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 differ ent in vitro and in situ methods to estimate the extent and rate of de gradation in the rumen. The methods used to study degradation kinetics were: (1) DM disappearance when a sample of each hay was placed in ny lon bags and incubated in the rumen for different lengths of time, (2) gas production (GAS) at different incubation times when hays were inc ubated in vitro with buffered rumen fluid, (3) DM disappearance at dif ferent incubation times in a cellulase solution (CEL), (4) sugars (SUG AR) release to the supernatant after incubation in the cellulase solut ion and (5) optical density of the supernatant at lambda=280 nm las in dicator of phenolic compounds release) after solubility in the cellula se (PHEN). All kinetic data were fitted to first-order kinetic models. From the relationships between gas production and DM degradation in s itu it was estimated that 0.295 mi (SE 0.0335) gas were produced per m g of DM degraded for incubation times beyond 24 h, although this ratio was significantly lower for short incubation times. There were signif icant (P<0.01) correlations between fractional degradation rates estim ated by in situ, GAS and CEL methods, whereas digestion rates estimate d by the SUGAR method were only correlated (P<0.01) with the in situ d egradation rates. Estimated values obtained with method PHEN were not si,significantly (P>0.05) correlated with those obtained by other meth ods. 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 differ ences 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 paramet ers, although prediction equations were different depending on the bot anical composition of the hay. (C) 1998 Elsevier Science B.V.