Prediction of supply of amino acids to meet requirements

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The amino acids supplied in the CNCPS model depend on accurate determination of dry matter intake (DMI), ingredient content of carbohydrate and protein fractions and their content of essential amino acids in the rumi-nally undegraded protein, microbial growth on the fibre and non-fibre carbohydrates consumed and the amino acid content of rumen microbes and their intestinal digestibilities, and the unique rates of digestion and passage of the individual feed carbohydrate and protein fractions that are being fed. First limiting in the CNCPS is accurate determination of DMI, and whenever it is possible the actual DMI should be used instead of the predicted one for the group of animals being evaluated. Then, the predicted DMI may be used as a benchmark for diagnostic purposes, or when actual DMI is not available, including grazing ruminants.

The interactions of DMI, digestion and passage have several implications. First, the growth rate of each microbial pool that digests respective available carbohydrate fractions, and thus the absorbable microbial amino acids produced, will depend on the special characteristics and intake of the feeds being fed, which in turn determines the demand for the nitrogen source required by each pool (Russell et al., 1992; Sniffen et al., 1992; Tedeschi et al, 2000c). Secondly, the percentage of cell wall carbohydrate and protein that escapes digestion will change, depending on level of intake, particle size, specific gravity, rumination and degree of lignification (Sniffen et al., 1992). Thirdly, the site of digestion and, depending on the rate of whole tract passage, the extent of digestion will be altered (Sniffen et al, 1992; Fox et al, 1992). Variable rates of digestion and passage have similar implications for protein fractions in feeds (Sniffen et al, 1992). Those readily available will be degraded in the rumen, whereas those more slowly degraded will be partially degraded in the rumen and partially degraded postruminally, the proportion depending on rates of digestion and passage of the protein fractions in the feeds (Sniffen et al, 1992).

Predicting the energy content of the ration is accomplished by estimating apparent TDN of each feed and for the total ration as described by Russell et al (1992), Sniffen et al (1992) and NRC (2000), and utilizing equations and conversion factors to estimate ME, NEm, NEg and NE, values, as described by Fox et al (1992), NRC (2000) and Fox et al (2000b). To calculate apparent TDN, apparent digestibilities for carbohydrates, proteins and fats are estimated. These apparent digestibilities are determined by simulating the degradation, passage and digestion of feed-stuffs in the rumen and small intestine. Also, microbial yields and faecal composition are estimated. Feed composition values needed include: neutral detergent fibre (NDF), lignin, crude protein (CP), fat, ash and neutral detergent insoluble protein (NDIP) as a percentage of the diet DM, and starch and sugar expressed as a percentage of non-fibre carbohydrates.

Rumen microorganisms are categorized into those that ferment fibre carbohydrates

(FC) and non-fibre carbohydrate (NFC), as described by Russell et ai (1992) and the NRC (2000). Generally, FC microorganisms ferment cellulose and hemicellulose and grow more slowly, and utilize ammonia as their primary nitrogen source for microbial protein synthesis. In contrast, the NFC microorganisms ferment starch, pectin and sugars, grow more rapidly and can utilize ammonia and amino acids as nitrogen sources. The FC and NFC microorganisms have different maintenance requirements (the CNCPS uses 0.05 and 0.15 g of carbohydrate per g of microorganism per hour, respectively). Although the CNCPS does not compute specific amino acid requirements for microbial growth, the efficiency of growth of NFC digesting bacteria is optimized at 14% peptides as a percentage of NFC (Russell et ai, 1992). Thus the degrad-able protein requirement is for supporting optimal utilization of NFC and FC to meet respective microbial growth requirements, and microbial growth is reduced when they are deficient (Tedeschi et ai, 2000c). The rate of microbial growth of each category is directly proportional to the rate of carbohydrate digestion, so long as a suitable nitrogen source is available. The extent of digestion in the rumen depends on digestion of FC and NFC feed fractions and how rapidly the feed passes out of the rumen. The extent of digestion thus depends on factors such as level of intake, particle size, rate of hydration, lignification, and characteristics of each carbohydrate and protein fraction.

The ME, MP and amino acids derived in each situation will primarily depend on the unique rates of digestion and passage of the individual feed carbohydrate and protein fractions that are being fed. Digestion rates are feed specific, and depend primarily on type of starch and protein, degree of lignification, and degree of processing. Extent of ruminai digestion is a function of competition between digestion and passage, and varies with feed type (forage vs. grain) and particle size. There are four nitrogen fraction requirements that must be met in evaluating a diet with the CNCPS; two microbial categories (ammonia for the FC and peptides and ammonia for the NFC microbial pools), and two animal pools (MP and essential amino acids). In evaluating a diet, one must be able to determine how well all four requirements are being met.

One of the critical factors affecting microbial growth is rumen pH. The CNCPS describes physical characteristics of feeds as related to their effectiveness in stimulating chewing, rumination and increased rumen motility based on their total cell wall content and particle size within classes of feeds (peNDF). The peNDF value in the CNCPS is defined as the percentage of the NDF retained on a 1.18 mm screen as described by Mertens (1997). Factors other than particle size that influence the peNDF value are degree of lignification of the NDF, degree of hydration, and bulk density. Pitt et ai (1996) described the relationship between CNCPS peNDF values, rumen pH and FC digestion. Total microbial yield and FC growth rate rapidly decline below a pH of 6.2, which relates to a diet peNDF content of 20%. The CNCPS reduces microbial yield 2.5 percentage units for each percentage drop in diet peNDF below 20%. Thus the diet peNDF must be accurately predicted to accurately predict microbial amino acid production and cell wall digestion.

Feed composition in the CNCPS is described by carbohydrate and protein fractions and their digestion rates, which are used to compute the amount of FC and NFC available for each of the two microbial pools (Sniffen et ai, 1992). Digestion and passage rates have been developed for common feeds, based on data in the literature, as described by Sniffen et ai (1992). All of the carbohydrate and protein fractions needed to predict the amounts of degradable carbohydrate and protein fractions available to support rumen fermentation can be determined in feed testing laboratories, using the Van Soest et ai (1991) system of feed analysis and proximate analysis. Included are NDF, CP, soluble protein, neutral and acid detergent insoluble protein, fat and ash. The CNCPS feed library contains over 150 feeds that are described by these analyses, as described by Sniffen et ai (1992). Included are digestion rates for sugars (CHO A), starch and pectin (CHO Bl), available NDF (CHO B2) and fast (ProtBl), intermediate (ProtB2) and slow (ProtB3) protein. Total carbohydrates are computed as 100 - (protein + fat + ash), using tabular or analytical values. Then carbo hydrates are partitioned into FC and NFC by subtracting NDF from total carbohydrates, with the available fibre being NDF - NDFProtein - (Iigninx2.4). Data from the literature are used to establish the distribution of sugars and starch in the NFC fraction.

The growth of two microbial pools (FC and NFC) is then predicted, based on the integration of rates of digestion and passage, which in turn determines the nitrogen requirements of each pool, microbial protein produced and MP available from this source, carbohydrates escaping digestion and digested postruminally and ME derived from the diet. Passage rates are a function of level of intake, percentage forage and peNDF value. Simultaneously, the degraded and undegraded protein pools are predicted, which are used to determine nitrogen balance for each of the microbial pools, feed protein escaping undegraded and digested postruminally, and MP and amino acids derived from undegraded feed protein. The protein fractions are expressed as a percentage of the CP. The 'A' protein fraction is NPN (non-protein nitrogen) and the 'Bl' fraction is true protein that is nearly all degraded in the rumen; these pools are measured as soluble protein. The 'C protein fraction is measured as acid detergent-insoluble protein (ADIP) and is assumed to be unavailable. The 'B3' or slowly degraded protein fraction can be determined by subtracting the value determined for ADIP from the value determined for NDIP. The 'B2' fraction, which is partly degraded in the rumen, depending on digestion and passage rates, can then be estimated as the difference between CP and the sum of soluble + B3 + C. Amino acids in feed-stuffs are described by their concentration in the undegraded protein, as described by O'Connor et al. (1993). Intestinal digestibility of the amino acids is assumed to be 100% in the Bl and B2 and 80% in the B3 protein escaping ruminal degradation.

Microbial composition of essential amino acids is used to calculate the supply of amino acids from bacteria, as shown in Equations [21.6] and [21.7],

REBAAj = (AABCWj x 0.01 x REBCWp

DIGBAA,.

where REBAA(. is the amount of the Ith bacterial amino acid appearing at the duodenum (g day-1); REBCWj. is the bacterial cell wall protein appearing at the duodenum as a result of fermentation of the f1 feedstuff (g day-1); AABCWj is the i* amino acid content of rumen bacteria cell wall protein (g 100g-1) (Table 21.3); AABNCW, is the Ith amino acid content of rumen bacteria non-cell wall protein (g 100g-1) (Table 21.3); REBTP^. is the bacterial non-cell wall protein appearing at the duodenum as a result of fermentation of the f1 feedstuff (g day-1); and DIGBAA; is the amount of the 1th absorbed bacterial amino acid (g day-1).

Essential amino acid composition of the undegradable protein of each feedstuff is used to calculate supply of amino acids from the feeds: AAINSPy x 0.01 x (REPB1,

REFAAs

j=iv where AAINSP^. is the f* amino acid content of the insoluble protein for the f1 feedstuff (g 100g-1); REFAAj is the amount of 1th dietary amino acid appearing at the duodenum (g day-1); REPB1. is the rumen escaped Bl protein from the f1 feedstuff (g day-1); REPB2j is the rumen escaped B2 protein from the jth feedstuff (g day-1); REPBSj. is the rumen escaped B3 protein from the f1 feedstuff (g day-1); and REPC^. is the rumen escaped C protein from the fi1 feedstuff (g day-1).

Total metabolizable amino acid supply from microbial and feed protein is computed as shown in Equations [21.9]-[21.11],

DIGFAA,

where REAA(. is the total amount of the Ith amino acid appearing at the duodenum (g day-1); DIGFAA. is the amount of the ith absorbed amino acid from dietary protein escaping rumen degradation (g day-1); AAAS(. is the total amount of the i* absorbed amino acid supplied by dietary and bacterial sources (g day-1).

Table 21.3. Amino acid composition of rumen microbial cell wall and non-cell wall protein3.

Ruminai bacteria13

Table 21.3. Amino acid composition of rumen microbial cell wall and non-cell wall protein3.

Ruminai bacteria13

Amino acids

Cell wall

Non-cell wall

Mean

sd

Methionine

2.40

2.68

2.60

0.7

Lysine

5.60

8.20

7.90

0.9

Histidine

1.74

2.69

2.00

0.4

Phenylalanine

4.20

5.16

5.10

0.3

Tryptophan

1.63°

1.63

-

-

Threonine

3.30

5.59

5.80

0.5

Leucine

5.90

7.51

8.10

0.8

Isoleucine

4.00

5.88

5.70

0.4

Valine

4.70

6.16

6.20

0.6

Arginine

3.82

6.96

5.10

0.7

aAmino acid composition as % of protein.

bAverage composition and so (standard deviation) of 441 bacterial samples from animals fed 61 dietary treatments in 35 experiments (Clark et al., 1992). Included for comparison to the cell wall and non-cell wall values used in this model.

°Data were not available, therefore, content of cell wall protein was assumed to be same as non-cell wall protein (O'Connor etal., 1993).

aAmino acid composition as % of protein.

bAverage composition and so (standard deviation) of 441 bacterial samples from animals fed 61 dietary treatments in 35 experiments (Clark et al., 1992). Included for comparison to the cell wall and non-cell wall values used in this model.

°Data were not available, therefore, content of cell wall protein was assumed to be same as non-cell wall protein (O'Connor etal., 1993).

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