In vitro and in situ evaluation of secondary starch particle losses from nylon bags during the incubation of different cereal grains

• Secondary starch particle losses occur during in situ degradation of soft cereal grains.
• Reducing the pore size of in situ bags decreases secondary starch particle losses.
• No secondary starch particle losses occurred when evaluating corn grains.
• In situ evaluation of ruminal starch degradation is not recommended for oat grains.

Three experiments were conducted to study the loss of secondary starch particles from bags used for the in situ incubation of different cereal grains. Two in vitro and one in situ study were combined. Ruminally cannulated Jersey cows were used for rumen fluid collection and in situ degradation studies in randomly assigned repeated measures designs, each including three replicates per treatment. An in vitro time course study was conducted to determine whether secondary starch particle losses occur during ruminal incubation. Ground wheat was weighed and placed in bags with a pore size of 50 μm, then washed, dried, and incubated for 0.5, 1, 2, 3, 5, 8, 16, and 32 h in a modified RUSITEC-system. Bag residues and samples of freeze-dried fermenter fluids were enzymatically analyzed for starch content. Using the same technique used for the first in vitro experiment, but with an incubation time of only 8 h, ground wheat, barley, and corn grains were incubated in bags with pore sizes of 50, 30 (with the exception of corn), 20, and 6 μm. In the in situ experiment, ground wheat, barley, corn, and oats were rumen-incubated in bags with pore sizes of 50, 20, and 6 μm for different time periods. Then, the grains and bag residues were enzymatically analyzed to determine their starch content, and the degradation characteristics of the materials were calculated. The in vitro trials showed that incubating wheat and barley in bags with pore sizes of 50 and 30 μm leads to a substantial degree of secondary starch particle loss during incubation. These losses were not detectable when the samples were placed in bags with pore sizes of 20 and 6 μm. No secondary starch particle losses occurred in corn, regardless of pore size; thus, corn can be studied in situ even when using bags with 50 μm pore size. Due to the high washout losses the in situ method is not suitable for the measurement of starch degradation in oats using the pore sizes tested in the present study. Because of the methodological problems associated with pore sizes <50 μm, no recommendations can be provided for the evaluation of wheat and barley. Thus, caution must be taken when the in situ technique is used for ruminal grain starch degradation measurements, as there are substrate-related errors possible that must be taken into account.