Estimation of fescue hay intake in horses using the n-alkanes method

The aim of the work was to evaluate the n-alkane method for measuring forage intake in horses. Four Quarter horses were housed in individual loose boxes and fed with fescue hay and concentrate in three equal feeds at 08.30, 13.00 and 17.00 hrs. After a preliminary period of 12 days, each horse received, for 8 days, 3 g/d of shredded paper impregnated with n-dotriacontane (C32, 209 mg/g), and 3 g/d of shredded paper impregnated with n-hexatriacontane (C36, 164 mg/g), mixed with the concentrate, as external markers. The most present n-alkanes in the forage, n-hentriacontane (C31) and n-tritriacontane (C33), were used as natural markers. Individual faecal samples were collected three times a day. The n-alkane concentrations in the faeces varied widely between days (P<0.001 for C31, C33 and C36; P<0.05 for C32); moreover, dosed n-alkanes did not show the expected enrichment phase during the initial days following marker administration. The variability was reduced when the ratio between natural and dosed n-alkanes was considered, although statistical analysis revealed that only the C31 to C32 ratio did not vary between days. The reduced between-days fluctuation was more evident starting from the 4th day from marker administration; the forage intake was calculated from this day onwards, using individual marker concentration of a single day (day 4), or of pooled days, i.e. the mean value of two (days 4 and 5), three (days 4 to 6), four (days 4 to 7) and five (days 4 to 8) days. The estimated intake was generally lower than the actual amount of forage ingested. The best estimate was obtained with C31/C32 pairs. Increasing the days of sampling from 1 to 4 caused a marked reduction of the standard deviation for all the estimates; only a modest reduction of variability was observed for the further increase of the period to 5 days.