference between grasses and legumes was so marked that azo-staining may provide a method for distinguishing grass and legume particles in feed or faecal samples. The apparent discrepancies between grasses and legumes in the effect of lignin on digestibility may be explained by the grass lignin having been grossly underestimated as ADL. When Heteropogon contortus and Astrebla squarrosa were incubated in dacron bags in the rumen the ratio of ADDL to ADL in the residual fibre decreased markedly in the first 48 h, indicating a higher apparent digestibility for ADDL. From feeding experiments with sheep the apparent digestibility of ADL and ADDL were 18 and 38% for Astrebla squarrosa and 14 and 26% for Stylosanthes hamata. As isolated lignin corresponding to ADDL was inhibitory in uitro to a mixed rumen microbial population at a concentration of 0.12 mg m1-l. Measurement of ADDL could easily be included in fibre analysis by the sequential detergent method as it would require only UV absorbance measurement on the acid detergent filtrate. This would allow measurement of total lignin within the existing system.

Digestibility of fibre in ruminants may be improved by the introduction of highly fibrolytic strains of ruminal bacteria. This approach may be feasible if, for example, strains of Ruminococcus that are significantly more fibrolytic than the normal population of Ruminococcus are used for inoculation purposes. Introduced strains of bacteria, irrespective of ecosystem, often decline after inoculation, and in this study, highly fibrolytic strains of Ruminococcus were continuously dosed to ensure that measurements of fibre digestion were made in the presence of significant numbers of the introduced bacteria. During dosing the total culturable count increased significantly (P T 005), but declined post-dosing. The level of dosed Ruminococcus, and total Ruminococcus, Fibrobacter succinogenes and eukaryotes measured by 16S rRNA probes increased significantly (P T 005) during the dosing period, but also declined post-dosing. When in vitro nylon bag digestibility, feed intake or whole-tract digestibility was measured, no improvement could be measured.

In this manuscript, the authors have sought to gain a better understanding of the interactions between Escherichia coli and lactic acid bacteria (LAB) isolated from Rogossa MRS agar along the digestive tract of grain-and forage-fed cattle. E. coli from cattle receiving a high-grain diet were more numerous (P , 0?05) than from the high-forage diet and the highest numbers were in the faeces. Isolates on Rogossa MRS agar were always greater in the high-grain diet (P , 0?05) and contained a significant number of LAB. A random set of Rogossa MRS agar colonies was selected and artificial neural networks were used to develop a relationship between colony description and species which was validated using sequence analysis (16S rDNA). The neural networks correctly predicted species in more than 80 % of cases and was composed, primarily, of Lactobacillus vitulinus, Lactobacillus ruminis, Selenomonas ruminantium, Streptococcus bovis, Acidaminococcus fermentans and Megasphaera elsdenii. In conjunction with statistical diversity indices, it was demonstrated that diversity in the high-fibre diet was always lower and was a consequence of the dominance of Str. bovis. In contrast, the diversity in the high-grain diet was greater (P , 0?05) and was a consequence of the decline in Str. bovis. These data demonstrate that there is a positive relationship between coliform and LAB isolates throughout the digestive tract of cattle, and diet is the major factor regulating bacterial composition.

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