The total protein in feed ingredients used for ruminants is partitioned between the rumen-degraded protein and rumen-undegraded protein. Values are often assigned to protein ingredients on the basis of the rumen-degraded protein that they yield. That is hardly a new concept. However, what might be news to many are the common misconceptions concerning the measurement of the rumen-undegraded protein fraction, and how these misconceptions can impact diet formulations.
Three protein fractions
Of the various models available, the National Research Council (2001) protein evaluation scheme is the easiest to visualize with three protein fractions: the A fraction is mostly non-protein nitrogen and is assumed to be instantly degraded. The C fraction is unavailable and indigestible, and therefore, not degraded at all. The B fraction is the variable upon which rumen-undegraded protein depends: the extent to which it is solubilized by the rumen microflora is influenced by the rate of digestion (Kd) in combination with the rate of passage (Kp). Rumen-undegraded protein is calculated as the B fraction that is not digested in the rumen, along with the C fraction that leaves the rumen untouched. To put this in terms of an equation:
RUP = B fraction * (Kp/(Kd+Kp) + C fraction
This equation assumes that the entire portion of the B fraction that becomes soluble in the rumen is degraded to non-protein nitrogen. However, just because a protein is soluble, or becomes soluble while it remains in the rumen, should not be equated to the protein being completely broken down by rumen microflora. Table 1 provides a case in point. The Swedish research team of Helena Hedqvist and Peter Uden showed that proteins could be soluble, but not necessarily degraded. These scientists measured the Kd rates on the soluble fraction and found that the Kd values are actually quite variable among ingredients.
Impact of solubility
Does this matter? The results clearly show that it does. Hedqvist and Uden determined that the portion of the soluble protein that does not break down leaves the rumen with the liquid outflow, and it joins the fraction described by the National Research Council (2001) as rumen-undegraded protein. The effective protein degradability – or the amount that actually is degraded in the rumen – varied from over 70 percent of the protein for wheat DDGS and soybean meal to under 50 percent for rapeseed meal (canola meal) and linseed meal (flax meal), seen in Table 2. The inverse of the effective protein degradability is therefore the true rumen-undegraded protein fraction.
Even more of an eye-opener is the amount of rumen undegraded protein that the individual protein ingredients provide. By multiplying these new rumen-undegraded protein values by the percentage of protein for the ingredients provided in the report, the quantities of rumen-degraded protein per kg of feed can be calculated. The rumen-undegraded protein of lupins, rapeseed meal and linseed meal is actually as high as or higher than that of the highest crude-protein ingredient, soybean meal.
A more complex fractionation system
Model platforms based on the Cornell Net Carbohydrate and Protein System use a protein-fractionation system that is a bit more complex than the National Research Council, resulting in five protein fractions. The A fraction is considered to be non-protein nitrogen, and the C fraction is unavailable. The three B fractions are each assigned different rates of solubilization in the rumen and digestion in the intestines. The rumen-undegraded protein fraction is calculated as the sum of each fraction that escapes, and varies with the rate of passage of solids out of the rumen.
RUP = RUP = B1 * (Kp/(Kd1+Kp) + B2 * (Kp/(Kd2+Kp)+ B3 * (Kp/(Kd3+Kp) + C
The importance of this approach is that protein fractions can be routinely generated by the forage analysis laboratories that assist the feeding industry. This is shown in Figure 1 and has advanced the formulation of diets for ruminants.
Rates of digestion are still not well defined. Analysis laboratories are developing methods to measure Kd rates for each of the fractions – as the rates are equally as important as the measurement of fractions for determining rumen-undegraded protein.
Figure 2 provides an illustration of the Kd rates for one ingredient, canola meal. In the original versions of the Cornell Net Carbohydrate and Protein System, as illustrated by the Cornell-Penn-Miner platform, the rates (shown under the column marked %/h) for the A and B1 fraction are high. So, essentially all of those fractions are soluble in the rumen and assumed to be degraded. Likewise, the portions of the insoluble B2 and B3 fractions that become soluble in the rumen are assumed to be degraded.
New developments in methods
Several commercially available platforms now use the equations set forward in Cornell Net Carbohydrate and Protein System 6.1 (Lanzas et al., 2008) along with estimations of rates established by the Cornell Net Carbohydrate and Protein System development group at Cornell University. In this system, rates of digestion of the protein fractions were reduced relative to previous versions to compensate for the escape of soluble protein before it is degraded. The differences are illustrated for canola meal in Figure 3.
While this is a huge step forward in understanding the difference between rumen-solubilized and rumen-degraded protein, the tabular rate functions are currently based on very little analytical information. As more becomes available, accuracy will be improved.
A method to routinely determine rate functions has been under development at the USDA Forage Research Center under the guidance of Dr. Glen Broderick (Columbini et al., 2011). This method provides a blended Kd for use within the National Research Council model, and it can be modified to produce multiple rates for the more complex Cornell Net Carbohydrate and Protein System model. The industry can expect to see improvements in how models predict rumen undegraded proteins for ingredients used in feed formulation in the near future, as the newer models have exposed the need.
When proteins reside in the rumen, they are digested to a stage where they are soluble in the liquid fraction of the rumen. The rates at which these soluble materials are actually degraded to non-protein nitrogen are highly variable and are characteristic of the ingredients provided. Now that the feed industry is gaining an understanding of this, methods are being developed to assign more appropriate rates to the respective protein fractions used in more advanced formulation models.