The use of the N-alkane technique for measuring herbage dry matter intake in horses.
The use of n-alkanes as indigestible markers for the estimation of herbage dry matter intake in grazing ruminants, is reported to have significant advantages over other markers used for this purpose (Dove and Mayes, 1991). The use of n-alkanes to estimate herbage intake in horses has not previously been reported, and was therefore investigated in this study. A preliminary trial, carried out in order to investigate possible practical problems in applying the technique to horses, showed that administration of the external marker (C32) in the form of coated grass pellets was satisfactory. Practical difficulties likely to be encountered in carrying out further indoor feeding/faecal collection trials were highlighted during this trial. Four, mature, thoroughbred geldings were used in an indoor feeding and total faecal collection trial to determine the accuracy of intake estimates made using the C31:C32 and C32:C33 n-alkane pairs. The faecal recovery of C31, C32, C33 and C36 as well as the ability of these n-alkanes to provide estimates of diet digestibility were also investigated. Estimates of intake made using the C32:C33 pairs were not significantly different (P< 0.05) from measured intake when the horses were consuming fresh Lolium perenne or Pennisetum clandestinum, P. clandestinum hay or a mixture of concentrates and hay. Overall, error of intake estimate using this n-alkane pair in the total collection trial was 4.8 ± 7%. The C31 :C32 n-alkane pair gave estimates of intake for individual animals that gave significant differences (P<0.05) from measured values when the horses were consuming P. clandestinum hay and the hay + concentrate diet. However, mean intakes were not significantly different (P>0.05) from measured intakes for the fresh L. perenne or P. clandestinum, or hay diet. Overall error in intake estimate was 8.5 ± 16%. Faecal grab samples taken twice daily gave mean estimated intake values which were not significantly different from measured intakes However, estimates of the intake of certain individual animals were found to be significantly different from measured values, using either of the n-alkane pairs. The overall error in intake estimate was 15.1 ± 14% and 4.0 ± 14% for the C31:C32 and C32:C33 estimates respectively. The faecal recovery of the n-alkanes was significantly lower (P< 0.05) when horses consumed L. perenne than when consuming the P. clandestinum grass, grass hay or a mixture of hay and concentrates. The recovery of the C31 , C32 and C33 n-alkanes were similar to those reported for ruminants (Dove and Mayes, 1991). However, there was no significant increase in n-alkane recovery with increasing chain length, and the recovery of C36 was significantly lower than reported in previous studies with ruminants, suggesting differences in behaviour of n-alkanes in the digestive tracts of ruminants and horses. Single doses of C32 resulted in peak faecal concentrations of C32 being reached between 20 and 32 hours after marker administration, after which the levels of the n-alkane decreased rapidly. Complete C32 excretion appeared to be complete 56-64 hours after final dose administration. Single, daily doses of C32 appeared to be insufficient to produce a steady state of faecal C32 marker excretion in the horse, the extent of which may have been influenced by the diet being consumed, resulting in marked diurnal variation in faecal n-alkane concentrations. The results of this trial showed that reasonable estimates of herbage intake in the horse may be obtained using the C32:C33 n-alkane pair. The C31:C32 n-alkane pair was a less reliable indicator of intake but may still be used to provide intake estimates. However, more frequent administration of external n-alkanes than once daily dosing, may be necessary to improve the accuracy of intake estimates made using faecal grab samples, due to the diurnal variation found in this study. The low faecal recovery of the n-alkanes observed in this study made limited their use as digestibility indicators. However, no comparable data is available to confirm the results of this trial. Further investigation is needed with regard to the difference in behaviour of the n-alkanes in the digestive tracts of ruminants and horses. The frequency of external n-alkane marker administration, and the impact of diurnal variation in faecal marker concentration, needs further investigation if the n-alkane technique is to be used successfully with grazing horses.