Estimation of feed intake and digestibility in cattle consuming low-quality tropical roughage diets using molasses-based n-alkane boluses

A feeding experiment was conducted to measure the faecal recovery rates of n-alkanes and to evaluate molasses-based alkane boluses for feed intake and digestibility estimations in cattle consuming low-quality tropical roughages. The experiment was performed in a cross-over design with four experimental diets, four 21-day feeding runs and eight bulls. The animals received a measured amount of the experimental diets that resulted in little refusal throughout the experiment. After seven days of adaptation, the animals were dosed with molasses-based alkane boluses (each containing 200 g C32 and 150 g C36) twice daily at 07:00 and 18:00 h. Concurrent with the alkane dosing, faecal spot samples were taken twice daily until the end of each run. In addition, total faecal collections were performed over the last 5 days of each run. The mean faecal recovery rate of both natural and dosed n-alkanes ranged between 0.61 and 0.86, with the recovery showing an upward trend with increasing carbon-chain length. The recovery rate of dosed alkanes was considerably higher than that of adjacent odd-chain alkanes. Whilst diets did not differ (P≥0.23) in the recovery of even-chain n-alkanes, an effect of diet (P≤0.01) was observed in the recovery of odd-chain n-alkanes. The faecal concentration of dosed alkanes reached equilibrium 3.30 days into the alkane dosing. On the assumption of similar faecal recovery of adjacent n-alkanes, intake was underestimated by 12% (P<0.001) when C31/C33 and C33/C32 alkane pairs were used and by only 1.5% (P≥0.42) when C35/C36 was used. Correction for differences in the faecal recovery of adjacent n-alkanes considerably improved the intake prediction when C31/C32 and C33/C32 pairs were used. Digestibility of diets was accurately predicted using either C36 as external marker or C35 as internal marker corrected for incomplete recovery. The results showed that molasses-based boluses administered twice daily are suitable, and that knowledge of the faecal recovery rates of adjacent n-alkanes improves the reliability of the predictions.