1. The relationship existing between the gross digestible energy content and the chemical composition of twenty-four silages has been studied with nine Cheviot wether lambs over a period of 8 months.
2. Silages were made from grassland herbage and varied widely in the chemical composition of the ingoing material.
3. Each silage was fed to a group of three lambs for a 10-day preliminary- and a 9-day collectionperiod. Faeces and urine were collected in metabolism crates.
4. The silages were analysed for volatile acids and bases, for proximate nutrients and for some of the more precise chemical constituents as suggested in the scheme of analysis of Ferguson (1948).
5. A close relationship existed between the gross digestible energy contents of the silages and the lignin (Ellis) content.
6. The most accurate prediction of gross digestible energy was obtained by taking into account the content of lignin (Ellis), cellulose (Crampton & Maynard) and crude protein, in the silage.
7. In the case of hays (Walker & Hepburn, 1955) no increase in the accuracy of predicting gross digestible energy was obtained by estimating the lignin and cellulose content, rather than crude fibre alone. However, with silages the results give support to the opinion of Mitchell (1942) and Schneider et al. (1951) that analysis for more precise chemical constituents will increase the accuracy of predicting digestibility.
8. Crude fibre and cellulose were not related in any fixed way with the digestibility of energy. Crude fibre itself was digested to some 80% compared with 62% in hays.
9. Metabolizable energy was closely related to digestible energy.
10. The starch equivalents of the silages were calculated by conventional methods and also from the content of gross digestible energy.