In vitro ruminal methane suppression by lauric acid as influenced by dietary calcium

The effect of Ca supplementation on the methane-suppressing effect of lauric acid was investigated in an experiment based on a 2 x 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 (C-12:0), the predominant effective medium-chain fatty acid in coconut oil, was added at a level of 50 g kg(-1). Adding C-12:0 did not affect bacterial count, but eliminated ciliate protozoa from fermenters. Ammonia concentration in fermentation mixture declined and volatile fatty acid pattern changed with C-12:0. The apparent degradation rate of total organic matter was not altered by C-12:0, but fiber fermentation was depressed (P < 0.001). Effects of Ca on microbial counts and fermentation characteristics remained low. Without additional Ca, C-12:0 reduced the average daily methane release (mmol g(-1) organic matter degraded) by 76%. In comparison, C-12:0 only reduced methane production by 47% when additional Ca was included in the diet (interaction of C-12:0 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.