THE POTENTIAL FOR USING N-ALKANES IN TROPICAL FORAGES AS A MARKER FOR THE DETERMINATION OF DRY-MATTER BY GRAZING RUMINANTS

The intake of temperate forages by herbivores can be determined using the C33 alkane naturally present in the forage and dosed C32 alkane. To determine whether the technique can be used with tropical forages, the concentrations of C33 alkane were determined in seven species. The leaves of Brachiaria decumbens, Digitaria decumbens, Pennisetum glaucum and Stylosanthes scabra contained sufficient C33 alkane (> 50 mg/kg DM) for the estimation of dry matter intake. Low concentrations of C33 alkane were found in the leaves of Setaria sphacelata, Sorghum sp. and Leucaena leucocephala. Setaria sphacelata was found to contain high concentrations of C27 alkane and therefore intake could be estimated by using C28 as the dosed alkane although the intake would possibly then be underestimated by c. 8%. Leaves of Sorghum sp. contained > 50 mg C31 alkane/kg DM and, with C32 as the dosed alkane, intake would possibly be underestimated by c. 5%. Leucaena leucocephala contained insufficient alkanes to estimate forage intake. The concentration of n-alkanes in Pennisetum glaucum and Sorghum sp. leaves decreased with age. It was concluded that some tropical forages contain insufficient C33 alkane for the measurement of intake by the double alkane technique. In these species, a shorter chain length alkane can sometimes be used, but only with a reduction in accuracy in estimating intake.