Balancing Dietary Protein to Maximize Protein

Recovery in Milk

Hélène Lapierre1, Doris Pellerin2 and Jean-François Bernier2

1 Agriculture and Agri-Food Canada, Dairy and Swine Research and Development Center,
P.O. Box 90, Lennoxville QC, J1M 1Z3, Canada
2 Department of Animal Science, FSAA, Laval University,
Ste-Foy, QC, G1K 7P4, Canada

Take Home Messages


The cow, by her symbiotic relation with the microbes living in her rumen and by her own metabolism, possesses the wonderful ability to transform rougher feedstuff into a human food with a high nutritive value, the milk. However, the price of this ability is a certain inefficiency in the global use of the diet, which is also true for protein. Over the last years, the transfer of feedstuff protein into milk protein has received much attention for different reasons.

First of all, the consumers request products with more protein and less fat. Also, cheese consumption has increased and cheese yield is directly related to protein content in the milk. These market forces have increased the demand for protein. This trend was finally translated, or is going to be, in a pricing system which is based, in part, on the protein content of milk. Under actual conditions in Quebec, an increment of 0.1% in milk protein content results in additional income of $2,000 per year for a dairy farm having a quota of 10,000 kg of fat per year (27.4 kg/d) (24). The challenge is to achieve this result with substantial increases in order to leave the producer a higher net income.

There is also a general interest in decreasing nitrogen excretion by livestock. In some countries where animal production is more concentrated than in Canada, like The Netherlands, there is an actual need to decrease nitrogen excretion to reduce pollution both in the air and in the soil/water.

What is a Protein?

Each protein is a specific sequence of subunits, called amino acids. There are 20 amino acids and their different combinations create the thousands of known proteins in sequences that can reach over 1800 amino acids. Each of these amino acids contains at least one atom of nitrogen and this is the basic difference between protein and fat or sugar: protein contains nitrogen, whereas this compound is not present in fat or sugar. This is so true that analytical determination of protein is not really a measurement of protein, but rather the measurement of nitrogen content. This nitrogen content is afterwards divided by an average factor of nitrogen content in protein (0.16) to yield crude protein content. This is usually quite fair, but in some feedstuff like silages, up to 57% of total nitrogen could be present in a form not bound to protein (25).

To understand when and where balancing dietary protein can affect milk protein recovery in milk, we will follow together the conversion of dietary nitrogen to milk protein. Using studies where the fate of nitrogen has been followed (5, 13, 26), a cow averaging 24 kg of milk per day eats daily 405 g of nitrogen (2.5 kg of crude protein). From this amount, 129 g of nitrogen are excreted in the feces, which means that 276 g are absorbed in the gut: an average nitrogen digestibility of 68%. Of the nitrogen that is absorbed 40% is excreted in the urine and only 30% appears in milk. Is it possible to partition more of this nitrogen towards milk to the detriment of urine which might become a source of pollution? To answer these questions, we will discuss the metabolic fate of ammonia, urea and amino acids, the major nitrogenous compounds flowing through the gut of dairy cows.

Absorption of Nitrogen

If we follow the route of ingested nitrogen, we first get into the rumen. Dietary nitrogen compounds will have two different fates, according to their degradability. Low degradable compounds will be transferred to the abomasum and to the intestine where they could be digested. Rumen microbes will break degradable compounds and make their own protein from the products of the degradation. This new protein will then also be transferred to the abomasum and to the intestine to be digested. As a result of these fermentation and digestive processes, new nitrogenous compounds, quite different from the ingested compounds, will be absorbed through the gut wall and reach blood circulation into the portal vein. Through blood circulation, these nutrients will reach the cells of the different tissues and be available for metabolic utilization. It is feasible to quantify the flow of these nutrients reaching portal circulation by surgically implanting chronic catheters that will allow further blood collection (Figure 1).

Figure 1. Nutrient flow in dairy cows.

Despite the power of this tool to study nutrient supply, there have been only a very limited number of studies involving dairy cows. Figure 2 summarizes studies measuring the global flow, including portal flow, of nitrogenous compounds in dairy cows (5, 13, 26). Amino acids are provided by the digestion of feed or microbial proteins. Ammonia comes from amino acid degradation, directly from dietary non-protein nitrogenous compounds or from urea recycling, as we will see later. Amino acids represents on average 42% of the total amount of digested nitrogen (which disappeared from the gut). Ammonia represents up to 62% of digested nitrogen. Two facts emerge from these numbers. First, there is about 50% more nitrogen absorbed as ammonia, compared to amino acids. Secondly, nitrogen absorbed as ammonia plus amino acids is larger than the total amount of digested nitrogen. Where does this extra nitrogen come from? Looking at Figure 2, one can see that a third compound flows through the gut, urea, but it flows in the opposite direction of amino acids and ammonia. Urea from the blood goes back in the gut, namely in the rumen to be used by the microbes to build protein or to be transformed in ammonia. Urea contribution to the gut nitrogen is not negligeable, as it represents 23% of ingested nitrogen.

Figure 2. Global flow of nitrogen in dairy cows.

Other nitrogenous fractions also contribute to the gut flow of nitrogen. Peptide contribution is actually very controversial. Peptides are short chains of amino acids, in this case made of two or three amino acids. As peptide concentration is measured by difference between different fractions and not directly, there is a big discrepancy among different laboratories and peptide contribution to amino acid flux covers a large range from 0 (3) to 85% (30). The real significance of peptide absorption is not known yet and deserves further investigation as peptides can also be used directly used by the mammary gland (1).

Also, portal absorption does not quantify all nutrients available from the diet to the animal, as the measure is effectively done after the gut has used the nutrients it needed. In the only study where intestinal disappearance of amino acids was compared to portal apparition, the amount of amino acids appearing in the portal blood varied between 10 and 88% of the amount disappearing from the intestine (29). A good part of this discrepancy could be attributed to the gut metabolism.

Products resulting from the digestion of nitrogenous compounds are absorbed in the portal vein, mainly as ammonia and amino acids. Ammonia absorption is 50% higher than amino acid absorption. A part of the urea from the blood goes back into the rumen and this recycling may represent up to 23% of the ingested nitrogen.

Ammonia and Urea

After its absorption in the blood by the animal, will ammonia be useful? Not really, as ammonia is a toxic compound for the animal. All the ammonia absorbed in the portal vein will be transformed to urea by the liver. Ammonia detoxification by the liver represents around 65% of urea production by the liver (5, 26). The liver also produced urea from nitrogenous groups coming from the metabolism of peripheral tissues. In ruminants, urea can be recycled in the gut through the saliva or through portal-drained viscera. However, 62% of urea produced by the liver is excreted in urine, resulting in a net loss of nitrogen. In fact, the amount of nitrogen being absorbed as ammonia in the portal vein matches quite perfectly the amount of nitrogen excreted in urine (Figure 2). In addition to being a net nitrogen loss for the cow, ammonia imposes a double expense to the liver; the energy required to produce urea as well as the loss of an additional N. This latter fact is still under study, but there is some evidence that under high ammonia portal absorption the liver needs additional nitrogen in order to remove blood ammonia and produce urea (21).

Ammonia appearing in the portal vein needs to be detoxified by conversion to urea by the liver, increasing the energy expenditure of the cow. Here, the knowledge of the fate of absorbed ammonia does not allow us to change it, but it emphasizes its metabolic cost and the importance to consider the formation of this compound in the rumen when balancing a diet. Ammonia absorption should be reduced to its minimum, and this could be achieved by controlling the degradable vs. undegradable portion of the protein in the feedstuff, in relation to the degradability of the energy (carbohydrates) delivered to the microbes so that microbial protein synthesis is optimized.

Amino Acids

As stated previously, amino acids are the basic blocks used for building proteins. Each type of protein is built identically following a specific genetic message describing the sequence of amino acids to be incorporated in the protein. Amino acids can be compared to the letters of the alphabet: one by one, letters don't mean anything, as amino acids have very limited functions, but if they are joined in a specific sequence, letters can form a multitude of words with powerful meaning, as amino acids can build proteins with various roles such as structure protein, hormone, enzyme, and export protein in the milk.

Utilization at the Whole Body Level

How does the body use amino acids after their absorption into the blood circulation? Figure 3 summarizes the fate of amino acids once absorbed.

Figure 3. Fate of amino acids.

They reach a pool of free amino acids. They will leave this pool being used for protein synthesis or being oxidized (burned). For nitrogen utilization efficiency, oxidation of an amino acid is a loss. Then, the incorporation of an amino acid into a protein is not a static process, as there is a significant turnover of the proteins which once degraded will provide amino acids that will come back to the free amino acid pool. The protein accumulated in the muscle or secreted in the milk is the difference between the opposing processes of protein synthesis and degradation. Using a labelled amino acid, it is possible to quantify these phenomena, the extent of which is quite surprising!

As there is only limited data for dairy cows, we will first look at the utilization of an amino acid, leucine, in growing cattle (Figure 4). Total utilization of leucine averaged 43 mmol/h (7, 11, 15, 16, 18, 19, 20). Sixteen percent of this utilization is directed towards oxidation, which is a loss for protein synthesis. If we convert the remaining leucine utilization into protein, 1923 g of protein were synthesized every day. However, cattle used in these studies retained only 216 g of protein per day... which means that over 90% of the proteins synthesized were degraded. This tells us the potential impact of a reduction of oxidative loss or of degradation on protein retention. Can we reduce these losses?

Figure 4. Utilization of leucine in cattle.

In dairy cows, as expected, the total utilization of leucine is more than twice the value observed in growing cattle (Figure 4). The oxidation is quite similar, averaging 16%. The main difference is in the amount of protein left for human consumption which goes up to 35% of total utilization with milk secretion in dairy cows (2, 4, 23). Degradation represents only 70% of protein synthesis (4)... it is lower than in growing cattle, but there is still a lot of room for improvement.

Protein synthesis represents around 84% of the total utilization of leucine, the remaining 16% being oxidized. Protein harvested by humans represents at the most 11% of the synthesized proteins in growing cattle and up to 35% in the milk from dairy cows. Other synthesized proteins are degraded to their basic constituents, the amino acids. The total utilization of leucine for oxidation and for the synthesis of protein that will be degraded afterwards largely overruns the utilization directed towards final products like meat and milk.

Utilization by Different Tissues

In the previous section, we reviewed amino acid utilization in the animal as a whole. However, tissues have very different metabolic activity. There are two basic techniques to determine amino acid utilization by a tissue. We can measure the net utilization which means the difference between what goes out and what goes in. We can also measure utilization using labelled amino acids.

If we first look at net utilization, the first organ that amino acids go through after their absorption is the liver. All the blood draining the gut first flows through the liver at over 2 000 liters per hour (5, 26). Therefore, the metabolism of the liver is a determinant on the nutrients, including amino acids, that will be available to peripheral tissues. In dairy cows, 30% of the amino acids absorbed in the portal vein were removed by the liver (5, 26), for protein synthesis or to be oxidized. There are no data for dairy cows, but in growing cattle the liver can completely remove the increment of amino acid absorption obtained with a postruminal casein infusion (8). This can partly explain why the milk protein response to dietary protein supplement is too often much lower than expected.

After their passage through the liver, nutrients in blood circulation are pumped by the heart and then distributed to the peripheral tissues including the mammary gland. To increase protein synthesis by the mammary gland, we want to ensure that the blood flowing through the mammary gland will supply an optimum balance of amino acids. During the last years, the concept of an ideal profile of amino acids has emerged, indicating that not only a minimum amount of amino acids is required to optimize protein synthesis, but also an adequate proportion of each amino acid relative to the total amino acid content is needed. The utilization of amino acids by the mammary gland is not uniform: if we compare the amount of each amino acid removed by the mammary gland from blood circulation to the amount of the same amino acid secreted in the milk, there is a large difference depending on the amino acid. The amount of amino acid in milk compared to the amount removed by the mammary gland varies between 0.36 for arginine and 8.3 for aspartate and arginine (9). This ratio is higher for non-essential amino acids (2.62) and lower for essential amino acids (0.88; 10). The extraction of amino acids by the mammary gland is correlated with their blood concentration (10, 12). Also, protein synthesis is related to the extraction by the mammary gland of essential amino acids, mainly methionine, lysine, and phenylalanine (22). All this work conducted on mammary gland utilization of amino acids gives us an indication of which amino acid is more limiting for milk production and we can afterwards look for means to increase the proportion of this amino acid in the blood supplying the mammary gland.

Also, as done at whole body level, we can look at the total utilization of amino acids by different tissues in order to help us to define the needs for a specific tissue. Only two studies have been conducted in lactating dairy cows in relation to specific tissues. Total utilization of leucine by the gut and the liver represents over 45% of whole body leucine utilization (17), while mammary gland utilization represents 41% of total utilization which includes 29% devoted to milk protein secretion (2). These numbers indicate how important the metabolism is of these tissues, as together they account for over 80% of the total utilization of leucine. A better knowledge of their requirements will help us chose proper ingredients to balance diets in order to optimize milk protein secretion and avoid wastage of nitrogen and energy.

In dairy cows, the mammary gland, gut, and liver contribute to over 80% of total leucine utilization. Furthermore, liver metabolism dictates the nutrients that will ultimately be delivered to the peripheral tissues. Therefore, a better knowledge of protein metabolism of different tissues will allow us to balance diets more adequately, supplying an ideal profile of amino acids corresponding to the requirements of active tissues, including the mammary gland.

What are we Doing with this Information?

Not that long ago, protein requirements of dairy cows were expressed in terms of crude protein per day. The research conducted to improve dietary protein to maximize protein recovery in milk actually brings us to the following recommendations.

Reducing Ammonia Absorption

As previously seen, the knowledge of the fate of ammonia, once absorbed, does not allow us to change it, but it reveals its costs in terms of energy and nitrogen losses. The diet must be balanced in order to reduce ammonia absorption to its minimum while still maximizing bacterial growth. This involves an adequate balance between dietary degradable protein to feed rumen microbes and undegradable protein bypassing the rumen combined with a good synchronization between ruminal supply of degradable sources of both energy and nitrogen.

Reducing Amino Acid Oxidation

As previously discussed, amino acids in excess of the limiting amino acid(s) will be burned and, therefore, lost as a nitrogen source. The oxidation rate of leucine, for example, averages 16% of its total utilization. This oxidation could be reduced by supplying the first limiting amino acids which results in an increased protein synthesis. There is general agreement that the first two limiting amino acids in high producing dairy cows are lysine and methionine: their respective contribution to essential amino acids in the duodenal digesta should average 15 and 5% (27, 28). Proteins synthesized by rumen microbes have a constant essential amino acid profile, similar to that of lean body tissue and milk, and supply 50% or more of the absorbable amino acids when diets are adequately balanced (27). However, most of the feed proteins are deficient in one or more amino acids. This means that for high producing dairy cows where the diets include a larger proportion of rumen undegradable protein, it may be difficult to achieve an adequate balance through the selection of appropriate feedstuffs. Some companies offer supplements of rumen protected lysine and/or methionine. How may one or two amino acids limit the whole process of protein synthesis? Protein synthesis is done according to specific sequences and when one amino acid is lacking it cannot be replaced, as this would change the sequence: the synthesis of the protein is thus interrupted. If an amino acid is added, this will allow protein synthesis to be pursued, using all the other amino acids until another amino acid becomes limiting. When fed in an unbalanced diet, the cow will have to oxidize the amino acids which are in excess due to the limiting amino acid(s).

Increasing milk protein secretion is one thing, increasing net income is something else. A pricing system based on milk components is obviously needed to justify any added feeding cost to increase milk protein content. In order to evaluate the economical potential of rumen protected amino acid (RPAA) utilization, we first calculated the feeding cost related to adjustments of a corn-based (Table 1) and a barley-based (Table 2) diet to meet lysine and methionine requirements. In the first step, we simply included RPAA to balance diets to meet lysine and methionine requirements. This increased the daily cost per cow by about $1.00 for both diets. Then, we considered carbohydrate degradability and increased it to improve microbial protein synthesis, by grinding the corn in the first diet or by adding some corn in the second diet. This decreased the initial increment by about $0.10. Finally, we added protein supplements to improve the amino acid balance before calculating the amount of RPAA needed to meet lysine and methionine requirements. Then, the incremental feed cost averaged around $0.70 for both diets.

Table 1. Comparative feed costs for a corn-based diet (kg/d as fed) for a 600 kg cow producing 40 kg milk/d, with 3.2% CP and 3.7% fat, balanced to meet requirements for undegradable protein with 3 modifications to meet lysine and methionine requirements.
Ingredients ($1) Basal


+ RPAA2 +ground



+ground corn

+ protein suppl.4


Corn silage (30) 172 173 176 180
Legume silage (40) 147 147 150 153
Grass hay (100) 20 20 20 20
Corn (175) 68 66 65 66
Barley (170) -- -- -- --
Soybean meal (425) 15 14 14 17
Wheat distillers grain (445) 23 21 21 6
Corn gluten (595) -- -- -- --
Heated soybean (595) -- -- -- 7
RPAA5 (3000) -- 40 38 30
Cost ($/d) 437 549 535 506
Difference -- 112 98 69
Lysine content6 134 149 149 148
Methionine content6 46 51 51 50

1 $: price of the ingredient per ton, as fed.

2 RPAA (rumen protected amino acid) added to meet lysine and methionine requirements.

3 The corn in the basal diet has been ground and then calculations for RPAA requirements were made.

4 The corn in the basal diet has been ground, dietary sources of lysine and methionine have been added, and then calculations for RPAA requirements were made.

5 Protein supplement containing 10% of RPAA.

6 Expressed as percent of essential amino acids in the duodenal digesta.

Table 2. Comparative feed costs for a barley-based diet (kg/d as fed) for a 600 kg cow producing 40 kg milk/d, with 3.2% CP and 3.7% fat, balanced to meet requirements for undegradable protein plus 3 modifications to meet lysine and methionine requirements.
Ingredients ($1) Basal


+ RPAA2 + corn3


+ corn

+ protein suppl.4


Corn silage (30) -- -- -- --
Legume silage (40) 18.8 19.3 20.3 21.3
Grass hay (100) 5.0 5.0 5.0 5.0
Corn (175) -- -- 4.2 4.2
Barley (170) 10.7 11.1 6.4 6.3
Soybean meal (425) -- -- -- 0.6
Wheat distillers grain (445) 3.2 2.2 2.1 --
Corn gluten (595) -- -- -- --
Heated soybean (595) -- -- -- 1.0
RPAA5 (3000) -- 0.44 0.44 0.40
Cost ($/d) 4.69 5.66 5.58 5.42
Difference 0.97 0.89 0.73
Lysine content6 13.2 14.8 14.8 14.9
Methionine content6 4.7 5.0 5.1 5.0

1 $: price of the ingredient per ton, as fed.

2 RPAA (rumen protected amino acid) added to meet lysine and methionine requirements.

3 Corn has been added and then, calculations for RPAA requirements were made.

4 Corn has been added, dietary sources of lysine and methionine have been added and then, calculations for RPAA requirements were made.

5 Protein supplement containing 10% of RPAA.

6 Expressed as percent of essential amino acids in the duodenal digesta.

The annual economical impact of diet adjustments to meet lysine and methionine requirements for a dairy farm with 42 cows (quota of 10,000 kg of fat per year) is shown in Table 3. The basal hypothesis is based on improvement of performances observed with RPAA in Canadian herds (14), price of milk predicted according to Bourbeau (6), and daily cost of RPAA per cow fixed at $0.70, which corresponded to balancing the diet as well as possible with dietary sources of amino acids before adding RPAA.

Table 3. Economic annual impact of rumen protected amino acid utilization in a dairy herd1.

Variation with basal hypothesis2

Including increment in charges usually attributed to higher milk production3 Price of the RPAA lower than actual4
No additional quota5

Net income

Cash flow

- first year

- following years










Additional quota6

Net income

Cash flow

- first five years

- following years










1 Dairy farm averaging 42 cows with a quota of 10 000 kg of fat per year (27.4 kg of fat/d).

2 Basal hypothesis:

- improvement of the performances are the following: 1.55 l of milk/d, 0.14% CP and no variation in fat content (14).

- price of milk components are the following: protein: $8.475/kg, fat: $5.05/kg, lactose: $1.135/kg (6).

- the daily cost of RPAA per cow is $0.70 per day

- RPAA are added to the diet for 107 days, beginning 7 days before parturition

- labor not included in the calculations

- increment in charges for increased costs of feed, veterinary, and AI usually observed with increased milk production have not been included.

3 As basal hypothesis, but increment in charges for increased costs of feed, veterinary, and AI usually observed with increased milk production were evaluated to half of charges usually used.

4 As basal hypothesis, but the daily cost of RPAA per cow is $0.50 per day.

5 As no additional quota is bought, an additional cow was culled the first year.

6 Quota is bought at the price of $13 500 per kg of fat/d, and paid with a loan reimbursed in 5 years with an interest rate of 8%.

When using a technology to increase milk production, Canadian producers have two choices: keep the same quota and decrease the number of cows or keep the same number of cows and buy additional quota. We have tested the two possibilities. Keeping the same quota does not bring a significant increase in net income, while buying additional quota would give back a return of around 1.5 on investment. There is not enough information to positively assess if increasing milk production through RPAA feeding would yield the increment of charges usually attributed to higher milk production. There seems to be an indication that it would not be the case with RPAA, because of the improvement in amino acid balance. Net income increment would be decreased and even negative if this was the case. We also analyzed another possibility using a daily price per cow of $0.50 (increasing the demand might decrease the price allowing the production of larger quantities). Obviously, this situation was more interesting for the producer, even in the case where no quota was bought. In conclusion, balancing the diet to meet lysine and methionine requirements might be economically interesting, but it is better to achieve the best balance of lysine and methionine with dietary supplies (lysine can often be balanced without RPAA) before supplementation with RPAA.

Where are we Going with this Information?

From work done on net utilization of amino acids, we still need more information on the requirements of amino acids, namely by the mammary gland and by the liver. We also need to determine the interaction between the different types of feeds (proteins vs. carbohydrates) on urea recycling and on portal absorption of ammonia and amino acids in relation with amino acid content of duodenal digesta. Furthermore, in order to improve amino acid delivery to the mammary gland, we need to improve our ability to predict liver metabolism of extra portal supply of amino acids.

From work done on total utilization, we have seen that a major component of amino acid utilization is degradation of the protein after it is synthesized. Any avenue which would have a slight effect on degradation could have a huge impact on protein retention and/or secretion in the milk. More work is needed to determine how stress, genetic, and hormonal treatments could alter this component.

This will allow us to increase the accuracy of our estimation of the cow's requirement and thus more accurately deliver the required amino acids, decreasing the absorption of ammonia and excess amino acids. These adjustments will optimize milk production while reducing nitrogen loss resulting in decreased production costs as well as reduced losses of N to the environment.


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