TECHNICAL NOTE - COMPARISON OF IN-VITRO AND IN-SITU DIGESTIBILITY METHODS

Relative comparisons were made between the in vitro and in situ method s for determining NDF digestion kinetics. In a split-split-plot design , eight cows and six heifers (whole plot) with ruminal cannulas were f ed either alfalfa hay (20% CP) or bromegrass hay (8% CP; subplot) to d etermine differences between the methods (sub-subplot). Cattle were fe d 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 samp les were obtained for in vitro estimation of forage NDF digestion kine tics. In vitro incubation vessels contained a 1:4 (vol:vol) dilution o f ruminal inoculum to buffer and .5 g of NDF substrate. Dacron bags co ntaining .5 g of NDF substrate from alfalfa or bromegrass were incubat ed ruminally for 0, 3, 6, 12, 24, 48, 96, and 192 h to determine lag t ime, rate, and extent of digestion. In vitro samples were incubated fo r the same times and computations. Results from the four treatments (c ows and heifers fed alfalfa and bromegrass) indicate that on average t he lag time was 3.5 +/- .3 h less (P < .01), rate was .03 +/- .002/h f aster (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 kin etics. These results should be useful when one is interested in making relative comparisons in the digestibility of forages among studies th at used the in situ or in vitro method.