Because of the complexity of the rumen ecosystem, nutritionists have historically balanced rations for crude protein (nitrogen x 6.25) rather than specifically for amino acids.
Rumen protein needs are met according to soluble protein (SIP) and degradable protein (DIP) requirements. Additional protein beyond that supplied by the rumen microbes is provided in the form of undegradable protein (UIP).
Non-protein nitrogen (NPN) can be used to meet some of the crude protein needs. NPN is first converted to ammonia in the rumen and then used by the rumen microbes to make microbial protein.
A protein is defined as a chain of 50 amino acids or more. The cow absorbs and uses individual amino acids and requires specific amounts of each for maintaining her body, making a calf, making her own muscle and bone, and producing milk.
We know that dairy cows actually require amino acids and that we should balance rations for individual amino acids. Dairy nutritionists are beginning to balance rations for amino acids with more confidence. But, historically, it has been difficult to balance dairy cattle diets for amino acids because of the uncertainty of the rumen’s effects. First, it is difficult to predict the amount of microbial amino acids produced in the rumen. Second, it is difficult to predict how much of each amino acid from dietary protein will escape the rumen. Advanced computer models are now being developed which help nutritionists to use the research data and complex calculations necessary for balancing dairy rations for amino acids. (See Amino Acids article on milkproduction.com)
Because of the complexity of the rumen eco-system, nutritionists have simply balanced rations for the amount of crude protein required by the dairy cow. Crude protein (CP) is simply calculated by determining the amount of nitrogen in a feed and multiplying it by the factor 6.25. This is based on the assumption that protein contains 16% nitrogen (100/16 = 6.25). Crude protein requirements have been compiled by experts such as those in the U.S. National Research Council (NRC). Crude protein requirements vary with the weight of the cow and the amount of milk produced. For dry cows, there is an extra need for crude protein for the growing fetus that is added to the maintenance requirement. Growing animals have an extra need for crude protein for muscle development.
A 1300-pound (591 kg) cow producing 90 pounds (41 kg) of milk containing 3.5% milkfat requires 0.892 pounds (0.405 kg) of protein for maintaining herself and 7.56 pounds (3.44 kg) of protein for milk production. Her total crude protein requirement is 8.45 pounds (3.84 kg). If she consumes 50 pounds (23 kg) of dry matter each day, her diet needs to contain 16.9% crude protein. This is the requirement just for 90 pounds (41 kg) of milk. Most nutritionists will “challenge feed” cows in early lactation. They will feed a higher percentage of crude protein to support 5 -10% more milk than that supported by dietary energy. They know that cows will use some energy from their bodies to support milk production (hopefully not more than one body condition score) and cows may not eat 100% of the dry matter formulated for every day.
Three protein sources contribute to the amino acid pool available for absorption at the small intestine. First of all, rumen microbes pass down to the intestine. In one study where researchers isolated rumen bacteria, crude protein as a percentage of bacterial dry matter ranged from 39 to 56%. Amino acid nitrogen made up about 67% of the crude protein in the bacteria. Secondly, the dietary protein that is not degraded in the rumen is passed down to the intestine. Third, endogenous protein comes from sloughed off cells in the digestive tract. Rumen microbial protein actually contributes about 50-75% of the cow’s total protein supply and endogenous contributions are minimal.
The goal when feeding dairy cows should always be to maximize microbial protein production and then supplement with additional protein which is expected to escape rumen fermentation. This strategy provides an economical source of protein and, due to growth of the microbial population, digestibility of starch and fiber is increased to provide more volatile fatty acids for energy.
The Protein Fractions
Soluble Protein (SIP)
- comprised of ammonia (NH3), nitrate (NO3), peptides, and some true proteins
- almost instantly available to the rumen microbes
- present in large amounts in alfalfa silage
- measured by soaking a feed in a buffer for 1 hour
- is part of the DIP fraction
Degradable Protein (DIP or RDP)
- contains true proteins which are more slowly digested in the rumen, in addition to the soluble protein fraction
- measured in laboratories by incubating the feed in a dacron bag in the rumen of a fistulated cow or with an enzyme (which is intended to digest feed protein at a similar rate as the rumen microbes), for the amount of time that the feed is predicted to normally remain inside the rumen of the cow (16 hours for concentrates, 30 hours for forages).
Undegradable Protein (UIP or RUP) (Bypass Protein) (Escape Protein)
- all protein which isn't rumen degradable
- contains protein which is available for absorption at the small intestine as well as protein which will show up in the manure
UIP (%CP) + DIP (%CP) = 100%
SIP, DIP, and UIP are expressed by labs and in feeding programs in two different ways. Sometimes they are expressed as a percentage of the dry matter in a feed or ration. But, more often, they are expressed as a percentage of the crude protein in a feed or ration.
For example, if a feed contains 20% CP and 8% SIP (as a % of dry matter in the feed), it contains 40% SIP as a percentage of the crude protein in the feed.
Unavailable Protein (ADF-CP) (Bound Protein)
- of no use to the dairy cow (shows up in the manure)
- a result of extensive heating
- is part of the UIP fraction
- measured by boiling feed in an acid detergent solution and determining the protein content of the residue
Protein Fractionation Scheme:
WHY DO WE CARE ABOUT THE PROTEIN FRACTIONS?
Feed the Bugs
Our first objective is to produce as much microbial protein as possible. We need to provide nitrogen and amino acids to the microbes so that they can make microbial protein. The microbes take nitrogen, amino acids, and carbohydrates and produce their own body protein. The microbes end up being washed down to the cow’s intestine and are then digested and used as a source of amino acids for the cow.
For efficient growth, the rumen microbes require sources of protein (or nitrogen) and carbohydrate that are available at the same time. If too much protein is supplied without an available source of carbohydrate, the microbes will use the protein as a source of energy and waste the nitrogen found in protein. For instance, alfalfa silage and raw soybeans have a lot of readily available protein contained in them, so a source of readily available carbohydrate should be fed with those feeds. Processing soybeans slows down the rate at which the protein is degraded in the rumen so that it will more evenly match the rate at which energy becomes available in the rumen. As carbohydrate availability increases, more dietary protein can be incorporated into the microbes.
Soluble Protein (SIP) provides immediate protein for the microbes to use. Degradable Protein (DIP), since it includes SIP, provides the immediate protein for the microbes to use plus the long-term protein for the microbes to use. The right amounts of both SIP and DIP are needed to complement the rapidly and slowly digestible carbohydrate in the cow’s diet.
If more SIP or DIP is fed than the cow needs, the cow will simply waste that extra nitrogen. It will be excreted in the cow’s urine and there will be higher levels of urea nitrogen in the blood and in the milk. A ration may meet the cow’s crude protein requirements but if too much degradable protein is fed, the amount of protein absorbed from the cow’s intestine will not be adequate. It could be like feeding a 16% CP diet rather than a 17% CP diet according to the cow! On the other hand, feeding too little degradable protein will result in decreased microbial protein and less total carbohydrate fermentation in the rumen. Milk production could actually decrease if a ration with a lot of readily available carbohydrate was fed with the only protein supplement being a slowly degradable protein such as, corn gluten meal.
Feed the Cow
Our second objective is to provide enough undegradable protein (UIP) to supplement the microbial protein made in the rumen so that the cow has enough total protein to meet her absorbed protein requirements. Absorbed protein is all of the protein that is absorbed from the cow’s intestine. It includes that from the microbes as well as undegradable protein. Undegradable protein should be supplemented according to the difference between protein supplied by the microbes and that required by the animal for milk production. In general, diets designed for high production will contain 30-32% SIP, 60-65% DIP, and 35-40% UIP. According to the 1989 NRC, a 1300-pound (591 kg) cow producing 93 pounds (42.3 kg) of milk containing 3.5% milkfat, requires 5.32 pounds (2.42 kg) of DIP and 3.09 pounds (1.4 kg) of UIP. This equals 8.41 pounds (3.82 kg) of protein (16.8% in 50 pounds (22.7 kg) of dry matter) of which 63.25% is DIP and 36.74% is UIP. A tail-end, lower producing cow will require less UIP because her total protein requirements are less. Therefore, a higher percentage of her protein requirements can be met with microbial protein.
A common method of increasing the amount of UIP in feeds is by heating to denature the protein. Heat input is usually a product of both time and temperature. The idea is to make the proteins less digestible in the rumen but still able to be broken down by the acids and enzymes in the rest of the cow’s digestive tract. Unfortunately, sometimes too much heating causes a total Maillard reaction. In this reaction, proteins are permanently bound with carbohydrates and become lignified. In this form, proteins are indigestible. Unavailable protein should therefore be measured in heated feeds and forages such as hay crop silage, distillers’ grains, and roasted soybeans. Unavailable protein should be assumed to be useless to the cow and accounted for when balancing rations. Consistent processing methods help to reduce or at least know the potential levels of unavailable protein in feeds.
The Bugs Need Some Non-Protein Nitrogen But Not Too Much
Non-protein nitrogen is simply nitrogen that is not incorporated into amino acids and protein. Urea is one of the most common forms of non-protein nitrogen. Much of the soluble protein in forages like hay crop silage is also non-protein nitrogen. Non-protein nitrogen is often seen as a cheap ingredient to use in rations. For this reason, sometimes it is overused. However, in many rations, supplemental sources are needed and beneficial.
In the rumen, non-protein nitrogen is first converted to ammonia and then, can be used by the rumen microbes to make microbial protein. Certain rumen microbes have a definite ammonia requirement. When carbohydrate is available, the microbes can incorporate amino acids (from degradable protein) and non-protein nitrogen into microbial protein. Without sufficient carbohydrate, microbial protein is not produced, the amino acids are fermented to produce volatile fatty acids (an energy source) and ammonia, and the non-protein nitrogen is just converted to ammonia in the rumen. When excess ammonia is absorbed out of the rumen, it is converted to urea by the liver and either recycled back into the rumen in limited amounts or excreted via the kidneys. This conversion not only wastes protein, but it also costs the animal energy.
Urea and Reproduction: When too much ammonia is absorbed out of the rumen, blood urea nitrogen (BUN) levels and milk urea nitrogen (MUN) levels rise in the cow's body. High BUN or MUN levels have been associated with decreased fertilization and reduced embryo quality. This is evidenced by irregular heat cycles. It is recommended that especially during the breeding period, BUN levels not exceed 20 mg/dL and MUN levels not exceed 16-18 mg/dL. High BUN and MUN levels are often from excessive dietary SIP but can also be simply from overfeeding DIP or a lack of rumen fermentable carbohydrate. BUN levels fluctuate during the day. A representative sampling from several cows taken throughout the day is needed to evaluate BUN status. For this reason, some people prefer to use MUN.
Urea Toxicity: Since the efficient use of urea is dependent upon the level of rumen available carbohydrate in the ration to increase microbial requirements for nitrogen and provide for the conversion of urea into microbial protein, there is really not an absolute level of dietary urea that will cause toxicity. Usually large amounts of circulating ammonia will reduce dry matter intake before acute signs of toxicity are seen. The urea itself is not toxic. The ammonia produced from it is what is responsible for cell death.
Recommendations for Feeding Urea:
- Use urea to meet soluble protein requirements but do not exceed the requirement.
- Regardless of soluble protein and available carbohydrate levels in rations, the generally accepted limit is no more than 0.5 pound (0.23 kg) urea/cow/day and no more than 0.25 pound (0.11 kg)/heifer/day.
- Don't feed urea to dairy calves less than 3 mos. of age because their rumens are not fully functioning and they are not able to use the urea to form microbial protein.
- Remember that feed grade urea is 45% Nitrogen so it has a CP Equivalent (CPE) of 281% (45% * 6.25). It takes a lot less urea to provide an ounce of crude protein.
- Adapt the rumen microbes and cows to urea over a period of 2-3 weeks and make sure urea is mixed well in the feed if it is purchased as an ingredient.
Feed the Bugs Some Quality Proteins
Like the cow, microbes require sources of amino acids. Isoacids obtained from the breakdown of dietary amino acids are required for growth of fiber-digesting bacteria. Amino acids are known to be preferred over ammonia by many starch-digesting bacteria. A study was conducted where rumen microbes were grown in a laboratory with a set amount of carbohydrate available to them. There was a significant increase in cell protein synthesis when casein (milk protein that is a good source of isoacids and amino acids) was added as a protein source. This increase was observed even though ammonia (or free nitrogen) levels were more than adequate. Based on this work it is commonly recommended that two pounds (0.90 kg) of soybean meal or its equivalent be included in rations for high-producing cows.
Metabolizable Protein Supply
The NRC (2001) Nutrient Requirements of Dairy Cattle recommended the metabolizable protein (MP) system in order to more accurately predict the protein available for the cow’s use. Metabolizable protein is the protein that flows from the rumen, is digested, is absorbed from the small intestine as amino acids, and is available to be metabolized by the cow. MP is a combination of undegraded feed protein (UIP or RUP), rumen microbial protein, and endogenous protein. Metabolizable protein is therefore predicted using computerized ration programs. It is not analyzed directly as a nutrient in a laboratory.
The NRC (2001) Nutrient Requirements of Dairy Cattle predicts microbial protein production from the amount of predicted digested organic matter in the rumen. If the diet is limiting in ruminally degraded feed protein (DIP or RDP), predicted microbial protein production is limited. To predict degraded feed protein (DIP or RDP) and undegraded feed protein (UIP or RUP), feed proteins are broken down into 3 fractions (A, B, and C). The A fraction is assumed to be entirely degraded in the rumen. The C fraction is not degraded in the rumen. The amount of B fraction digested in the rumen is a function of its predicted rate of digestion (derived from in situ data) and its predicted rate of passage (estimated based on dry matter intake, percentage dietary concentrate, percentage dietary NDF and feed moisture content). The higher the rate of passage, the lower the amount of predicted rumen degraded feed protein. The intestinal digestibility of the RUP fraction varies by feed.
Metabolizable Protein Requirement
Metabolizable protein is needed for making milk, producing a calf, and growing. The NRC (2001) Nutrient Requirements of Dairy Cattle predicts MP requirements based on a cow’s body weight, dry matter intake, days pregnant, milk production, and milk protein content. A 1500-pound cow (682 kg) producing 99 pounds (45 kg) of milk per day (3.5% mf, 3.0% true protein) requires about 6.6 pounds (3000 grams) of MP per day. That would be 11% dietary MP if the cow ate 59 pounds (26.8 kg) of dry matter per day.
Milk Nitrogen Efficiency (MNE)
Nitrogen waste management is becoming an issue on today’s dairy farms. The goal is to reduce nitrogen output in urine and manure. Improved nitrogen management helps not only the environment but also potentially improves feed economics and the farm’s bottomline. The amount of feed nitrogen retained by the cow in the form of milk is called milk nitrogen efficiency (MNE). In one dataset made up of 334 dietary treatments that ranged in CP from 10.2 to 24.6%, MNE ranged from 16.2 to 45.2%.
Generally, MNE decreases as dietary CP is raised but more factors are involved in predicting MNE. Breed may have an impact on MNE, being somewhat higher for Jerseys. MNE has been found to be lower later in lactation. Increasing milk protein content raises MNE. Feeding more frequently tends to increase MNE. Raising rumen degradable starch levels in the diet increases MNE. Increasing rumen degradable protein and/or soluble protein in the diet reduced MNE. Improvements in dietary amino acid balance should improve MNE.
The partition of excreted nitrogen between feces and urine is also a concern. Studies have found that at higher CP intakes a larger proportion of nitrogen was excreted in the urine. Of course, higher amounts urinary nitrogen increase ammonia volatilization and potential odor problems. Milk urea nitrogen (MUN) levels can be used as an indicator of urinary nitrogen.
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Protein and Carbohydrate Nutrition for High Producing Dairy Cows
Rick Grant, University of Nebraska - Lincoln
Milk Urea Nitrogen Testing
Rick Grant, D. Drudik, and J. Keown, University of Nebraska - Lincoln
MUN as a management tool
M.F. Hutjens, Ph.D., University of Illinois
Feed Nutrients In: Feeding the Dairy Herd North Central Regional Extension Publication
J.G. Linn et al.