The use of dosed and herbage n-alkanes as markers for the determination of herbage intake

Abstract

SUMMARY The recovery in the faeces of the n-alkanes of herbage (odd-chain, C 27 –C 35 ) and of dosed artificial alkanes (even-chain, C 28 and C 32 ) was studied in twelve 4-month-old castrated male lambs. The lambs received three levels of cut, fresh perennial ryegrass or a mixed diet of perennial ryegrass (0·70) and a barley-based concentrate (0·30) (500–900 g D.M./day). C28 and C32 n-alkanes (130 mg each), absorbed onto shredded paper, were given once daily for 17 days to test whether the recoveries of herbage and dosed alkanes were similar to enable their use as markers for determining the herbage intake of grazing sheep. Stearic and palmitic acids (130 mg each) were given with the dosed alkanes to half of the animals with the objective of facilitating emulsification of the dosed alkanes within the digestive tract. With the exception of C 27 n-alkane, the faecal recoveries of all alkanes were unaffected by diet, feeding level or emulsifying agent. Faecal recovery of odd- chain herbage n-alkanes increased with increasing C-chain length. The recovery of the dosed C 28 n-alkane was slightly greater than the recoveries of both C 27 , and C 29 n-alkanes of herbage. The recoveries of the dosed C 32 n-alkane and the herbage C 33 -alkane were the same. The mean herbage intake estimated using C 33 and C 32 n-alkanes was identical to the actual herbage intake. Other alkane pairs gave slight underestimates of herbage intake ranging from 3·5% for the C 28 –C 29 pair to 7·6% for the C 27 –C 28 pair. No cyclical pattern of n-alkane excretion throughout the day was observed. Examination of daily variations in faecal alkane concentrations indicated that the start of alkane dosing should precede the sampling of faeces by at least 6 days. These results suggest that accurate estimation of herbage intake in grazing sheep is possible from the simultaneous use of dosed C 32 and herbage C 33 n-alkanes as markers. The method may be particularly useful in enabling unbiased estimates of herbage intake to be made in animals receiving supplementary feed.