A comparison among analytical methods to assess fatty acids and conjugated linoleic acids (CLA) content and repeatability of ruminant faeces

Methods to determine fatty acids (FAs) and CLA contents of faeces should limit isomerisation, provide a good repeatability of the measures, avoid the use of harmful substances. Three methods of FAs extraction from faeces for GC analysis were compared: Est-DFtol, based on extraction and esterification of FAs contained in dry faeces using Na-methoxide, methanolic-HCl and toluene as solvent; Est-EEtol, based on acid-base extraction and esterification of FAs on the faecal ether extract (EE), using toluene as solvent; and AEst-EEhept, based on an acid catalyzed esterification of FAs contained in EE, using n-heptane as solvent. Faeces were collected from bulls receiving 0, 8 and 80 g/d of rumen protected CLA (rpCLA). The faeces of 9 bulls (3 for each dose) were analysed in triplicates by each method. Methods were compared by linear regression. The measurements performed with Est-EEtol and AEst-EEhept regressed against those of Est-DFtol, evidenced, in particularly for CLA isomers and their sum, positive intercepts and slopes significantly lower than the unity. The proportions of c18:2,t9,t11 found with Est-DFtol and AEst-EEhept were correlated to the dose of rpCLA (R = 0.87 and 0.51, respectively), whereas those found with Est-EEtol did not (R = 0.17). The EstDFtol method is recommended because it minimizes the isomerisation of the polyunsaturated fatty acids and yields a more accurate measurement of the FAs profile.