Cortes et al., 2005. Rev. Bras. Zootec., 34 (5): 1468-1474

This trial was carried out to determine the composition of n-alkanes (C24 to C36) in hypothetical diets comprising of pure Brachiaria brizantha Stapf. cv. Marandu and Arachis pintoi Koprov & Gregory. cv. Amarillo and mixtures of these two spececies with 15%, 30%, 45%, or 60% of Arachis pintoi); it also intended to identify the combination of alkanes that allows to calculate the botanical composition of diets with the smallest residual value (real less estimated values). The forages were sampled in the summer. The n-alkanes were extracted for the direct saponification method and identified and quantified by the gas chromatography analysis. Alkane C34 was used as internal marker. The proportions of A. pintoi in the diets were estimated by the minimization of z (sum of the squares of the deviations) between the real proportion of the analyzed alkanes and the pre-established proportions (treatments), using Duncan et al. (1999) equation. The prevalence of odd carbonic chains was detected. The total concentration of n-alkanes decreased as the proportion of A. pintoi increased in the mixtures. Good estimates of the botanical composition of the mixtures of A. pintoi with B. brizantha were obtained from the alkanes C29, C31, C33 and C35. The alkane C35 was fundamental for the quality of the estimates. The results indicate the great potential of the technique for studies of grazing animal diet.