Fermentation kinetics of stems of sorghum and millet genotypes

Abstract

Gas production profiles were obtained from in vitro fermentation of stems of six genotypes of sorghum and millet grown at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), India. The ranking of sorghum and millet genotypes by cumulative gas production was consistent throughout the 96-hour fermentation period. However, differences were proportionally 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 observations of these substrates, and fermentation kinetics parameters were calculated 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 (+ or -) 50 percent, were stable, showing no dependence on the starting values of the model parameters. However, in millet stems, the C parameter has shown a tendency to converge near unity. The stability of the final values of the parameters of the model in this study suggests the potential applicability 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 between 0 and 3 h. In both, sorghum and millet the asymptotic gas of the first phase (A1) was 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, respectively). The estimated maximum fractional rate of substrate digestion in the second phase (Rm2) 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). Estimated 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 kinetics parameters. The degree of variability between genotypes observed for kinetics parameters related to both, phases 1 and 2 suggest that these parameters have potential for discriminating feed quality differences between crop residues or plant parts. These results point out the difficulty in finding a single feed quality parameter to be used to rank different crop residues of different genotypes. Additional studies are needed to associate these parameters with voluntary intake and in vivo rumen outflow rates with which these phases could be associated.