Why Amino Acid Digestibility Matters

Amino acid digestibility has been a popular topic in my past blogs and trade journals as an important measure of protein quality and that higher digestibility is better.  However, what do these values mean and how do differences in digestibility matter in practical situations?

Production animals consume proteins in their diet every day. The protein is typically supplied by grains and oilseeds, but animal ingredients can also be used.  Protein consists of long chain  amino acids, the building blocks of all proteins; including those in meat, milk, eggs and animal fiber, like wool.  Animals must break down the intact proteins from the diet, absorb amino acids across the intestinal wall into circulation (blood) and then use these building blocks to construct protein at the various tissues.  If the proper amount and balance of these building blocks are not present, then protein synthesis will not be maximized and animal performance, as measured by body weight gain, will be reduced.

The challenge arises when less than 100% of the amino acids in feed ingredients are available for productive purposes.  This is what digestibility attempts to do – estimate the percentage of each amino acid available for protein building.  Various animal models have been developed to measure digestibility (for example, see here).

Adding further complexity, digestibility varies based on the ingredient in question, field conditions, composition of other nutrients in the ingredient and diet, animal age, genetics and production level; climate and ingredient processing conditions.

I have blogged in the past about how processing conditions with our equipment results in higher quality soy ingredients (see here and here) and the positive effects of extrusion on available energy levels in ingredients (see here and here).  But, what does this mean in terms of diet formulation?

I recently formulated the following broiler diets for one of our prospects:

Broiler Starter (0-7 d), Cobb SBM + oil ExPress ®
Corn 44 % 52 %
Solvent soybean meal (SBM) 46 % 0 %
Veg oil 6 % 0 %
ExPress ® soy meal 0 % 44 %
Dicalcium phosphate 1.9 % 1.9 %
Limestone 1.4 % 1.4 %
Salt 0.5 % 0.5 %
DL-methionine 0 % 0 %
L-lysine 0 % 0 %
Mineral/vitamin premix 0.2 % 0.2 %
TOTAL 100 % 100 %
ME (kcal/kg) 3,120 3,120
CP (%) 26.2 % 25.7 %
Digestible Lys (%) 1.34 % 1.33 %
Digestible Met (%) 0.36 % 0.38 %
Digestible M + C (%) 0.72 % 0.78%


To prove a point, I limited the number of ingredients.  Without using any synthetic amino acids (DL-methionine and L-lysine were set to 0) in the complete diet, I achieved higher digestibility levels of methionine (Met) and methionine plus cysteine (M + C) when ExPress ® soy meal was used versus commodity, solvent-extracted soybean meal.  The digestible lysine levels were kept equal.

This is because ExPress ® soy meal has higher digestible amino acids than solvent-extracted soybean meal.  In this example, less meal was required when it is was processed one way versus the other.  Therefore, we can conclude that ingredients with higher-digestible amino acids are higher quality and support equal or greater performance at lower amounts in the diet.  Digestibility, therefore, matters tremendously.