Method 102 Determination of digestibility using the mobile bag technique


• Monofilamentous polyester fibre - 0.41 pm pore size, (Sericol Ltd, Westwood Road, Broadstairs, Kent CT10 2PA, UK), folded in half and double-sealed along two of the three open sides using a fabric heat-sealer to make bag sizes of 40 x 10 mm and 60 x 10 mm.

• Nasogastral stomach tube - approximately 10 mm ID x 2500 mm long flexible PVC tubing with rounded end for insertion.

Note: There are several Home Office regulations under the Animals (Scientific Procedures) Act 1986 which must be complied with in the UK. The premises used for the research project will require a certificate of designation to authorize its use for the specified animal species and procedure. A project licence must be obtained for the specific procedure/s being undertaken; details must be given regarding the background, objectives, benefits, plan of work, list of procedures, severity of impact on the animal, and whether any pain, suffering, distress or lasting harm may occur. Finally, a personal licence will need to be obtained for the person performing the procedure, and details provided of the techniques, animals, and whether anaesthesia will be administered. It is also wise to take veterinary advice before proceeding. For further information and application forms, consult the Home Office website pages:

Procedure. Grind approximately 100 g foodstuff through a 1-mm mesh and sieve to remove all particles (about 0.5-2.0% by weight) less than 45 pm. Number the bags with a waterproof pen, dry the bags at 60°C for 8 h, cool in a desiccator and weigh. Transfer 130 mg foodstuff into the 40 x 10 mm bags and 200 mg into the 60 x 10 mm bags, double-seal the open end and reweigh to confirm no sample has been lost. Also weigh a separate sample of foodstuff for DM determination by heating at 60°C for 48 h, and a sample for NDF, or other nutrient components, if required. If weighing the same foodstuff, take a further portion for DM after every batch of 22 bags. Samples should also be taken and subjected to only the washing cycle to give a zero time value and an estimate of water-soluble components plus loss of any fine particles. Load the stomach tube by inserting the batch of bags with a brass rod with rounded ends or similar, being careful not to puncture the bags. Using a hand pump, flush the sample bags into the stomach using approximately 750 ml warm water during the morning meal after ingestion of approximately 1 kg hay, then continue the meal.

Retrieve the bags from the faeces at the set times; if there is any delay between retrieval of bags and washing, or between washing and drying, store the bags at 4°C to reduce microbial activity. Briefly rinse under running tap water to clean the bulk of the material adhering to the outside of the bags, then wash the bags in cold water in an automatic washing machine (Hyslop et al., 1999) for 45 min with four rinses (4 min immersion in water plus 3.3 minutes agitation per rinse) to remove the remaining adhering microbial and exogenous debris from the outer surface, also endogenous and free microbial contamination from within the bag. (Note: hand-rinsing in tap water may lead to significant variations, and ^5 short term rinses, e.g. 1 min agitation plus 2 min spin have been suggested (Broderick and Cochran, 2000); machine washing undigested samples in bags reveal there could be a significant DM loss from this process, but this loss would include water-soluble components. A less severe washing methodology may need to be developed.) Dry the bags in a forced-draught oven at 60°C for 48 h, cool in a desiccator and weigh to determine the disappearance in DM. Remove the residue for determination of NDF etc., if required. The residual DM from bags containing the same foodstuff retrieved from a particular animal on the same occasion may be pooled and well mixed for subsequent analyses.

Data analysis. The data may be analysed using the techniques presented by 0rskov and McDonald (1979) or Dhanoa (1988). The former authors suggested fitting the rate of disappearance (p) of DM, CP, NDF, etc., to an equation of the form:

where t is the incubation time. With increase in t, p increases, but at a reducing rate. This is an empirical equation to fit the incubation data, where a, b, and c are constants fitted by an iterative least-squares procedure. It is also possible to conceive that these constants represent the following parameters: a is the rapidly soluble fraction b is the slowly degradable fraction c is the fractional rate constant at which b will be degraded per unit time

For a protein supplemented feed, 0rskov and McDonald (1979) found a = 20, b = 80, c = 0.082 and k = 0.046 h-1.

The effective degradability, or effective percentage degradation (ED), becomes:

where k represents the fractional outflow rate passing through the gut per hour, which is measured by regression analysis. In ruminant studies, these authors determined k in a separate experiment using a chromium marker to render indigestible the feed supplement particle to which it was attached. ED therefore provides an estimate of the degradability of the feed component under the specified feeding conditions.

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