Accuracy of the n-alkane technique for intake estimates in beef cattle using different sampling procedures and feeding levels

Abstract

Abstract The present study aimed to evaluate the reliability of the n -alkane technique for estimating herbage intake in beef cattle receiving two different feeding levels (low and high) and to test the effect of different faeces sampling procedures (total faeces samples or rectal grab samples at 8 h intervals) on the n -alkane faecal recoveries and hence on intake estimates. Two consecutive experiments were performed with 11 non-lactating beef cows of “Asturiana de los Valles” breed, fed with lucerne hay at two feeding levels: 1.0 or 1.7 kg DM/100 kg body weight (BW). Animals received a daily dose of paper pellets containing C 24 , C 32 and C 36 n -alkanes as external markers with the purpose of using different n -alkane pairs of adjacent chain length (C 23 /C 24 , C 25 /C 24 , C 31 /C 32 , C 33 /C 32 , C 35 /C 36 ) for intake calculations. A 5-day equilibrium period was sufficient for external marker concentrations and n -alkane pair ratios to reach steady state in faeces. There was no effect of different sampling times (every 8 h) on the faecal excretion of n -alkane pairs C 31 /C 32 and C 33 /C 32 . Grab samples obtained at the time of dosing (0830 h) gave the best estimate of n -alkane pair ratios in total faeces collection. There was a general trend of increasing n -alkane faecal recoveries with increased chain length, although n -alkanes C 30 , C 31 and C 33 showed lower faecal recoveries than expected. n -Alkane recoveries were in general lower at the low feeding level, where an increase in individual variability of natural n -alkane recoveries was observed. At both feeding levels, the n -alkane pairs C 23 /C 24 and C 35 /C 36 gave accurate estimates of the treatment average intake, although the natural n -alkane (C 23 or C 35 ) faecal recoveries showed high individual variability, which could be in part due to analytical bias caused by their extremely low concentration in the diet and faeces. However, the n -alkane pairs C 31 /C 32 and C 33 /C 32 gave the greatest deviations of intake estimates, due to the high discrepancy between the faecal recoveries of natural and dosed n -alkanes. These results demonstrate that in beef cattle, feeding level may have some influence on the relative faecal recoveries of the n -alkane pair used for intake calculations. This effect, together with the high individual variability of n -alkane recoveries, especially under low feeding level, may produce significant deviations of the individual intake estimates.