In a rumen simulation technique (RUSITEC), the availability and timing of energy (maltose) supply to amino acids/peptides from thawed (frozen) grass was examined in order to determine if continuous (synchronous), rather than transient (asynchronous, with maltose infused 6 h prior to, or 6 h after feeding RUSITEC with grass), availability of energy was required for optimum ruminal fermentation.  The addition and pattern of energy supply (synchronous or asynchronous) did not influence either fibre (DM) degradation or microbial numbers, although there was an indication of increased total volatile fatty acids (TVFA) and acetate production in the continuous (synchronous) maltose supply.  However, the supply of energy (maltose), irrespective of the pattern of supply, improved the capture of ammonia. The effects of amino acid supplementation on mixed microorganisms fermenting a range of substrates (maize and grass silages, barley straw, avicel and xylan) that usually form part of ruminant diets were examined using gas syringe incubations.  Gas production, measured at 4, 6, 8, 12 and 18 h incubation, increased by 15.6, 18.7, 18.9, 15.0 and 5.4% respectively, in the xylan substrate, suggesting xylan fermentation was stimulated by amino acids supply.  This implied xylanolytic organisms within the mixed population benefited more from the amino acids.  A subsequent in vitro (syringe) experiment was conducted to identify amino acids that may be simulatory, using a deletion approach where individual amino acids were deleted from a complete mixture of all 20 amino acids normally found in protein.  Amino acid additions, either as the complete mixture or with single amino acid deletions, stimulated microbial growth and fermentation rate compared to only ammonia as the N source.  Although the individual deletion of aromatic amino acids (notably tyrosine and tryptophan), as well as leucine, seemed to decrease fermentation rate, microbial yield was not affected.  The mixed microbial population achieved the highest growth rate and fermentation when complete mixtures of amino acid were provided.