The in situ dry matter (DM) and neutral-detergent fibre (NDF) degradability kinetics of eight forages (four grass hays and four legume hays, harvested at two different dates) were compared to assess the fitting ability of a first-order and a Gompertz model.
The Gompertz model fitted DM degradability data as well as the first-order model and differences between fitted and observed data for the two models were very small but the Gompertz model proved to be statistically superior for the NDF degradability data, especially for the early hours of incubation.
A numerical but not significant difference was observed in the estimated rapidly available fraction for DM and NDF, which zvas respectively lower (mean values 24·4 v. 27·8%) and higher (mean values 5·8 v. 1·8%) with the first-order model. More pronounced differences were observed for the estimates of total potential degradability of NDF, which were often significantly lower with the Gompertz model (average values for the eight forages 75·1 v. 72·3%;.
The sigmoidal shape of the Gompertz model was more biologically appropriate to describe the initial phases of NDF degradation and was thus applied to the cellulose and hemicellulose degradability data.
As the harvesting date progressed through the season, a decrease of the immediately available fraction of DM and nitrogen was generally observed but the effect of harvesting date was not so evident for fibre fractions; the differences within forages were very low. Correlation coefficients between lignin content and total potential degradability of fibre were always high (for NDF, r = −0·96; for hemicellulose r = −0·95; for cellulose r = −0·79; P < 0·001), while the acid-detergent fibre content influenced DM and nitrogen total potential degradability (r = −0·91 and −0·82, respectively).
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