Protein evaluation methods and systems in ruminants- Overview

Balanced energy and protein feeding to animal is essential for optimum production from animals and low methane, nitrogen excretion into environment. Balancing diets for crude protein without taking in account protein quality or rumen degradability often led to overfeeding of nitrogen and less than optimal production. So various countries Viz. Europe, North America, and Australia developed their own protein feeding systems (ARC 1980, AFRC 1984, AFRC 1992, CSIRO 1990, INRA 1978, Madsen. J. 1985, NRC 1985, NRC 2001, Rohr, K. 1987, Russel1, J. B 1992, Sniffen. C. J. 1992,). Although the basic framework of all the proposed new systems is similar, the units and factors used to calculate the protein values differ. The new protein feeding systems bring together the results from work by nutritionists, biochemists, bacteriologists and physiologists. All these protein feeding system have their own benefits and limitations in predicting protein requirements or in describing dietaryprotein in terms of CP or digestible CP, rumen degradable (RUP, RDP) and metabolizable protein.

In ruminant the protein need should be consider separately for rumen microorganisms and the host animal i.e. the N requirements of the rumen microorganisms and the amount of amino acidsN of dietary, microbial, and endogenous origins that is available for host animal. 

DCP is truly a crude description of the protein value of feeds. DCP is simply the difference between the intake of crude protein (N x 6.25) and the CP excreted in the corresponding faeces. Although this concept has use in the feeding of non-ruminant animals it is inadequate and has low predictive value for ruminants. It does not distinguish NPN in feed from protein and other nitrogenous materials, nor between proteins having different chemical and physico-chemical characteristics. It takes no account of the fact that the crude protein in faeces is predominantly microbial including some arising from fermentation in the hind gut, not undigested dietary material, nor of the fact that the type and quantity of the protein available to the animal for absorption often differs substantially from feed CP.

The Agricultural Research Council in the first edition of 'The Nutrient Requirements of Farm Livestock: Ruminants' (ARC 1965) proposed a modified DCP system termed “available Protein”. The available Protein system does not take account of the close link between energy and protein and has been abandoned by the ARC in favour of a new system. In NRC (1971, 1978) dietary protein requirements was presented in the crude protein and metabolic requirements as digestible protein. 

On the basis of degradation pattern the crude protein entering in the can be split into rumen degradable nitrogen (RDN) and undegraded dietary nitrogen (UDN). It is a better system to calculate requirement levels, especially for high-yielding cows which have been shown to benefit from protein that escape microbial degradation in the rumen and is absorbed as amino-acids in the small intestine than only digestible crude protein. Inorganic nitrogen sources from plants as well as other non-protein nitrogen, such as urea, are completely degraded by microbes in the rumen. RDN is broken down by rumen microbes and used for their protein synthesis by the microbes. Later in the digestion process the microbes are themselves digested and the microbial protein becomes available to the animal. This microbial synthesis is only optimal when the animal receives sufficient energy supplements. Therefore, if sufficient RDP is not available, the rate of digestion of fibrous as well as concentrate rich diets will be reduced. This leads to a reduction in intake, lower energy supply and reduced milk production. On the other hand, some protein nitrogen can resist microbial breakdown in the rumen and can pass directly to the cow’s intestine. This feed protein fraction is called by-pass protein which is especially profitable for high-yielding cows. At a low level of productivity a cow can meet her protein requirements entirely from microbial protein and the diet only needs to contain degradable protein. This explains why such a cow can be fed with urea or chicken manure instead of high quality protein can meet the protein requirements. It is therefore important to have the optimum balance of UDP and RDP in the diet. 

After microbial and abomasal digestion of the feed protein, the total digestible true protein available to the animal for metabolism is called metabolizable protein (MP). The metabolizable protein (MP) feeding standard for ruminant was presented by Burroughs et al. (1974) for a limited number of feedstuffs. This standard is an amino acid (AA) system of evaluation involving animal requirements and their dietary fulfillment. Measurement units of MAAs are employed as specific quantities of absorbable AAs in the postruminal portion of the digestive tract. Feature of this standard is its ability to predict the quantity of urea or non protein nitrogen (NPN) that can be useful with given amounts of a feedstuff or combination of feedstuffs. It has two components:

It is synthesized by the rumen microbes from fermentable energy, amino acids and NPN sources. About 0.25 part of Microbial Crude Protein (MCP) is present as nucleic acids which cannot be used by ruminants and remaining 0.75 parts as Microbial True Protein (MTP) which is digestible.

            DMTP (g/d) = 0.75 x 0.85 x MCP

                                 = 0.6375 MCP (g/d)

The NRC (2001) also proposed 80% digestibility of the true microbial protein, which was considered to have 20% nucleic acid. The resulting value obtained is also 64, and comparable to AFRC (1993) value 63.75.

            It is the fraction of feed protein which is not degraded in the rumen (UDP) but digested and absorbed in lower intestines. The proportion of digestible fraction in UDP varies from zero to 0.9. The digestibility of UDP can be predicted from the acid detergent insoluble nitrogen (ADIN) content of the feed.

Thus, MP (g/d) = 0.6375 MCP + RUP

This system envisages a demand for amino acids by the animal's tissues that must be met by absorption of amino acids from the small intestine. These amino acids are provided from two main exogenous sources namely microbial protein and unfermented feed protein.

Some considerations in metabolizable system are as follow

The quantity of amino acids that has to be absorbed to meet the body's needs for maintenance, defined as endogenous urinary N losses and the N in hair, wool and from the skin, plus the needs for the required production (growth, milk and reproduction). The supply to the small intestine and to the body of microbial amino acids; calculated from a knowledge of the ME intake commensurate with the required level of production and the quantity of amino acids available from unfermented dietary protein flowing into the small intestine.

 

 

Digestible crude protein (DCP) does not describe about the amino acid nitrogen (AAN) supplied to the ruminant animal because DCP neglects the complex N conversions in the ruminant digestive tract. The German Working Group on Protein Evaluation developed system in which the animal's tissue requirements are contrasted with the amounts of protein being available in the small intestine (Ausschuß für Bedarfsnormen, 1986). After knowing the animal tissue requirements, the absorbability and the efficiency of utilization of amino acids the necessary amount of true protein at the proximal duodenum is determined. As the proportion of duodenal N present as AAN is almost constant the latter can easily be converted into crude protein (CP). CP requirements at the duodenum are then contrasted with the amounts of crude protein reaching the small intestine. Duodenal CP flow, which comprises microbial CP and rumen-undegraded feed protein (UDP) is estimated by means of a regression equation. If protein in duodenal digesta falls short of the requirements, UDP has to be increased by including feedstuffs with a higher portion of by-pass protein. Some of the points taken into consideration are as follow

INRA working group described the PDI system (1975 and 1977). It was published In early 1978 in the book "Alimentation des Ruminants" (Jarrige et al., 1978 ; Vérité et al., 1979). The PDI values were given in the feed tables and the recommended dietary allowances. DCP system was totally replaced by the PDI system in field level in Germany.

The PDI system was developed to solve practical problems concerned with protein feeding rather than describing research model. It covers the whole range of situations encountered on the field when considering feedstuffs, types of diets and different kinds and levels of animal production. So it compromises the accuracy and feasibility for practice. Thus, protein value had to be assessed on a relatively small number of feed characteristics, representing those main factors of variations on which one can act 1n the field. Further variations between feeds or technological and conservation processes had to be assessed by easily available parameters: low-cost, easiness of measurement 1n routine laboratory the system was intended to be a tool not only for rationing but also for trade. For this, Individual value (or set of values) was to be given to every feedstuff.

The PDI system estimates the quantity of amino nitrogen (N X 6.25), except that from endogenous secretions, which 1s truly absorbed in the small Intestine. A main feature of this system 1s that It assigns two PDI values to each feed, the actual value being determined by the kind of diet used (see later). It takes into account both the contributions of the dietary protein which has escaped fermentation in the rumen, and of the microbial protein which arises from fermentation.

For a diet the PDI content is the sum of two fractions i.e.

1 PDIA: The dietary protein undegraded in the rumen but truly digestible in the small Intestine (PDIA)

 2. PDIM: The microbial true protein which 1s truly digestible in the small Intestine (PDIM). 

In Italy, some modification in the INRA (PDI system) system was done and used for the protein feeding in ruminant. The following modifications have been introduced:

A Nordic working group (NKJ, 1985) has proposed a protein evaluation system for ruminant. In this protein values of the feeds are expressed in the units AAT and PBV (Madsen, 1985). AAT is the amount of amino acids truly absorbed in the small intestine and PBV is the protein balance in the rumen.

The Nordic system deviates from other systems in especially three respects: (1) The system has a value which express the protein supply of the microorganisms relative to their need. (2) The amino acid proportion in undegraded protein is 0.85 for concentrates and 0.65 for roughages where nearly all other systems use a factor of 1.0 (3) The microbial protein synthesis in the rumen is related to the amount of totally digested carbohydrates.

The proportion of amino acid nitrogen in undegraded feed protein from different roughages of 0.65 as used in the Nordic system is probably an oversimplification of the actual situation; the value will probably vary among different roughages according to their amino acid nitrogen content in the original feed (Hvelplund and Madsen,1985) . The value of 1.0 as adopted in most of the protein evaluation systems (ARC, 1984; NRC, 1985; Verité et al., 1987) is too high and leads to an overestimation of the contribution of amino acid nitrogen from undegraded feed protein.

The system directly predicts 1) the amino acid supply of the ruminant body and 2) the nitrogen supply of the rumen microorganisms. These values are given to the single feeds and can in the feed planning be added separately, as the other nutrients in the feeds. The system can be extended to include a prediction of the absorption of single amino acids, e.g. AAT (methionine) or AAT (lysine), if new information about the requirements of single amino acids makes it desirable. The AAT-PBV system has been proven to predict the production better than the DCP system, and it is intended to implement the system in practice within 1 to 5 years depending on the country in question. 

A direct measurement of the AAT and PBV value of all available feeds using fistulated animals is not realistic, and therefore formulas were developed by which the AAT and PBV were expressed by factors which are either constants, or variables which can be related to analysis on the feeds:

PBV g/kg DM = g crude protein / kg DM× degradability in the rumen – microbial protein produced / kg DM 

"Absorbable Protein in the Intestine" system of protein feeding is largely based on the French PDI-system was widely introduced in Switzerland in 1984. The main difference to the French System consists in determining a fixed allowance of nitrogen for the microorganisms, related to energy intake. This simplification is plausible and helped to introduce the system in practice without major difficulties.

The protein production potential of the feed is expressed by the API-value of a particular feed is calculated using the following formula:

API (g) = Absorbable protein in the intestine

DOM (kg) = Digestible organic matter

D = Degradability of CP in the rumen

CP (g) = Crude Protein

It was assume that the amino acids (AA) of microbial origin absorbable from the intestine are related to the digestible organic matter as follows

Currently we have many tools to use in feed evaluation, to help in a proper estimation of the feed value of a feedstuff. For many conventional feeds these methods can give rather exact estimates of protein value. Various countries Viz. Europe, North America, and Australia developed their own protein feeding systems (ARC 1980, AFRC 1984, AFRC 1992, CSIRO 1990, INRA 1978, Madsen. J. 1985, NRC 1985, NRC 2001, Rohr, K. 1987, Russel1, J. B 1992, Sniffen. C. J. 1992,). In the countries of origin the systems seems to be well accepted by feed manufacturers, advisors and farmers. All these protein feeding system have their own benefits and limitations in predicting protein requirements or in describing dietaryprotein in terms of CP or digestible CP, rumen degradable (RUP, RDP) and metabolizable protein. In the countries of origin the systems seems to be well accepted by feed manufacturers, advisors and farmers.