NDF degradability of hays measured in situ and in vitro

The NDF degradability (dNDF) of hays measured by an in situ method (nylon bag technique) was compared with in vitro fermentation, using the DaisyII incubator (Ankom®, Tech. Co., Fairport, NY, USA). Eighteen hays were produced from mountain areas (about 700 m.s.l.) from plots subjected to different cutting frequencies (two to four cuts per season) and types of N fertilisation (slurry and slurry plus mineral). Hay samples from each cut were incubated (in situ) in the rumens of three cows (incubation times: 2, 4, 8, 16, 24, 48 and 72 h; sample weight: 15 mg/cm2 of free bag area) or inserted (in vitro) into three different digestion jars (jar volume: 2 l; bag size 5 cm×3 cm; sample weight: 250 mg per bag) that were placed into the DaisyII incubator for 48 h. rages showed large variation in effective dNDF (assumed rumen turn over rate: 3% per hour) which ranged from 38 to 43% for hays with two cuts to 58–65% for the hays with four cuts per season. The dNDF obtained in situ (Y, effective) and in vitro (X, after 48 h of incubation) were highly correlated (P<0.01) and the regression equation was Y=0.74(±0.05)X+6.39(±2.90), n=18, S.D.=±1.96, r2=0.94. riability (coefficient of variation, CV) of the in vitro measurements (among jar repeatability) was 2.8%, which is close to that generally found for some chemical analyses of feedstuffs and lower than that obtained for the in situ measures (among cow repeatability, CV: 3.7%). The DaisyII incubator produces repeatable in vitro dNDF data which are highly related to those obtainable with the reference in situ procedure.