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14th May 2020

Protein Nutrition


Crude Protein (CP)

Crude protein is defined as the basic protein content of a feed, it is calculated by determining the nitrogen content of a feed.

It is not really an accurate definition of the real protein content of that feed because it inevitably includes a proportion on Non-Protein-Nitrogen (NPN). It also assumes that the protein all contains 16% nitrogen. Therefore CP = Nitrogen content divided by 0.16 (or multiplied by 6.25).

Digestible Crude Protein (DCP)

Crude protein was never going to be a particularly reliable measurement of useful dietary protein. In some feed with low “Q” values, the digestibility of the protein was poor. In order to improve the reliability of dietary predictions the CP was multiplied by the “D” value to give the DCP.

Rumen Degradable Protein (RDP) / Undegradable Protein (UDP)

Unfortunately, the DCP measurement included a significant rogue element, Non Protein Nitrogen or NPN!

NPN can be used with variable efficiency in the rumen compared to true proteins. Rumen ammonia which is a form of NPN can give rise to high blood and milk urea’s if not managed properly.

In the 1970’s research suggested that dietary protein was:

  • either broken down into ammonia (NH3) and carbon fragments in the rumen, and then used by the rumen microbes for protein synthesis. (making more rumen bugs)
  • or it passed through the rumen undegraded and was then available for digestion in the abomasum and small intestines.

It took about a decade before this information could be usefully correlated into a coherent ration formulation system that truly represented an improvement on the old DCP system. Henceforth protein was either rumen degraded (RDP) or undegraded in the rumen (UDP).

The Metabolisable Protein System

Protein metabolism in the ruminant is another part of the science of animal nutrition which is still undergoing debate and change. Different countries are using different systems which is a pretty reliable indication that so far nobody has got it exactly right.

In 1992 the UK adopted the, “Metabolisable Protein” system (MP System) as a significant improvement on the older DCP and the RDP/UDP systems. 1n 2002 Feed into Milk (FiM) was launched as a further improvement to the MP system.

The reason for adopting the MP system concerned the fact that the degradability of the protein varied according to the rate at which it passed through the rumen. For example, high yielding Holsteins tended to have large intakes and more rapid throughput than low yielders and the rumen degradability of say soya protein could be as low as 40% in the high yielders but as high as 85% in the low yielders.

The method of determining a system that could be used to calculate the degradability of protein more accurately, involved dangling the food source in little porous bags made out of indigestible material, in the rumen for set periods of time.

Personally, I can’t imagine how bizarre this process would appear to the layman, but it certainly produced some very useful data indeed.

The resulting figures showed the rate at which the different proteins were being degraded.

The figures produced were tabulated originally as the A, B and C fractions but modern developments in the early part of the new millennium developed the definition of the A fraction and now we use S, A, B, and C, fractions.

S, fraction is protein which is released as soon as the feedstuff enters the rumen.

A, fraction is the protein that is not released as soon as the feed enters the rumen but is degraded before it leaves the rumen.

B, fraction is the rumen Bi-pass protein.

C, is the rumen outflow rate.

The scientists had to be able to use this data to predict animal performance from the different feedstuffs.

In order to do this, they had to work out some sort of measure of animal performance to gauge against the rumen outflow rate. They came up with “Animal Production Level” (APL).

This measurement is simply the total energy requirement of the animal divided by the energy needed for maintenance.

For example:-

The APL for the Holstein cow giving 35 Kilo’s of milk referred to on page 15, is as follows: –

Total ME requirement = 235.25 MJ

ME required for maintenance = 65.5 MJ

APL = 235.25 / 65.5 = 3.6

This figure of 3.6 can now be used to determine the relative degradability of the different protein sources in this cow. (around 8% per hour on the graph below.)

Figure 1

The relationship of APL to rumen outflow rate.

APL and its link to rumen outflow rate

From this information it can be seen that an animal feeding at maintenance has a rumen outflow of 0.02 or 2% of its rumen contents per hour. this increases to about 10 % for dairy cows producing about 40 litres of milk. The graph is not very well supported by masses of trial work at this time, but it does seem to be a reasonably good measurement on which to base predictions.

Most of the nutrition models have used this principle but have then gone on to use their own correction factors, which explains why we don’t always get the same answers.

The RDP/UDP system assumed that the protein that was not degraded in the rumen, was digested in the small intestine, and the bit that wasn’t digested at all was a constant fraction of the protein

This system began to show its limitations when farmers found that some of the raw materials that they were using, notably Maize Gluten and Dry Distillers Grains, often failed to live up to expectation. This was because the protein had been denatured by heat during manufacture and handling and, after reacting with sugars, became insoluble, and indigestible in the small intestine.

As a rule of thumb, when purchasing products lie Maize Gluten, Distillers grains, and cereal bi-products, “The yellower, the better”.

The indigestible protein can be measured by determining the amount of Nitrogen in the Acid Detergent Fibre (or ADF for short). this is referred to as the “Acid Detergent Insoluble Nitrogen” (or ADIN for short).

ADIN can be converted into “Acid Detergent Insoluble Protein” (ADIP) quite simply. Once this has been done it is easy to see how much of the protein expressed in a feedstuff analysis will be wasted.

A graphic example of this wastage factor can be found in rape seed meal where up to about 10% of the protein can be ADIP and therefore wasted.

The MP system is more accurate than the old RDP / UDP system because it automatically deducts this ADIP from the total UDP. The remaining fraction of UDP is known as Digestible Undegradable Protein (DUDP) or (DUP).

The other main element of doubt in the RDP/UDP system was that the RDP didn’t all degrade at a constant rate. We have already seen that the protein can be split up into A, B, and C fractions.

The A fraction can vary from as low as 8% in soya to around 65% in grass silage. In the case of values as high as 65%, it is probable that the rumen microbes won’t be able to cope with all of the ammonia that is being released.

Ammonia is a very important feed nutrient for the rumen microbes. They can use it to build more microbes, as long as they have access to an energy source.

Ammonia can be absorbed into the blood stream through the rumen wall very easily, so it is important to supply enough rapidly available energy in order to use the ammonia before it is wasted.

Sugar is probably the best source of energy under theses circumstances, but it should be fed at a constant rate in order to avoid acidosis.

The resultant improvement in microbe population will serve to help animal productivity.

The yield of microbes is in effect supplying the animal with extra protein in a usable form. This is known as Microbial Protein (MCP).

Excess of ammonia, results in absorption into the blood stream as urea and is then excreted via the kidneys.

This element of waste can be very significant on high protein silages and on grass swards where animals graze soon after top dressing with nitrogen.

Good diets need to recognise this dilemma and provide the animal with better quality protein and/or, sugar to help the rumen microbes thrive in the potentially very rich nutrient source.


There is just one last piece to the jigsaw of protein metabolism; “Effective Rumen Degradable Protein”

ERDP is a measure of the amount of RDP that is actually available for microbial metabolism and growth. The old RDP/UDP system assumed that all of the RDP was available.

In fact, some of the ammonia released from the “A” fraction of the protein, is absorbed across the rumen wall into the blood stream as urea.

This Blood Urea can be measured as “Blood Urea Nitrogen” (BUN) and has a relationship with infertility in the female ruminant. The BUN is normally filtered in the kidneys and excreted in the urine, but some is found in the milk of the lactating animal.

“Milk Urea Nitrogen” (MUN) is now commonly included on the milk data returned to the dairy farmer. High MUN levels are a good indication of dietary imbalance and potential infertility problems. Low MUN levels are usually indicative of a dietary under supply of protein.

The “S and A” protein fraction is roughly defined as: –

QDP (Quickly Degraded Protein)

SDP (Slowly Degraded Protein)

The ERDP is calculated as 80% of the QDP plus the SDP.


MPN (Metabolisable Protein from rumen Nitrogen)

This new measure effectively replaces ERDP it has to be said that it is a much improved measure since it makes a further distinction of the old ‘A’ fraction into ‘S’ and ‘A’ where S is instantly available and A is rapidly available.

There should never be more than 200 grams of MPN more than that of the MPE.

MPB (Metabolisable Protein from Bypass protein)

replaces DUP, essential in high yielding cow diets where demand increases dramatically, because the output of MPN is limiting.

If you have stuck with it so far, you have by now got a good grasp of the main elements of protein metabolism. Well done! Now go and have a break, when you come back have another look at what you have just read. Getting to grips with this bit of theory will help you with your decision making when you start to balance up your own diets.

It is important to try and understand the abbreviations because they are in extensive use in virtually all of the definitive works on the subject. Once you have got to grips with the reasoning behind the definitions, will be much easier to make sense out of diet sheets and feed analyses.

Protein Metabolism

The ruminant, like all animals, needs protein in the same way it needs energy, for Maintenance, Growth, Pregnancy, and Milk production.

As already mentioned, the FME/MPE is needed by the rumen microbes, to fix the ERDP/MPN in the form of microbial protein (MCP) and subsequently metabolisable protein (MP).

The FME/MPE content of the forage fraction of the diet, will have a direct influence over the amount and type of supplementation of energy and protein in the diet.

  • Low FME/MPE forages will generally be low in ERDP/MPN and need heavy supplementation with high FME/MPE and high ERDP/MPN feedstuffs.
    In plain English, forages of poor quality with low sugar and protein, don’t supply the rumen bugs with enough fermentable nutrients to insure an optimum production of more rumen bugs. The rumen bugs are needed for digesting in the abomasum and small intestine. The bugs that are being digested are effectively the main source of nutrients (Food) for the cow, so most of the time we want to design rations that get the best possible production of rumen bugs.

High FME/MPE forages will require much less supplementation but tend to coincide with low NDF levels and, as a result may well need supplementing with fibre sources. In plain English, forages that are good quality and highly fermentable, tend to cause frenzied reproduction of the rumen bugs. A by-product of this activity is acid production. If there is too much acid being produced, the rumen can shut down while the animal cuds to produce the alkali saliva needed to neutralise the acidity. In extreme cases Rumen Acidosis can be fatal. So, we have to add some nice scratchy fibre to make sure that the animal can cud well enough to regulate the rumen acidity back up to a neutral Ph.

As a rule, Silages with high acid loading will need greater supplementation of both ERDP/MPN and FME/MPE from concentrates, (ie Soya bean meal and Sugar Beet Pulp).

Microbial Protein Synthesis.

I love this bit of ruminant nutrition because it is so critical to getting good performance out of all ruminant livestock.

Anything we do to bring about an increase in the synthesis of microbes in the rumen, effectively increases the supply of nutrients to the animal. Live and dead bugs entering the abomasum and hind gut are effectively the main proportion of nutrient supply to the animal.

The basic concept of effective ruminant nutrition necessary to supply the genetic potential of highly bred modern ruminants; relies fundamentally on optimising the fermentation output capability of the animal’s rumen.

The temptation on the part of some nutritionists to overload the animal with bypass proteins and energies in the misguided belief that they are able to some how meet the animals requirements more effectively; is frankly, an expensive way of trying to hide the inability they have to get the best performance out of the rumen.

The rules for optimising Microbial Crude Protein (MCP) output are in fact, really simple:-

If the supply of protein (Nitrogen) is too low, the MCP production = ERDP supply.

If the supply of energy is too low the MCP synthesis is proportional to the FME supply.

The yield of MCP per Megajoule of FME is called “Y”.

“Y” is variable and its value increases as production (APL) increases. (see table).

Figure 2

The relationship between APL and the yield of MCP per MJ of FME


The Metabolisable Protein (MP) System.

The AFRC 1992 concluded that the MP system should be adopted as standard practise for the UK Industry.

The representation in figure 3 below (AT Chamberlain & J M Wilkinson (1996)) is a good representation summary of how the system elements are related to each other.


  • Q D P is 80% available to microbes.
  • Q D P + S D P = ERDP.
  • M C P production depends on availability of FME and the value of Y.
  • Only 64% of M C P is absorbed.
  • U D P consists of indigestible waste (A D I P) and digestible Bypass protein (D U P).
  • D U P varies according to A P L (the higher the A P L; the faster the rumen outflow rate, the greater the D U P fraction, and the smaller the ERDP fraction of any given feedstuff).
  • 90% of D U P is absorbed.
  • Higher A P L = Higher F C E.
  • Metabolisable protein is eventually converted into Net Protein (NP). The efficiency of conversion varies according to the final use.

Figure 3

The metabolic pathways of protein utilisation

Crude protein figure 3

The figures shown for each end use represent the relative efficiency of conversion of each metabolic pathway.

The Metabolisable Protein system is a big improvement on the old Digestible Crude Protein (DCP) system but, it has its limitations.

  • It is not very likely that the efficiency of uptake of Q D P will always be 80%.
  • Nutritional data for many feeds is still incomplete.
  • ADIN tests are very easily corrupted by nitrogen from the detergent, although laboratories have improved their techniques considerably.
  • FME availability is likely to vary with rumen outflow rates, so high AP levels are likely to suffer from lower than expected FME levels. I have certainly found this to be true in very high yielding herds and fast growing livestock. The assumption that FME evaluation is a fixed figure is probably wide of the mark in these situations. The FiM MPE figure seems to be much more accurate since it takes better account of relative speed of fermentation and rumen outflow rates.

These areas of concern mean that the practical use of the system is still one that is open to a deal of subjective interpretation (artistic license).

I personally favour the increase in use of sugar sources in many “complete” diets in order to get over the “normal” shortfall in FME. I also tend to lead feed energy and protein by up to 8% in high output situations.

It is difficult to imagine how we can progress much from this, or even the FiM systems of rationing when feedstuff variability and hence analytical confidence in feedstuff nutrient composition, means that any development in the accuracy of the rationing system will be diluted by the questionable nature of the data being used, so I think there is a real need to be subjective when evaluating feedstuffs. Realistic interpretation of silage analysis is especially important if we stand any chance of building a reliable ration from any of the current models in use.

Protein Requirement Tables

Note the tables 12,13, and 14; below show the ARFC 1992 standard calculated requirements for cows, (see Appendix 1 a to 1 f for sheep). The values shown are additive.

ie: – Take the values for maintenance, growth, and lactation and add the sum of these values to derive the total MP requirement.

Table 12
Protein requirements for maintenance (g/day)(AFRC, 1992)

Relationship between Weight (kg) and Protein Requirement (MP)
400 216 550 275 650 312
410 220 520 264 650 312
420 225 530 267 660 315
430 229 540 271 670 319
440 232 560 279 680 322
450 236 570 282 690 326
460 240 580 286 700 329
470 244 590 290 710 333
480 248 600 293 720 336
490 252 610 297 730 340
500 256 620 301 740 343
510 260 630 304 750 347
640 308

Note:-1 Values do not vary in theory, but will in practise be approximate due to the effects of “Q” values and APL.

Note:-2 Requirements for growing stock are noted in chapter 6

Table 13
Protein (MP) requirements for weight change (g/day) (AFRC, 1992)

Weight Change MP
-1.50 – 197
-1.25 – 164
-1.00 – 131
-0.75 – 98
-0.50 – 66
-0.25 – 33
0.00 0
0.25 61
0.50 122
0.75 183
1.00 245
1.25 306
1.50 367

Table 14 shows in bold print the metabolisable protein requirements for standard 3.3% milk protein production.

Table 14
Protein (MP) requirements for lactation (g/day) (AFRC, 1992)

Milk Protein (%/L)
Yield (L) 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8
2 80 83 85 88 91 94 97 100 103 105 108
4 160 165 171 177 182 188 194 199 205 211 217
6 239 248 256 265 274 282 291 299 308 316 325
8 319 331 342 353 365 376 387 399 410 422 433
10 399 413 427 442 456 470 484 499 527 527 541
12 479 496 513 530 547 564 581 598 615 633 650
14 558 578 598 618 638 658 678 698 718 738 758
16 638 661 684 707 729 752 775 798 821 843 866
18 718 744 769 795 821 846 872 897 923 949 974
20 798 826 855 883 912 940 969 997 1026 1054 1083
22 878 909 940 972 1003 1034 1066 1097 1128 1160 1191
24 957 992 1026 1060 1094 1128 1162 1197 1231 1265 1299
26 1037 1074 1111 1148 1185 1222 1259 1296 1333 1370 1407
28 1117 1157 1197 1237 1276 1316 1356 1396 1436 1476 1516
30 1197 1239 1282 1325 1368 1410 1453 1496 1539 1581 1624
32 1276 1322 1368 1413 1459 1504 1550 1596 1641 1687 1732
34 1356 1405 1453 1502 1550 1598 1647 1695 1744 1792 1841
36 1436 1487 1539 1590 1641 1692 1744 1795 1846 1898 1949
38 1516 1570 1624 1678 1732 1786 1841 1895 1949 2003 2057
40 1596 1653 1710 1766 1823 1880 1937 1994 2051 2108 2165
42 1675 1735 1795 1855 1915 1974 2034 2094 2154 2214 2274
44 1755 1818 1880 1943 2006 2068 2131 2194 2257 2319 2382
46 1835 1900 1966 2031 2097 2163 2228 2294 2359 2425 2490
48 1915 1983 2051 2120 2188 2257 2325 2393 2462 2530 2598
50 1994 2066 2137 2208 2279 2351 2422 2493 2564 2635 2707
52 2074 2148 2222 2296 2371 2445 2519 2593 2667 2741 2815
54 2154 2231 2308 2385 2462 2539 2616 2692 2769 2846 2923
56 2234 2314 2393 2473 2553 2633 2712 2792 2872 2952 3032
58 2314 2396 2479 2561 2644 2727 2809 2892 2975 3057 3140
60 2393 2479 2564 2650 2735 2821 2906 2992 3077 3163 3248

Balancing Protein Sources

Most feedstuffs contain some form of protein, the exception being the inorganic mineral sources.

As we have seen already, there are basically two types of protein, Effective Rumen Degradable Protein (ERDP){FiM MPN}, and Digestible rumen Undegradable Protein (DUDP){FiM MPB}(or bypass protein). Both types of protein are very important.


ERDP/MPN is commonly sourced from forages. Spring grass, for example can contain as much as 93% of its protein as ERDP/MPN! This percentage deteriorates as the season progresses, but even mid summer grass is typically 85% ERDP/MPN.

The evolution of the rumen was designed to produce microbial protein by allowing the rumen bugs to multiply. This they do by using the ERDP/MPN from feedstuffs along with the fermentable energy from the same feeds.

It is important to make sure that diets contain enough ERDP/MPN to keep the rumen bugs reproducing at the optimum rate.

For sheep fed on grass this is not usually a problem but sometimes dairy and beef animals fed on straw, wholecrop, or maize silage based diets, may be deprived of ERDP/MPN from the basic forages.

In these cases it is very important to introduce protein feeds rich in ERDP/MPN.

This is not always easy since many protein supplements also contain good levels of DUDP/MPB, and whilst the overall supply may be good it is vital that the rumen bugs get enough ERDP/MPN and that money is not wasted by feeding more DUDP/MPB than is required.


DUDP/MPB is usually only needed as a supplement in very high output systems. Ewes at lambing and just after, will benefit considerably from inclusions of up to 25% of their protein as DUDP/MPB.

Lamb and calf creeps should also include DUDP/MPB at around 20%.

High yielding dairy cows may need as much as 37% of their dietary protein as DUDP/MPB.

The DUDP/MPB fraction of the protein is coincidentally the portion that tends to contain the richest sources of essential, and in some cases, performance limiting amino acids. Diets may well be balance for ERDP/MPN and DUDP/MPB but sometimes they are limited in performance by lack of amino acids like lysine or methionine. Amino acid balancing, is a growing part of ruminant nutrition, especially in pedigree, high output diet situations.

Rationing Tips

  • Ewes only require DUDP/MPB supplementation if they are carrying twins or triplets near to lambing and just after. The addition of 50 grams of high quality fish meal (currently banned), Prairie meal or protected soya meal, is tremendously effective. Trials at the Rowett Research Institute in the mid 1980’s showed a marked improvement in colostrum quality and milk yield from ewes that were fed on fish meal.
  • The UK ban on feeding fish meal to ruminants is unfortunate since it is difficult to source equivalent products. Some fish meal replacers are getting quite close but few are as good as the real thing.
  • Dry cow rations benefit from the addition of around 500 to 600 grams of protected soya meal from 3 weeks before calving. This is believed to assist the animal to mobilise some of its back fat prior to calving and improve colostrum yield and quality. It has also been shown that animals come into milk production much faster and are generally easier to get back in calf. The effect is similar to the supplementation of fish meal in sheep diets.
  • Animals fed on low protein forages may be supplemented with Non Protein Nitrogen from feed grade Urea. This source of nitrogen can be converted by the rumen bugs into microbial protein, in the presence of water and a suitable energy source, like molasses. It is now possible to buy ready mixed molassed urea supplements. These supplements are a safe option because they contain a regulated slow release formula that prevents a rapid build up of ammonia in the rumen.
  • Feeding urea on its own has to be done very carefully indeed since it is very easy to induce serious health problems arising from excessive absorption of ammonia into the blood stream. Sheep have died from inclusions of as little as 8.5g per day and yet others can thrive on as much as 50 g per day. Unfortunately. it is not possible to prescribe accurate limits for any animal.
  • The little table below shows the desired ratios of ERDP/MPN to DUP/MPB in various ruminant diet situations expressed as percentages.
Ewe 6 weeks to lambing 90% 10%
Ewe 2 weeks to lambing 85% 15%
Pedigree lowland ewe 2 weeks to lambing (twins) 80% 20%
Ewe 2 weeks after lambing 80% 20%
Ewe at weaning 85% 15%
High yield dairy cow (40 litres) 65% 35%
Mid yield dairy cow (25 litres) 75% 25%
Late yield dairy cow (12 litres) 85% 15%
Dry cow 3 weeks to calving 80% 20%
Lamb or Calf 5 weeks of age 80% 20%
Heifer 15 months 85% 15%
Intensive bull beef 8 months 85% 15%

Like most raw materials, buying proteins should be very carefully undertaken. The following points should be considered:

  1. How much, and what quality of protein exists in the forages that need balancing? Have you had the forages analysed?
  2. Is the price of bought in protein supplements representative of good value for money? What is the cost per tonne of dry matter, of any potential supplement compared to other feeds? This helps to put the price of succulents like Brewers grains and Distillers draft into perspective.
  3. What is the protein quality like? DCP, ERDP/MPN, DUP/MPB, ……..
  4. Is there a particular benefit from any essential amino acids? Lysine, Methionine, Leucine….
  5. How will the bought in feed need to be stored and fed?
  6. Is the feedstuff traceable back to source? Modern feed systems should be able to trace all feedstuffs back to origin. This is a growing requirement of UK buyers and is true of all bought in feedstuffs not just proteins. The current UFAS approval scheme is growing in its popularity as a trade coordinated quality and traceability standard demanded by agricultural livestock product customers.