CONCEPT OF THE USE OF NPN AND BY-PASS PROTEIN IN RUMINANT DIETS
Before going in detail lets have a look in the important terminology used in this post.
Definition –
By-pass proteins are defined here as those dietary proteins that pass, intact, from the rumen to the lower digestive tract.
- Digestible by-pass protein is that portion of the by-pass protein that is enzymatically hydrolysed in, and absorbed as amino acids from, the small intestine.
- Over-protected protein is that protein of the by-pass protein that is neither fermented in the rumen, nor digested in the small intestine.
- Metabolisable protein is the digestible by-pass protein plus the digestible protein in the microbes that enter the small intestine.
- Fermentable carbohydrates are those parts of the feed carbohydrate that are degraded by microbial action in the rumen to volatile fatty acid (VFA) plus that entering into the microbes that grow with the energy (ATP) released when VFA are produced.
Protein digestion in ruminants————-
In different production systems, ruminants consume many types of carbohydrates, proteins and other plant and animal constituents. All digestible carbohydrates are fermented to volatile fatty acids (VFA) plus methane and carbon dioxide by microbial action. Proteins are degraded by microbial enzymes in the rumen to give the same three end-products (i.e. VFA, CO2 and CH4) plus ammonia . In all cases a proportion of the substrate metabolised by microbes is used for synthesis of the microbes. The microbial fermentation of soluble protein in the rumen is an unavoidable consequence of the ruminant mode of digestion. In the absence of other forms of N, it ensures a supply of ammonia nitrogen for micro-organisms from which they synthesize the protein in their cells. Under many circumstances, it is a wasteful process because high quality proteins are broken down to ammonia, absorbed as such, converted to urea in the liver and this is excreted in the urine.
EFFICIENCY OF MICROBIAL GROWTH ON PROTEIN
Protein degradation to VFA leads to a relatively low availability of ATP (`energy’) to rumen microbes and therefore protein that is degraded in the rumen is inefficiently used for the growth of micro-organisms. In comparison with carbohydrate when protein is degraded in the rumen, only half the ATP (the energy currency of the microbes) is produced in fermentation of protein relative to the same amount of carbohydrate. The breakdown of carbohydrate in the presence of adequate ammonia and sulphur and other minerals supplied by, for instance urea/molasses blocks, results in more microbial protein being produced than from an equal amount of protein fermented in the rumen. It indicates that from a highly soluble protein such as leaf protein, less than 10% of the protein in the diet is available to the animal. Quite clearly therefore with readily soluble and fermentable protein; whilst little escapes the rumen if the protein is in high concentrations the protein to energy ratio in the nutrients arising from the rumen may be decreased.
Factors that influence the availability of by-pass protein————-
For a variety of reasons a proportion of the dietary protein passes from the rumen into the small intestine without alteration. On reaching the small intestine this by-pass protein is digested by enzyme hydrolysis and absorbed into the body as amino acid. The conditions under which some dietary protein may escape the rumen for digestion in the lower alimentary tract include:
- When a protein meal has been made highly insoluble by heat treatment.
- The protein meal contains tannins (2-4%) which bind to make an insoluble tannin – protein complex which is not degraded in the rumen but is degraded in the abomasum/small intestine. – Chemical treatment has been applied, e.g. formaldehyde treatment .
- When a relatively soluble protein meal is fed in very high quantities and is either in a finely ground form or is rapidly fragmented into small particles which move quickly through the rumen. For example, when clover or lucerne (that do not contain tannins) are fed at levels below 2.5% of liveweight (on a dry matter basis), it is probable that no dietary protein escapes to the lower tract. However, at levels above this, some protein escapes because of the rapid movement of digesta out of the rumen. The amount of by-pass protein can be as high as 30% of the total protein in the feed if this is highly digestible
- When heat is applied to a mixture of soluble protein and xylose, when a modified browning reaction can insolubilise the protein.
Microbial protein synthesis in the rumen—————–
Ammonia, peptides, amino acids and amines form the nitrogenous substrate for the synthesis of microbial cells but ammonia is the most important source of N for the microbes that ferment forages. Ammonia is used by many species of rumen micro-organisms as their sole source of nitrogen for protein synthesis .This assessment of the role of ammonia in the rumen can be misleading if it is unqualified. Firstly some species of bacteria and protozoa commonly found in the rumen cannot grow or survive unless small quantities of peptides, amino acids or branched chain fatty acids are provided in the diet and are present in low concentration in rumen fluid .A high level of rumen degradable protein in the diet may support high levels of all N-nutrients needed by bacteria and may cause specific populations of microbes to develop in the rumen as compared to diets where urea alone supplies the fermentable N. A deficiency of rumen ammonia results in a low microbial growth rate which may reduce digestibility of fibre and lower intake of feed. The requirements for ammonia for microbial activity Estimates of the critical level of ammonia in the rumen fluid for efficient digestion has been reported to be as low as 50 mg N/l or as high as 200 mg N/l. However, recent studies have shown that, when ammonia concentrations fall below about 200 mg N/l, the rumen, microorganisms are inefficient and are likely to respond to dietary NPN supplements particularly to UMB Intake of straw by cattle has been shown to be increased by increasing urea levels in the diet until the level of ammonia reaches 200 mg N/l .Recent studies with buffaloes fed forage based diets showed that, given a period of access to molasses/urea blocks, these animals learn to modify their intake according to the protein content of the basal diet .
Can the rumen microbes supply all the protein needs of the ruminant?—————
Even when ammonia and other nutrients are supplied, the quantities of microbes that leave the rumen in digesta do not supply sufficient protein to meet the needs for productivity in ruminants (i.e. moderate to high growth rates and milk yields). In such a situation, the deficiency symptoms indicate an insufficient supply of essential amino acids to the tissue. Under these conditions supplementation with a protein meal (which has a high content of by-pass protein) to supply additional dietary amino acids increases both the level and efficiency of animal production
Protein (or Amino Acid) Requirements of Ruminants—————-
In the past, the protein requirements of ruminants and evaluation of the protein value of feeds for ruminants have been based on digestible crude protein (N x 6.25). This is now recognised as a misleading concept. The use of digestible crude protein has arisen largely because it was considered that cattle and sheep could obtain their essential amino acids from microbes produced in the rumen. This in turn led to suggestions that extensive use could be made of non- protein nitrogen in high carbohydrate feeds and that a special role of ruminants could be to convert non-protein nitrogen to high quality animal protein. These have now been superseded by new concepts which take into consideration that when amino acid requirements are high, insufficient digestible microbial protein is available from the rumen to meet these needs. It is now necessary to assess the requirements for N by ruminants in terms of the amount of ammonia (or NPN) and amino acids needed by the rumen microbes, and the amount of digestible by-pass protein needed by the animal to augment the total protein (amino acids) available to the animal and to create an efficient metabolism. The sum of the two sources of digestible protein represents the metabolisable protein. Protein or amino acid requirements relative to energy requirements of ruminants are, however, influenced by a number of factors and cannot be stated with any degree of accuracy. The requirements are influenced by:
- physiological state of the animal,
- rate of growth and milk production,
- body composition as influenced by previous dietary and health history,
- basal feed (particularly fat content),
- proportions of the different amino acids absorbed,
- patterns of rumen fermentation (i.e. acetate:propionate ratio),
- availability of volatile fatty acids,
- requirements for glucose for essential purposes,
- environmental heat or cold stress, and
- the extent of the work load of the animal.
With all these unknowns, the need for by-pass protein under conditions pertaining to small-holder cattle can only be assessed in feeding trials aimed at developing response relationships. The effects of physiological state of the female goat on the utilisation of protein and, therefore its requirements, is well illustrated by the data shown in Figure 2.
Metabolisable Protein Available to Ruminants————————–
Metabolisable protein available is the sum of digestible dietary bypass protein plus digestible protein from microbes reaching the lower tract. On most straw based diets the metabolisable protein is mainly of microbial origin (i.e. there is no by-pass protein in the diet). The amount of protein available therefore depends on the efficiency of microbial growth in the rumen. This in turn depends on several factors:
- the presence of all the essential nutrients in the balances and amounts needed by the rumen microbes to grow e.g. ammonia, sulphur, phosphorus, trace minerals, amino acids, peptides, etc.,
- a source of fermentable dry matter, i.e. the feed consumed,
- to a small extent the rate of digesta turnover and therefore feed intake. However, this depends on degradability of the feed, type of carbohydrate and the physiological status of the animal.
- buffering capacity of the rumen and pH of the rumen fluid which largely depends on diet, and
- the balance of micro-organisms in the rumen. If supplementation with carbohydrate promotes protozoal population this can actually decrease the protein to energy ratio in the nutrients available from the rumen .As an example of how the balance of microbial protein to VFA energy can be altered in a cow given a straw based diet, the effects of an inefficient rumen (low rumen ammonia supply) and an efficient rumen (optimum rumen ammonia) are shown in Table 2. (see Leng, 1982 for the assumptions and calculations). The point is that the P:E ratio in the nutrients absorbed is altered according to how efficiently the rumen organisms are digesting the feed or how much by-pass protein there is in the diet.
Effect of increasing ammonia concentrations in the rumen of cattle on N deficient diets—
In most situations, adding urea to a low protein diet, such as that based on a cereal crop residue, increases intake of the basal diet in addition to improving microbial growth and digestibility (Table 3). Table 3. The effect of infusing urea into the rumen of a cow given straw based diets (Campling et al., 1962).
The potential effects of providing a UMB to ruminants on low protein forages (which is intended to provide urea and other nutrients) include the following:
- Increased digestibility of straw
- Increased feed intake
- Increased absorption of total nutrients
- Increased P:E ratio in the nutrients absorbed
The effects of supplementation of by-pass protein —————–
Supplementing a diet of crop residues fed to cattle with a by-pass protein improves the P:E ratio in the nutrients absorbed (see Table 1 and 2). This has a large influence not only on the level of production but on the efficiency of feed utilisation (i.e. the amount of feed required per unit of milk production or growth, is lowered). Stated in another way, animals produce less metabolic heat when P:E ratios are well balanced to requirements. This is well illustrated by research shown in Table 4 where straw intake has been maintained constant and efficiency of utilisation of the feed is improved by supplementation. In other studies the increased efficiency is not readily discernible as the effect of such supplements is to increase forage intake .
CONSTRAINTS TO APPLICATION OF THE BY-PASS PROTEIN TECHNOLOGY
Even though the application of UMB/by-pass technology is highly promising, a few constraints are still to be overcome before it can be widely applied with confidence. Some of these are given below and indicate areas for intensive research:
- The information regarding the degradabilities of protein in all raw materials used in cattle feed are not yet available and may be quite variable depending on source, manufacturing conditions and presence of other compounds.
- Easy laboratory tests for protein degradability are still not available and there is still some considerable disagreement as to which method provides the best indication of the content of by-pass protein in a protein meal.
- There are insufficient data from feeding trials available on milk production per unit input of by-pass protein under the systems commonly used by small farmers.
- There are no response relationships for milk production for economic analysis of the feeding of by-pass proteins which covers at least two lactations. This is important as by-pass protein supplementation on these diets often improves the body condition of cattle and therefore reproductive performance. The second lactation after introduction of these systems may show the greatest economic response.
- Many protein meals are undegradable in the rumen. However, their digestibility in the intestines may be very low. This applies particularly to protein meals with high tannin content. Such protein meals are not good sources of protein to the animal since much of the protein is lost in faeces.
- For the most efficient utilisation of by-pass protein for production, the essential amino acid to total N ratio must be high.
- The limits of responses to by-pass protein resides in the digestible energy content of the diet and at low digestibilities, high level feeding of a by-pass protein meal will result in amino acid degradation as an energy supply.
FEEDING OF BYPASS NUTRIENT TO DAIRY CATTLE FOR MORE MILK PRODUCTION
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Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
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