THE USE OF THE RUMEN DEGRADATION CHARACTERISTICS OF HAY AS PREDICTORS OF ITS VOLUNTARY INTAKE BY SHEEP

Eleven sun-cured hays with crude protein (CP) and neutral-detergent fibre (NDF) concentrations ranging from 57 to 207 and 428 to 744 g/kg dry matter (DM) respectively, were used to study the relationship between the DM and NDF degradation characteristics and the voluntary DM intake (VDMI) by sheep. The VDMI was measured in the last 10 days of a 21-day period using 50 mature ewes in five groups of 10 ewes per group, with a mean live weight (LW) of 55 kg and ranged from 41.2 to 69.8 g/day per kg LW0.75. Disappearance rates of DM and NDF were measured by incubating samples in nylon bags in the rumens of three hay-fed rumen-cannulated ewes for 3, 6, 9, 15, 24, 48 and 72 h and by fitting the exponential model Y = a + b (1 - e(-ct)) to the results. Potential degradabilities (defined as a + b) for DM and NDF ranged from 679 to 868 and from 445 to 808 g/kg respectively. The VDMI was significantly correlated with the soluble fraction of DM (aDM; P < 0.05) and the rate of degradation of DM (cDM; P < 0.01) and NDF (cNDF; P < 0.01). The inclusion of aDM and cDM in a multiple regression model resulted in the equation: VDMI = 21.3 + 0.0733 (s.e. 0.01577) aDM + 138 (s.e. 26.0) cDM (R2 = 0.897; residual s.d. = 2.79), with a significant increase of the variance explained by one single degradation characteristic (P < 0.05). The prediction was not improved by adding the constant cNDF to the equation (P < 0.05). These results indicate the possibility of predicting the VDMI of hays by sheep from the rumen soluble fraction (aDM) and the rate of degradation (cDM) of DM.