The use of internal markers to estimate herbage digestibility and intake: 1. Potentially indigestible cellulose and acid insoluble ash

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Abstract
Concentrations of potentially indigestible cellulose (PIC) and acid insoluble ash (AIA) were determined in samples of feed and faeces in a study to determine their value as internal markers. PIC and AIA were then used to predict the organic-matter digestibilities (OMD) of samples of perennial ryegrass (Lolium perenne) and lucerne (Medicago sativa)of known OMD which had been determined using wether sheep. The mean values for OMD (%) measured in vivo were 67·2 and 62·8 for ryegrass and 58·0 and 55·8 for lucerne at intakes of 15 and 25 g D.M./kg live weight, respectively.The in vitro digestibility technique, using rumen liquor, gave OMD values of 69·5 and 57·4 for ryegrass and lucerne which are in close agreement with the in vivo values at the lower level of intake.The mean differences between OMD measured in vivo and that estimated using PIC were: -1·1 (± 0·65) and -1·6 (± 0·46) percentage units for ryegrass and lucerne respectively, and using AIA were: -1·4 (± 0·57) and -3·5 (± 1·61).For the two feeds tested, OMD was predicted more precisely by PIC than by AIA or the in vitro digestibility technique. However, the PIC technique required the use of rumen-fistulated animals and 10 days to digest samples of faeces; this may make the technique impractical if large numbers of determinations of PIC are required.