Abstract The in-situ procedure is commonly used in ruminant nutrition research to evaluate digestibility. A core assumption of this procedure is that material that washes out of the in situ bag is completely digested, but there are no methods to evaluate the nutrients that wash out of the bag. This project addressed this question by utilizing a tabletop washing machine to simulate the rumen washout of nutrients in the in situ bags and collecting the water from the spin cycle in 250ml bottles. Each spin cycle contained all 60 bags of one sample with 15L of water. The bags were washed for 5 cycles. Each cycle was 1 minute of agitation and 2 minutes of spin. The bottles were then centrifuged at 5,000 x g for 10 minutes to recover the solid nutrients from the washout fraction. After centrifugation, the liquid portion was decanted off and the pellets combined by sample type, frozen, freeze dried, and weighed. The residue of the feed in the in-situ bags was dried in a 100º C oven for 24 h and weighed. Composites of the original sample, the residue remaining in the in situ bag, and the washout fraction were analyzed for CP and NDF. There were duplicate samples of alfalfa (ALF; 19.0% CP; 55.2% NDF), corn (DRC; 4.48% CP; 9.72% NDF), distillers grains (DGS; 27.6% CP; 36.3% NDF), and blood meal (BM; 99.1% CP). Analysis of NDF was not conducted on the BM. There were 60 large in situ bags (10cm x 20cm, 50µm pore size) per sample, each with 5 grams of ingredient (AS-IS basis) heat sealed into the bag. The proportion of the original sample that was retained in the bag (residue) and collected in the washout were quantified and the proportion that was unaccounted for was determined by difference. Data were analyzed using the GLM procedures in SAS utilizing an unstructured design. The model contained the sample type and a tukey adjustment was used for mean separation with differences declared at P 0.10. Data are shown in Table1. The proportion of DM, CP, or NDF retained in the bag did not differ across the different feed types (P 0.17). However, a greater proportion of DM and NDF was collected from the washout fraction of DRC using this procedure compared to other feed types (P 0.10). These data suggest that nutrients are not equally retained in the residue fraction of in situ bags and a portion of the washout fraction can be collected via centrifugation of the water used to wash the in situ bags.
Lewis et al. (Wed,) studied this question.