The effect of Ca supplementation on the methane-suppressing effect of lauric acid was investigated in an experiment based on a 2 × 2-factorial arrangement using the in vitro system Rusitec. Additional Ca was supplemented at 1 g kg-1 diet in the form of compounds of relatively low solubility in rumen fluid. Lauric acid (C12:0), the predominant effective medium-chain fatty acid in coconut oil, was added at a level of 50 g kg-1. Adding C12:0 did not affect bacterial count, but eliminated ciliate protozoa from fermenters. Ammonia concentration in fermentation mixture declined and volatile fatty acid pattern changed with C12:0. The apparent degradation rate of total organic matter was not altered by C12:0, but fiber fermentation was depressed (P < 0.001). Effects of Ca on microbial counts and fermentation characteristics remained low. Without additional Ca, C12:0 reduced the average daily methane release (mmol g-1 organic matter degraded) by 76%. In comparison, C12:0 only reduced methane production by 47% when additional Ca was included in the diet (interaction of C12:0 and Ca, P < 0.05). The present results suggest that the dietary content of soap-forming Ca has to be kept low in order to achieve a high methane-suppressing effect of lauric acid. Key words: Methane, lauric acid, lipids, calcium, Rusitec, ruminants
