Digestion kinetic parameters from an in vitro gas production method as predictors of voluntary intake of forage by mature ewes

Kinetics of digestion of 12 forages previously fed ad libitum to mature ewes over a period of 8 weeks was studied in an automatic gas production system. Samples (≈400 mg) were incubated in vitro with a mixed solution of rumen fluid for 72 h and the volume of gas produced was recorded every 20 min. Kinetics of fermentation was described by the exponential equation Y=a+b(1−e−c(t−L)) and by a mono- and a biphasic Michaelis–Menten equation Y=Ai/(1+(Bi/t)Ci). Constants of the exponential model were not significantly correlated to voluntary dry matter intake (DMI) and digestible dry matter intake (DDMI). The DMI and DDMI were best predicted through stepwise regression analysis using the constants of the biphasic Michaelis–Menten equation. The gas production constants A1/(2×B1) and C2 explained 60% of the variation of DMI, while A1/(2×B1), C2, A2/(2×B2) and A2 could account for 94% of the variation of DDMI. In vitro gas production at 8 h explained 76 and 48% of the variation observed for DDMI and DMI, respectively.