Bypass protein supplements are excellent tools for creating diets that are low in total protein, but still meet the nutritional requirements of high producing dairy cows. One of the challenges these supplements commonly face is maintaining a balance between bypassing the rumen while retaining high intestinal digestibility.

Perhaps the most extreme illustration of this can be found in blood meals, where intestinal digestibility can range from a low of 20% to a high of 98%. Similarly, distillers grains can range in intestinal digestibility from 25%-75%.

While many consultants and producers have seen this effect in the field, the variation has been difficult to quantify. Rumen degradability and intestinal digestibility of protein is intrinsically difficult to measure by laboratory procedures, due to factors like the variability of commercially available enzymes, and the small particle size of many protein supplements. While protein digestibility in forages may be reasonably estimated through AD-ICP and ND-ICP schemes, protein supplements are so low in ADF and NDF that this is not a realistic approach. 

Recently a group of researchers at Cornell University published a new in vitro method for measuring intestinal digestibility of protein.  The features of this new analysis include a move away from bag technology that can bias digestibilities higher when small particles escape bags undigested; or lower when microbial digestion is impeded by small pore size of the bags.

Another important feature is a move towards using physiological enzymes that are less variable in proteolytic activity. Finally, this new assay leaves open the possibility of analyzing the residue for individual amino acids to calculate amino acid digestibility. 

With these advances, commercial laboratories are now able to provide RUP and intestinal digestibility values for a wide range of feeds and protein supplements. Besides being an effective analysis for quality control testing of ingredients, the values generated will be useful as inputs into CNCPS-based ration balancing programs.

One commonly referred to quote in the lab industry comes from Dr. Peter Robins: “it is not uncommon to be unable to measure things we know to be true.”

While we’ve known for a long time that bypass protein products vary greatly in digestibility, we now have a tool that can quantitatively measure these differences.

Feed Type                                 RUP %          Intestinal Digestibility %

SBM                                           32                  76

SBM – Heat Treated                64                  84

Blood Meal                                95                  75

Heat Damaged Blood Meal     93                 0