Method 73 Determination of modified acid detergent fibre MAD fibre

This modification to the ADF method was designed to improve the relationship between the ADF and digestibility in ruminants. The sample drying temperature of 95°C, however, means it is unsuitable for assaying heat damage and unavailable protein (Van Soest, 1982). The method below is based on that given in MAFF/ADAS (1986, pp. 93-94), with Crown Copyright permission. See Chapter 4 for further discussion on fibre extraction procedures.

Reagents.

• Acetone, commercial 'drum' grade

• Sulphuric acid-CTAB solution - dissolve 10 g of cetyltrimethylammonium bromide in 1 l of 0.5 M sulphuric acid using a magnetic stirrer, and filter if necessary.

Procedure. Transfer 1.000 g of the dried (95°C) and ground (1 mm) sample into a 500-ml short-neck round-bottom flask with ground glass socket size 34/35. Add 100 ml of the sulphuric acid-CTAB solution, transfer to a macro Kjeldahl heating unit and connect a coiled reflux condenser through which a steady stream of cold water is flowing, and bring to the boil within 1 min on full heat. Reduce the heat and continue to boil gently for 2 h. Reduce any excessive foaming while boiling, or at the filtration stage, by addition of 1-2 drops octan-2-ol, and swirl every 5 min to wash sample particles from the flask wall back into the acid. If any bumping has caused sample particles to enter the condenser, squirt a wash-bottle into the top of the condenser to wash them back into the acid using the minimum amount of water. Filter hot through a previously dried (place in cool muffle, raise to 500°C and maintain for 30 min) and weighed 60 ml borosilicate porosity No. 1 (P160, 100-160 pm) filter crucible using gentle suction. Wash the residue with 3 x 50 ml portions of almost boiling water and then with acetone. Allow to air-dry in a fume cupboard until no smell of acetone can be detected. Dry the crucible and contents overnight in an oven at 102°C. Allow to cool in a desiccator and weigh. Retain the crucible and contents for determination of ash-free MAD fibre, as detailed below.

Procedure (ash-free MAD fibre). Place the crucible and its contents of MAD fibre in a cool muffle furnace. Increase the temperature of the furnace to 475-500°C; keep at this temperature for at least 30 min and until ashing is complete. It is important not to overshoot the temperature, because the sintered crucibles will be damaged at ^ 515°C. Remove the crucible from the muffle furnace, cool in a desiccator and weigh.

Calculations. Subtract the weight of the empty crucible from the weight of the crucible plus MAD fibre and multiply by 1000 to give the g kg-1, or by 100 to give the % MAD fibre in the sample DM.

Subtract the weight of the crucible plus ash from the weight of the crucible plus MAD fibre and multiply by 1000 to give the g kg-1, or by 100 to give the % ash-free MAD fibre in the sample DM.

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