Some studies use the mobile bag technique in fistulated animals, but no fis-tulation is necessary when the small bags of feed are discharged into the stomach via a nasogastral (naso-oesophageal) tube. It was first used in equids by Machelboeuf et al. (1995), and subsequently by Hyslop and Cuddeford (1996). An assessment of this technique for studying the dynamics of fibre digestion in equids has been made by Tomlinson (1997). She used rye grass hay, dehy drated grass, dehydrated lucerne and grass chaff, and concluded that the MBT is a useful tool for studying the dynamics of total tract digestion in equids, and is sensitive enough to distinguish differences in digestibility between the four foodstuffs. A typical graph of disappearance of DM and NDF from mobile bags containing ryegrass hay subjected to total tract digestion by a mature Welsh cross pony gelding is shown in Fig. 10.1.
The MBT is one of the in sacco methods, and these have been reviewed by Noziere and Michalet-Doreau (2000). The bags are made from polyester, nylon or Dacron material woven with a controlled pore size and heat-sealed into small pouches. The latter is a compromise between retention of the ground foodstuff and sufficient permeability to allow complete penetration of the food particles by the gastric juices, enzymes and microbial population. A range of between 40 pm and 60 pm is suitable for most purposes (Nocek, 1988). The sample size should be ^1.0 g, and milled to 1 mm. Machelboeuf et at. (1995, 1996), however, used 200 mg milled to 3 mm, and sealed them in 60 x 10 mm bags. Cherian et at. (1988, 1989), found that a pre-digestion stage gave closer agreement with conventional methods when measuring the apparent digestibility of protein for swine. This was achieved by soaking the feed, with agitation, for 4 h in a solution of pepsin with an activity of 3771 IU l-1 in 0.01 M HCl (pH 2). The exposure of samples to equine saliva before insertion into the stomach may affect the digestibility, but is yet to be investigated. There is, however, little amylase in equine saliva (M. Moore-Colyer, Aberystwyth, 2001, personal communication).
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