Effect of bag pore size on the loss of particulate matter and on the degradation of cell wall fibre

The loss of particulate dry matter and water-soluble dry matter from roughage has been studied in vitro with nylon bags with 5-, 6-, 10-, 20- and 36-μm apertures. Degradation of the acid detergent fibre (ADF) fraction and the disappearance of lignin were subsequently studied in situ using nylon bas with 5-, 6-, 10- and 20-μm apertures. There are only small and non-significant differences in the loss of particulate dry matter between nylon bags with 5-, 6- and 10-μm apertures. Increasing the aperture from 10 to 20 μm significantly (P < 0.0001) increases the loss of particulate dry matter. The amount of water-soluble dry matter is also substantially increased from nylon bags with 10- to those with 20-μm apertures. There are significant differences in the degradation rate of the ADF fraction between nylon bags with 5- and 10-μm apertures (P < 0.0001) and between nylon bags with 10- and 20-μm apertures (P < 0.0003). Mainly this is reflected in the digestion during the first 24 h. The loss of particulate dry matter in situ, measured as lignin loss, is quite similar for nylon bags with 5- and 10-μm apertures. Between nylon bags with 5- and 20-μm apertures on the other hand, the difference in lignin loss is highly significant (P < 0.0001). It is concluded that for samples milled to pass a 1-mm screen, a membrane aperture of 10 μm is a good compromise considering inflow and washout.