Alkanes as internal markers to estimate digestibility of hay or hay plus concentrate diets in horses

Dry matter intake (DMI), dry matter digestibility (DMD), and fecal output ( FO) are difficult to measure directly in the field, and indirect methods us ing external and internal markers have thus been developed. An experiment w as conducted consisting of two digestion trials with two periods in each tr ial to examine the use of five odd-chain alkanes (C25 to C33) of plant cuti cular wax as internal markers to estimate DMD of hay or hay plus concentrat e diets in horses. Eight mature Thoroughbred geldings were housed in 4- x 4 -m stalls and randomly assigned to one of two mixed grass/legume;hays (Diet s 1 and 2) in Trial 1 and to mixed grass/legume hay plus one of two concent rates (Diets 3 and 4) in Trial 2. After the first 12-d period was conducted , dietary assignments for each group were switched for the second period in each trial. Each period consisted of a dietary accommodation from d 1 to 7 and total fecal collection from d 8 to 11. Results indicated that fecal re coveries of odd-chain alkanes were 88 to 90% for Diet 1, 75 to 92% for Diet 2, 71 to 81% for Diet 3, and 71 to 82% for Diet 4. Alkane recoveries were not related to alkane chain lengths. Digestibilities calculated from alkane concentration data adjusted using the mean fecal recovery of individual od d-chain alkanes (D-A1) were not significantly different from the digestibil ities estimated from total collection (D-TC) for Diets 1 and 2 in Trial 1 a nd Diets 3 and 4 in Trial 2. When adjustment was based on the mean recovery of all alkanes (D-A2; estimated by linear regression), all DA2 estimates f or horses offered all diets were similar to DTC Results indicate that accur ate mean estimates of DMD can be obtained by using plant wax alkane markers and adjusting for the mean recovery of five odd-chain alkanes in a diet.