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 Est- DFtol method is recommended because it minimizes the isomerisation of the polyunsaturated fatty acids and yields a more accurate measurement of the FAs profile.

CLA; faeces; fatty acid; gas chromatography; repeatability

Published by University of Zagreb, Faculty of Agriculture, Zagreb, CROATIA
Sponsored by Ministry Science and Education of the Republic of Croatia