The usefulness of the n-alkane technique to estimate intake, digestibility, and diet composition in sheep fed mixed diets was tested in an experiment with 16 ewes consuming 4 different proportions of barley grain and straw ranging from 15:85 to 60:40. Two cases were considered: (1) straw as one component or (2) straw separated into leaves and stems. The relationship between estimated (Y) and observed (X) proportion of grain in the diet was highly significant (1: Y = 0.2355 + 0.9945X, r2 = 0.9999; 2: Y = –2.472 + 1.0063X, r2 = 0.9999). Actual intake was overestimated in (1) by 3.43–12.40%, the largest discrepancies corresponding to the lowest proportions of grain. In (2) actual intake was either overestimated (2.69%) or underestimated (5.38–6.85%), without a clear effect of the diet. A similar pattern was observed for digestibility, which was overestimated in (1) by 1.65–9.59% and overestimated, (by 2.07%) or underestimated (by 3.80–4.62%) in (2). Due to the small discrepancies between observed and estimated values, it may be concluded that the n-alkane technique might be a valid method for estimating intake, digestibility, and diet composition in sheep consuming mixed diets, for the two latter provided that faecal recovery of individual n-alkanes is calculated with enough accuracy.
Two experiments were carried out at Mt. Cotton, The University of Queensland, from November 1992 to July 1993, to study the effect of extent of digestion or feed type (grass or legume) on particle kinetics in the rumen. Small (0n5-1n18 mm) Yb-labelled grass or legume particles, either digested or undigested, were injected into the rumen of sheep fed on different diets, and their retention time in the reticulo-ruminal compartment measured. In Expt 1, four intact wethers were fed on either pangola grass hay, chaffed lucerne hay, pelleted lucerne hay or commercial pelleted concentrate. Digested particles from the faeces of animals fed on pangola or lucerne and undigested material from the same diets were wet-sieved and the fraction 0n5-1n18 mm collected, labelled with Yb-acetate and injected into the animals together with a solution of Cr-EDTA. Faecal samples were taken and analysed for marker concentrations. In Expt 2, four similar animals, fitted with duodenal and ruminal cannulae, were fed on different proportions of pangola grass hay and lucerne hay, and Cr-EDTA and the above mentioned labelled particles were injected through the rumen cannula. Samples were taken from the duodenum and analysed for marker concentrations.The results indicated that diet characteristics rather than extent of digestion or particle type had the greatest influence on rates of passage of both liquid and particulate phases. Different proportions of pangola and lucerne did not result in marked differences in either the volumes of rumen contents or the rates of passage of the solid phase marker but altered the rates of passage of Cr-EDTA. Increasing the proportion of legume increased intake and decreased retention time markedly, with no additive effects on digestibility.Particles of the same small size escaped with the same fractional passage rate within each diet, irrespective of type (grass or legume) or status (undigested or digested), indicating identical kinetics within each rumen type.It was concluded that rumen conditions as influenced by diet type have most influence on water and particle kinetics and that extent of digestion of the small particles used in our experiments was not important. Particles of legume or grass of the same size behaved similarly within a diet type.
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