Fermentation kinetics of stems of sorghum and millet genotypes

Gas production profiles were obtained from in vitro fermentation of stems o f six genotypes of sorghum and millet grown at the International Crops Rese arch Institute for the Semi-Arid Tropics (ICRISAT), India. The ranking of s orghum and millet genotypes by cumulative gas production was consistent thr oughout the 96-hour fermentation period. However, differences were proporti onally greater during the initial 3 and 6 h of fermentation. The multiphase model described by Groot et al. [Groot, J.C.J., Cone, J.W., Williams, B.A., Debersaques, F.M.A., Lantinga, E.A., 1996. Anim. Feed Sci. Technol. 64:77-89] was used to fit the in vitro fermentation gas observatio ns of these substrates, and fermentation kinetics parameters were calculate d using the fitted model. The final estimates of the model parameters (A, B , C), tested by varying the initial estimates obtained with the monophasic model by +/-50%, were stable, showing no dependence on the starting values of the model parameters. However, in millet stems, the C parameter has show n a tendency to converge near unity. The stability of the final values of t he parameters of the model in this study suggests the potential applicabili ty of the multiphase model when only nine gas observations over a period of 96 h were available. However, the indeterminacy in the parameters of phase 1 for some millet stems indicates the need of an intermediate gas value be tween 0 and 3 h. In both, sorghum and millet the asymptotic gas of the first phase (A(1)) wa s negatively correlated with NDF (r = -0.82, p < 0.05; r = -0.80, p < 0.05, respectively) and lignin (r = -0.86, p < 0.05; r = -0.95, p < 0.01, respec tively). The estimated maximum fractional rate of substrate digestion in th e second phase (R-m2) showed a strong inverse relationship with lignin (r = -0.93, p < 0.01) in millet but not in sorghum. On the other hand, the time at which the rate of fermentation reached its maximum in phase 1 (t(max1)) was negatively correlated with ADF and lignin (r = -0.88, p < 0.05 and r = -0.87, p < 0.05, respectively) in sorghum, whereas in millet only t(max2) (phase 2) was negatively correlated with lignin (r = -0.88, p < 0.05). Esti mated differences in the contribution from the different phases to similar cumulative gas at 48 h in stems of different sorghum genotypes indicate the need to complement measurements of total gas production with selected kine tics parameters. The degree of variability between genotypes observed for k inetics parameters related to both, phases 1 and 2 suggest that these param eters have potential for discriminating feed quality differences between cr op residues or plant parts. These results point out the difficulty in findi ng a single feed quality parameter to be used to rank different crop residu es of different genotypes. Additional studies are needed to associate these parameters with voluntary intake and in vivo rumen outflow rates with whic h these phases could be associated. (C) 1999 Elsevier Science B.V. All righ ts reserved.