This project sought to develop improved methods for estimating nutritive value applicable to LDC diets. Two simple laboratory assays have been shown to indicate the inhibitory effect of tannins on rumen microbes. The project has used in vitro gas production as a core technique to evaluate feeds and feed mixtures. It has demonstrated that: the technique is inhibited by tannins; is very sensitive to differences between feeds; is sensitive to interactions between feeds in mixtures and responds to nitrogen supplementation. Strong relationships were found between the extent of degradability of pasture samples during in vitro gas production and in the rumen (as measured by the nylon bag technique). Relationships between gas production, other in vitro methods and in vivo digestibility were, however, less strong for fodder tree leaves.
During a study on Nepalese fodder tree leaves, no strong positive correlations were found between farmers' rankings of fodder tree leaf nutritive value and in vitro digestibility. Farmers' rankings appeared to be related to protein supply and dung quality (which was inversely related to in vitro digestibility). This was a surprise as it had been expected that digestibility would be the primary positive indicator of fodder tree quality. The in vitro gas production method was modified to provide information on the nitrogen status of feed mixtures. The ability of the method to ferment feed mixtures under controlled nitrogen limiting conditions, while being able to monitor fermentation kinetics where nitrogen scarcity has major effects, gives in vitro gas production advantages over existing nutritive value assessment methods for studying the supplementation of feeds. The approach has been applied to feeds used in three in vivo supplementation experiments and one single feed experiment to help validate the technique, and the initial screening of fodder mixtures from N W India and Costa Rica to illustrate its use.
Variable responses were found in both intake and digestibility during in vivo trials. The interactions observed in vitro appeared to correspond to interactions observed in vivo in some trials, but were at times difficult to interpret. Strong correlations were found between intake and the gas production rate constant (b), and between digestibility and the gas production lag time (T+) for roughages and supplemented roughages in the final, most comprehensive trial. A more mechanistic approach to integrating and interpreting in vivo and in vitro data is required, together with a more detailed integration and consideration of data obtained by this and collaborating projects.
The gas production method did not appear, in practice, to give reliable indications of the effects of all the major relevant anti-nutritive factors in fodder trees. Two simple bioassays were found to be potentially useful for screening fodder trees for such factors. These methods need to be used more widely to assess their usefulness in evaluating tree fodders. Laboratory methods aimed at indicating the supply of bypass protein in tanniniferous feeds appeared unconvincing and require further development.
The project covered technically complex fields of work using new approaches; there is considerable scope for further strategic research in this field. The gas production method is being actively researched for application to intensive feeding systems by several other groups in the UK and elsewhere; doubtless there will be further developments in the technique. The most recent findings of this project indicate that useful indicators of performance are obtainable for supplemented roughages. The gas production method appears to be particularly suitable for investigating the key areas of interest in improving LDC diets, where traditional in vitro techniques are inappropriate for providing the required information, and is sufficiently developed to start applying it to practical problem solving.
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