Technical note: comparison of in vitro and in situ digestibility methods.

Relative comparisons were made between the in vitro and in situ methods for determining NDF digestion kinetics. In a split-split-plot design, eight cows and six heifers (whole plot) with ruminal cannulas were fed either alfalfa hay (20% CP) or bromegrass hay (8% CP; subplot) to determine differences between the methods (sub-subplot). Cattle were fed the diets for 28 d; d 1 to 20 were used for adaptation, and on d 21 to 28 the in situ experiment was conducted. On d 24 ruminal fluid samples were obtained for in vitro estimation of forage NDF digestion kinetics. In vitro incubation vessels contained a 1:4 (vol:vol) dilution of ruminal inoculum to buffer and .5 g of NDF substrate. Dacron bags containing .5 g of NDF substrate from alfalfa or bromegrass were incubated ruminally for 0, 3, 6, 12, 24, 48, 96, and 192 h to determine lag time, rate, and extent of digestion. In vitro samples were incubated for the same times and computations. Results from the four treatments (cows and heifers fed alfalfa and bromegrass) indicate that on average the lag time was 3.5 +/- .3 h less (P < .01), rate was .03 +/- .002/h faster (P < .01), and extent was 6.0 +/- .5% greater (P < .01) for the in situ than for the in vitro method for determining NDF digestion kinetics. These results should be useful when one is interested in making relative comparisons in the digestibility of forages among studies that used the in situ or in vitro method.