Long, long ago and far, far away, crude protein (CP) was used as the sole measure of the protein value of ingredients. Up until some 45 years ago, it was felt that this (CP [N x 6.25], along with total digestible nutrients (TDN) as a measure of energy), was all that was needed to balance a lactation diet.

While research in the 1960s showed that microbial protein (MCP) could support up to 10,000 pounds of milk production, even by the 1970s production had moved beyond this level. Nitrogen balance trials and production trials gave us an indication of how much protein was needed to supplement that supplied by MCP.

It was recognized that different protein sources had different effects on milk yield and milk components. These protein sources were used in rations formulation based upon production requirements and cost.

Over the next 30 years, milk yield per cow in the U.S. more than doubled, reaching 20,000 to 30,000 pounds of milk annually. The MCP supplies a decreasing amount of the protein requirement of the modern cow.

The challenge of balancing the protein needs of cows is daunting. Enough rumen degradable protein (RDP) must be fed in the form of non-protein nitrogen (NPN), peptides and amino acids (AA), and degradable protein to meet the needs of the micro-organisms in the rumen without overfeeding them and reducing their efficiency.


At the same time, enough rumen undegradable protein (RUP) must be fed to supplement the MCP to provide adequate metabolizable protein (MP) and AA to help meet the requirements for lactation, growth and reproduction.

Metabolizable protein

Metabolizable protein is the true protein and component AA that are absorbed from the intestine of the cow. It comes primarily from the MCP, which comes from the micro-organisms that grow in the rumen and pass into the abomasum (true stomach) and then to the intestine; RUP, which is from the feed protein that escapes rumen degradation; and endogenous protein.

Feed ingredients are divided into several types of proteins and NPN compounds based on their structure and solubility. The four primary types of proteins are albumins, globulins, prolamines and glutelins. These four protein types and NPN make up about 65 percent of the fractions found in feeds.

The remaining insoluble nitrogen includes proteins from cell wall constituents and heat-denatured proteins associated with NDF. Peptides, free AA, nucleic acids and ammonia are variable low molecular weight NPN compounds found in feed ingredients.

Forages contain the highest levels of these variable NPN levels, and preserved and ensiled forages contain higher levels than fresh forages.


Crude protein values of feed ingredients reflect the nitrogen content and nothing more. Crude protein tells nothing of the type of protein or of its value to the rumen micro-organisms or to the cow if it should escape rumen degradation.

RDP supplies peptides, free AA and ammonia for microbial growth and production of MCP. Most of the AA passing to the small intestine comes from MCP.

The remaining source of available AA comes from RUP. Understanding how CP is degraded in the rumen and how this is determined is necessary to understanding the MP system.

The 2001 NRC edition of the Nutrient Requirements of Dairy Cattle (NRCDC) divides feed protein into three fractions based upon their reaction to being placed in in situ testing procedures to derive the data on the feedstuff. This involves placing a known amount of the feedstuff into a nylon bag and incubating it in the rumen of a fistulated cow for a given length of time, generally 16 hours.

Fraction A is the portion of the feed CP that disappears immediately during the pre-soak period. It is completely degraded and is considered to be the soluble protein fraction and 100 percent degradable. It includes NPN compounds, such as urea and nitrates, small true protein products, including free AA, and peptides. All of Fraction A is included in RDP.

Fraction B is the portion of the CP that disappears from the bag during rumen fermentation after Fraction A has been removed. This represents that portion that is potentially degraded in the rumen.

Due to rate of passage out of the rumen and the digestion rate of the protein, a portion of Fraction B will be RDP, and a portion will be RUP. The digestion rate is determined by removing bags at different times during ruminal exposure and measuring the disappearance of protein in the sample.

The rate of passage is affected by dry matter intake, the amount of concentrate in the dry matter and the NDF content of the forage in the dry matter. A higher digestion rate means a greater proportion of the CP will be degraded in the rumen, while a faster rate of passage means less CP will be degraded in the rumen.

Fraction C is that portion of the CP remaining in the bags following the end of the designated time of the in situ testing. Fraction C is considered to be 100 percent RUP.

The digestibility of RUP is not constant and varies with different feed ingredients. Many of these values were included in the 2001 NRC. Digestibility values on different feedstuffs and whole rations may be determined through live animal testing where the bags from in situ testing are placed into the intestine of the cow and recovered excreta following passage through the cow.

In the last few years, in vitro determination of RUP digestibility has also been developed. This has become more popular as cost and time constraints limit live animal testing.

MCP production is dependent upon bacterial and protozoa cell growth in the rumen. Nitrogen from RDP and energy from organic matter degraded in the rumen determines the cell growth in the rumen and the resulting MCP.

The 2011 NRCDC shows how TDN and RDP are used to determine the amount of MCP produced. MCP is assumed to contain 80 percent true protein and 20 percent nucleic acids. The true protein portion is considered to be 80 percent digestible in the intestine. The same methods for determining RUP digestibility may be used to determine MCP digestibility.

Methionine and lysine

MP digestibility can now be determined by adding the RUP digestible protein and the MCP digestible protein together. If the methionine (Met) and lysine (Lys) values of the RUP are known, and using average values of MCP Met (2.68 percent) and Lys (8.2 percent), then the MP digestibility of Met and Lys supplied by the feedstuffs can be calculated.

Why is this important? For more than 35 years in the poultry industry and 25 years in the swine industry, the optimization of individual AA has been common practice. It has increased animal performance with an improvement in feed conversion efficiency.

It has also allowed the reduction of feeding of excess nitrogen-containing products and thereby the reduction of nitrogen loading into the environment. By formulating to individual digestible AA needs into a diet for a post-ruminal dairy cow of high genetic merit, these same performance improvements can be achieved.

Many trials have demonstrated that Met and Lys were likely the first two limiting AA for milk components and yield. Other limiting AA may include histidine, leucine and isoleucine. As many do not currently formulate for the first two limiting AA, it is not practical to incorporate constants on the third and fourth limiting AAs.

However, the proper sequencing of the limiting AA is important and will allow the nutritionist to reduce protein inputs while maintaining performance.

The role played by the rumen in transforming synthetic AA into MCP has prevented the practical supplementation of lactating diets with synthetic AA. The development of rumen-protected (RP) AA has allowed the supplementation of high-yielding cows with additional Met and Lys to improve milk protein efficiency.

Cows fed RP Met and Lys consumed 7.2 percent less N and produced 0.6 percent more milk total N, excreted 9.9 percent less urinary N and 18 percent less fecal N. This work illustrated that feeding low CP with balanced AA diet could improve the efficiency of nitrogen utilization and reduce the environmental impact without compromising the profitability of milk production.

Over the years, the understanding of protein in dairy diets has advanced from simply formulating 16 or 18 percent CP in diets to understanding the MP system, which formulates for the production of MCP and supplying RUP to the intestine.

Today, individual AA supplementation with the use of RP AA is becoming the norm for high-yielding dairy cattle to maintain production while reducing feed inputs and lowering the impact on the environment and optimizing the bottom line for a producer.  PD

References omitted but available upon request. Click here to email an editor.

Robert C. Hill is the director, and over tech support and new product development with HJ Baker Feed Products Group. Email Robert Hill.