PREDICTION OF APPARENT DIGESTIBILITY AND VOLUNTARY INTAKE OF HAYS FED TO SHEEP - COMPARISON BETWEEN USING FIBER COMPONENTS, IN-VITRO DIGESTIBILITY OR CHARACTERISTICS OF GAS-PRODUCTION OR NYLON BAG DEGRADATION

The study compared using chemical components (i.e. crude protein (CP), neutral- and acid-detergent fibre or lignin (NDF, ADF and ADL) g/kg dry matter (DM), the in vitro digestibility (Tilley and Terry, 1963), the in situ (nylon bag) DM degradation (g/100 g DM) and gas production (ml/200 mg DM) techniques to predict voluntary daily intake (g DM per kg MO(0.75)) and in vivo apparent DM digestibility (DMD) of 10 graminaceous hays individually offered ad libitum to four Merino male sheep. Gas production or DM degradation were determined after 6, 12, 24, 48, 72 or 96 h incubation and their characteristics described using the equation p = a + b (1 - e(ct)). Intake and in vivo DMD of the hays were variable and poorly related (r = 0.52; P > 0.05). The in situ DM degradation was significantly (P < 0.05) related to in vivo apparent DMD at 48 to 96 h incubation (r = 0.76 to 0.75) and to intake at 24 to 96 h (r = 0.71 to 0.75) incubation. However, fibre components, the in vitro digestibility or gas production were either related to daily intake or in vivo apparent DMD, bur nor to both on the same occasion. Accurate prediction of intake (r = 0.90; P < 0.05) and in vivo apparent DMD (r = 0.88; P < 0.069) were achieved using NDF, ADF, ADL and CP in a multiple regression. Using the (a + b) and the rate (c) of in situ DM degradation, both in vivo apparent DMD (r = 0.77; P < 0.05) and intake (r = 0.83; P < 0.05) were predicted with accuracy. However, using the (a + b) and (c) of gas production, only intake was predicted accurately (r = 0.87; P < 0.01). The lower performance of the gas test was attributed to the small contribution to gas production and higher buffering capacity resulting from protein fermentation. When data of the graminaceous and other data from leguminous hays were combined, the most accurate prediction of both intake and apparent digestibility was by using characteristics of in situ DM degradation followed by those of gas production. The latter was more accurate than using chemical components or the in vitro digestibility. Addition of CP in the multiple regression improved the prediction of intake and in vivo apparent DMD from characteristics of gas production. It was concluded that despite the need to overcome the problem of protein fermentation in the gas test, accurate prediction of both intake and apparent digestibility can be achieved simply from the degradation characteristics of foods.