The estimation of dry matter intake (DMI) using the alkane pair technique has been validated in ruminants, but not in equids. The current paper reports the finding of three comparative validation studies carried out using a total of 12 cattle, 29 donkeys and 10 horses during which directly measured intake was compared to estimated intake using the alkane pair technique. Two methods were developed to dose the even chain alkanes that were used as external markers. Study I, carried out in Zimbabwe, compared the accuracy of estimated intake with measured intake in cattle and donkeys using hexatriacontane (C36) as the external marker. Studies II and III were carried out in the UK with horses and donkeys and compared the accuracy of estimated intake with measured intake using dotriacontane (C32) as the external marker. Study III also tested the effect on the accuracy of intake estimates of two marker dosing levels (mean daily dose of 224 mg per animal and 448 mg per animal) and two dosing frequencies (2× and 3× daily). Twice daily dosing of even-chain alkane at the lower dose level provided an estimate of DMI similar to that obtained by thrice daily dosing at this low level. The higher dose level given twice daily tended to produce large variation in faecal concentrations of dosed even-chain alkanes, this variation was reduced when dosing frequency was increased to thrice daily. The accuracy of estimated intake improved progressively as the number of faecal sampling days was increased from one to six with no significant difference between estimated intake based on day 5 or 6 of faecal sampling.
The results of all three studies indicate that the alkane pair technique provides a robust method of estimating intake in equids with no significant difference between measured and estimated values in all but one case. Using C31 as an internal marker provided a more accurate estimated intake than using C33 as the internal marker in all cases. Faecal recoveries of alkanes in equids do not appear to show the same influence of carbon chain length that has been observed in ruminant studies.