Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 27, 1994
Publication Date: N/A
Interpretive Summary: Researchers traditionally use one of two methods to determine the rate and extent that a forage is digested in the runem. These are important indices of the nutrient's value of a forage. One method (in vitro) involves placing a forage sample into a test tube, adding ruminal fluid and a buffering agent, incubating this mixture for varying times, and analyzing the residue remaining. Another method (in situ) involves placing the forage sample into a porous nylon bag which is placed directly into the rumen of an animal via a cannula, incubating it for varying times, and analyzing the residue remaining. The objective of our study was to compare the results from these two methods under defined conditions. Our results indicate that with the in situ method, alfalfa and bromegrass samples begin to digest 3.5 hours earlier than the in vitro method. These forage samples also digested 3% faster per hour and are more extensively digested (6%) when the in situ method is used. These results should be kept in mind whe one is trying to compare the digestibility of forages between various research reports.
Technical Abstract: Relative comparisons were made between the in vitro and in situ methods for determining NDF digestion kinetics. Eight cows and six heifers with ruminal cannulas were fed either alfalfa hay (20% CP) or bromegrass hay (8% CP). Each of two sampling periods, consisted of 28 d, d 1 to 20 for adaptation and d 21 to 28 for in situ analyses. On d 24 ruminal fluid samples were obtained for in vitro analyses of forage NDF digestion kinetics. This consisted of a 20% ruminal fluid inoculum, 0.5 g NDF substrate, and buffer. Dacron bags containing 0.5 g 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 analyses. Results from the four treatments (cows and heifers fed alfalfa and bromegrass) indicate that on average the lag time was 3.5 h less, rate was 3.0%/h faster, and extent was 6.0% greater for the in situ vs in vitro method for determining digestibility.