Nutritionists are moving beyond evaluating feed sources by crude protein or the quantity of protein. Protein is being evaluated based upon the quality of the protein. The amount and quality of protein available varies by feed source.
Nutritionists aim to put together a diet that has a balanced amount of these protein pools while keeping the cost of the diet “in check.” On-farm forages (i.e., alfalfa silage, grass hay, alfalfa hay) are economical feed sources. There are lower transportation costs associated with these feeds. Alfalfa and grass sources are typically high in rumen-degradable protein (as shown in Figure 1).
A common protein concentrate, canola meal, has a greater amount of rumen-undegradable protein (RUP) and rumen-undegradable protein digestibility (dRUP) relative to alfalfa silage (Figure 2).
Thus, less protein is degradable in the rumen, and the bypass protein is quite digestible. The challenge for nutritionists is to find a supplemental protein source that has a high level of RUP available for absorption (i.e., a high level of dRUP), as such sources complement readily available alfalfa and grass sources, but the nutritionists can’t break the bank doing it.
The remainder of this article will explain a method for evaluating the value of protein in a feed source. In other words, determining cost per unit of protein that can be used by the cow for productive functions (i.e., milk production).
Evaluating protein concentrate sources involves knowing some values and doing some math.
You’ll need three values:
- Cost ($ per ton)
- Crude protein (CP, percent dry matter)
- dRUP (percent dry matter)
A calculator or a spreadsheet would also be helpful. I’ll forewarn you that the measure of dRUP might be tough to get from some sources, as this is a newer metric.
Table 1 shows an evaluation of three protein sources and provides the math needed to arrive at the calculated values.
Cost in $ per ton – One cannot simply look at the cost of the protein source alone because the amount and quality of protein present varies from ingredient to ingredient. This comparison is only valid if you’re comparing the same feedstuff from two different suppliers.
Cost per unit of CP – Evaluating protein sources on a cost per unit of CP ($ per pound CP) is one option. In the example, feedstuff A is the least expensive at 27 cents per pound, while feedstuffs B and C are more expensive. Thus, you’d buy feedstuff A.
- Cost per unit of dRUP – A more sophisticated manner to evaluate protein sources is to examine the cost per unit of digestible bypass protein or dRUP ($ per pound dRUP). In the example, feedstuff C is the best value at 44 cents per unit of dRUP, while feedstuff B is 92 cents and feedstuff A is 83 cents per unit of dRUP. Thus, you’d buy feedstuff C.
Based on my experience, this is how protein sources should be evaluated. Protein sources must contain digestible protein for the protein to be absorbed and utilized by the cow. If the protein is not digestible, it is essentially wasted. Feeding extra protein that is indigestible increases the total cost of the protein.
So let’s break down the terms and meanings to effectively evaluate protein sources based upon quality of the protein. Figure 1 illustrates the protein pools for alfalfa silage, and Figure 2 illustrates the protein pools for canola meal.
These figures illustrate how the different protein systems relate to one another. Pools are shown both on a dry matter and CP basis (i.e., CP = 100 percent).
- Crude protein (CP) is a common method used to measure protein. It is the nitrogen measured in a feed multiplied by 6.25.
Rumen degradability of protein is described with the following terms:
Rumen-degradable protein (RDP) – This is the portion of protein that is degraded in the rumen. It may be soluble in the rumen fluid or degraded more slowly by microbial bacteria or protozoa (aka degradable intake protein).
- Rumen-undegradable protein (RUP) – This is the portion of protein that is not degraded in the rumen. This protein will be comprised of a digestible and an indigestible portion. The digestible portion will be absorbed in the small intestine (aka bypass protein; undegradable intake protein or IUP)
RDP + RUP = CP
The digestibility of protein is described with the following terms:
RUP digestibility (RUPd) – This is the digestibility of the RUP pool, expressed on a CP basis (aka intestinally available protein).
Digestible RUP (dRUP) – This is the portion of the RUP containing protein that is available for absorption in the intestine, expressed on a dry matter basis. Think of it as similar terminology to neutral detergent fiber digestibility (NDFd) and digestible neutral detergent fiber (dNDF). That is, NDFd is expressed on an NDF basis (as a percent of NDF), and dNDF is expressed on a dry matter basis.
- Indigestible protein (IUN) – The portion of RUP containing protein that is unavailable to the animal. Thus, this pool is excreted as waste.
RUP = RUPD + IUN, if on CP basis
RUP= dRUP + IUN, if on dry matter basis
- Metabolizable protein (MP) – This is the protein that is absorbed in the form of amino acids. MP includes protein from the feed (primarily dRUP), rumen microbes (bacteria and protozoa, aka microbial protein) and endogenous sources (i.e., old cells or enzymes). MP is the protein that is used for productive functions in the cow.
Nutritionists will look at the amount of MP (in grams) and the MP-allowable milk, which is the amount of milk that is predicted to be produced from the MP amount. Thus, the amount of MP is the ultimate measure of the amount of quality protein.
However, there are many dietary factors that influence MP, like rumen fermentation (type and amount of protein and carbohydrate available) and dry matter intake. These other factors make MP difficult to assess without the use of ration formulation software.
As previously mentioned, dRUP is a primary component of MP that can be assessed without much variation. This reinforces the rationale for evaluating the value of protein sources on a cost per unit of dRUP ($ per pound dRUP, as shown in Table 1).
Robin R. Rastani
- Novita Nutrition
- Email Robin R. Rastani