Evaluation of Specific Variables Affecting In Situ Estimates of Ruminal Dry Matter and Protein Digestion1

Abstract

Specific variables associated with the in situ digestion of soybean meal (SBM) were identified and investigated. Soybean meal was incubated in situ for seven periods of time ranging from 1 to 48 h. Bags were introduced into the rumen in two ways: (1) all bags introduced at the same time and (2) bags introduced at different times (all removed at once). The latter procedure resulted in higher rate constants for both dry matter (DM) and nitrogen (N) disappearance. Dry matter disappearance rates of cottonseed meal were not different between four cows in similar stages of lactation. However, differences developed within cows over a 6-wk period. Correction for bacterial N did not significantly alter rate of SBM N disappearance compared with no correction. The amount of SBM DM and N washed from bags before ruminal exposure varied with sample grind and weight. Disappearance was higher for bags with porosities of 80 and 102 µm compared with 6, 20, 40 or 59 µm. Grinding SBM at 1, 2, 5 mm or unground did not influence rate constants for DM or N disappearance from polyester bags. As the ratio of sample weight to surface area increased (2.5, 12.6, 25.3 and 37.9 mg/cm2), DM and N disappearance rates decreased. Bag porosities of 6 and 20 µm gave lowest and 80 and 102 µm highest rate constants of in situ DM and N disappearance. For SBM, a sample weight to surface area ratio of 12.6 mg/cm2 and use of bags with porosities of 40, 59, 80 and 102 µm resulted in acceptable estimates of ruminal protein availability.