Comparison of different in vitro and in situ methods to estimate the extent and rate of degradation of hays in the rumen

Eleven sun-cured hays with a crude protein and neutral-detergent fibre contents ranging from 57 to 207 g kg−1 and 428 to 744 g kg−1 dry-matter (DM) respectively, were used to study the suitability of different in vitro and in situ methods to estimate the extent and rate of degradation in the rumen. The methods used to study degradation kinetics were: (1) DM disappearance when a sample of each hay was placed in nylon bags and incubated in the rumen for different lengths of time, (2) gas production (GAS) at different incubation times when hays were incubated in vitro with buffered rumen fluid, (3) DM disappearance at different incubation times in a cellulase solution (CEL), (4) sugars (SUGAR) release to the supernatant after incubation in the cellulase solution and (5) optical density of the supernatant at λ=280 nm (as indicator of phenolic compounds release) after solubility in the cellulase (PHEN). All kinetic data were fitted to first-order kinetic models. From the relationships between gas production and DM degradation in situ it was estimated that 0.295 ml (SE 0.0335) gas were produced per mg of DM degraded for incubation times beyond 24 h, although this ratio was significantly lower for short incubation times. There were significant (P<0.01) correlations between fractional degradation rates estimated by in situ, GAS and CEL methods, whereas digestion rates estimated by the SUGAR method were only correlated (P<0.01) with the in situ degradation rates. Estimated values obtained with method PHEN were not significantly (P>0.05) correlated with those obtained by other methods. Methods in situ, GAS and CEL resulted in similar relative ranking of forages according to their degradation rate (significant Spearman rank correlation coefficients), although there were significant differences in the absolute values estimated. Those differences were larger as the legumes proportion of the hays was greater. In situ degradation characteristics can be estimated from in vitro gas production parameters, although prediction equations were different depending on the botanical composition of the hay.