Comparison of the in situ and in vivo intestinal disappearance of ruminally protected methionine

We designed a switchback expriment to compare the intestinal disappearance of ruminally protected Met determined in situ or in vivo over three consecu tive periods. Four nonlactating Holstein heifers (477 +/- 36 kg) with cannu lae in the rumen, duodenum, and ileum were fed a diet based on timothy sila ge to meet requirements for maintenance (dry matter intake = 8.72 +/- 0.15 kg). A total of 16 bags, containing 1.5 g of ruminally protected Met, were incubated in the rumen (4.5 h) of each cow and transferred to an acid-pepsi n solution to simulate the abomasum (2.5 h). Following each incubation, bag s were recovered and three bags were dried at 55 degrees C and analyzed for Met. Remaining bags were introduced directly into the duodenal or ileal ca nnula for the in situ method, while for the in vivo method, the content of remaining bags was transferred into gelatin capsules before their introduct ion in the duodenal or ileal cannula. Spot samples of digesta were collecte d during a 96-h period, with Co-EDTA and Cr-mordanted fiber used as indiges tible markers to estimate in vivo digestibility. The disappearance of Met i n the small intestine determined in vivo tended to be higher than in situ ( 74.45 vs. 43.65 +/- 1.79%). Our results indicate that when used to assess i ntestinal availability of ruminally protected Met, the mobile nylon bag tec hnique can underestimate the true bioavailability of Met.