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The efficiency of myristic acid (14 : 0) as a feed additive to suppress CH4 emissions of ruminants was evaluated under different dietary conditions. Six sheep were subjected to a 6 x 6 Latin square arrangement. A supplement of non-esterified 14 : 0 (50 g/kg DM) was added to two basal diets differing in their forage:concentrate values (1 : 1.5 and 1 : 0.5), which were adjusted to dietary Ca contents of 4.2 and 9.0 g/kg DM, respectively. Comparisons were made with the unsupplemented basal diets (4.2 g Ca/kg DM). The 14 : 0 supplementation decreased (P<0.001) total tract CH4 release depending on basal diet type (interaction, P<0.001) and dietary Ca level (P<0.05, post hoc test). In the concentrate-based diet, 14 : 0 suppressed CH4 emission by 58 and 47 % with 4.2 and 9.0 g Ca/kg DM, respectively. The 14 : 0 effect was lower (22 %) in the forage-based diet and became insignificant with additional Ca. Myristic acid inhibited (P<0.05) rumen archaea without significantly altering proportions of individual methanogen orders. Ciliate protozoa concentration was decreased (P<0.05, post hoc test) by 14 : 0 only in combination with 9.0 g Ca/kg DM. Rumen fluid NH3 concentration and acetate:propionate were decreased (P<0.05) and water consumption was lower (P<0.01) with 14 : 0. The use of 14 : 0 had no clear effects on total tract organic matter and fibre digestion; this further illustrates that the suppressed methanogenesis resulted from direct effects against methanogens. The present study demonstrated that 14 : 0 is a potent CH4 inhibitor but, to be effective in CH4 mitigation feeding strategies, interactions with other diet ingredients have to be considered.
Machmüller et al. (Mon,) studied this question.