T echnical N ote : Comparison of 4 methods for determining in vitro ruminal digestibility of annual ryegrass

ABSTRACT Multiple IVDMD methods exist, but information comparing results obtained by different methods is scarce. This study aimed to compare 3 different IVDMD methods [Daisy II (DY), batch culture (BC), and the Ankom Gas Production System (GP)] at 4 incubation times (IT; 12, 24, 36 and 48 h). Additionally, results obtained at 24 h were compared with those obtained from dual-flow, continuous-culture fermentors (CF). Annual ryegrass at vegetative state was clipped from an ungrazed pasture, dried (60°C, 48 h), and ground in a Wiley Mill (1 mm). Three 48-h periods of each method were conducted using rumen inoculum from a cannulated Holstein cow. Ankom F57 acetone prerinsed bags containing 0.5 ± 0.01 g of sample were used for DY, BC, and GP. Apparent DM digestibility coefficients in CF were estimated in 3 periods (7 d of adaptation and 3 d of collection) started simultaneously with the other methods. Data were analyzed using the mixed procedure of SAS in a model including method and IT as fixed factors and period as a random factor, with IT as a repeated measure. Means within each IT were compared by the PDIFF function. Results indicated that DY predicted greater DM digestibility than GP and BC at IT greater than 12 h. Apparent DM digestibility estimated using CF was similar to that obtained with BC and GP at 24 h but less than DY. We conclude that different IVDMD methods yield different results, and caution should be exercised when comparing data obtained by different methods.