A Net Carbohydrate and Protein System for Evaluating Cattle Diets: 11. Carbohydrate and Protein Availability D. O'Connor*p2, P. J. Van D. G , Fox*, and J. B. Russell*#+

C. J. Sniffen*J, J.

Soest*,

*Department of Animal Science, Cornell University, Ithaca, NY 14853 and W.S. Dairy Forage Research Center, ARS, USDA, Madison, WI 53706 and U.S. Plant, Soil, and Nutrition Laboratory, Ithaca, NY 14853

ABSTRACT: The Cornell Net Carbohydrate and Protein System (CNCPS) has a submodel that predicts rates of feedstuff degradation in the rumen, the passage of undegraded feed to the lower gut, and the amount of ME and protein that is available to the animal. In the CNCPS, structural carbohydrate (SC) and nonstructural carbohydrate (NSC) are estimated from sequential NDF analyses of the feed. Data from the literature are used to predict fractional rates of SC and NSC degradation. Crude protein is partitioned into five fractions. Fraction A is NPN, which is trichloroacetic (TCA) acid-soluble N. Unavailable or protein bound to cell wall (Fraction C) is derived from acid detergent insoluble nitrogen (ADIP), and slowly degraded true protein (Fraction B3) is neutral

detergent insoluble nitrogen (NDIP) minus Fraction C. Rapidly degraded true protein (Fraction B11 is TCA-precipitable protein from the buffer-soluble protein minus NPN. True protein with an intermediate degradation rate (Fraction B2) is the remaining N. Protein degradation rates are estimated by a n in vitro procedure that uses Streptomyces griseus protease, and a curve-peeling technique is used to identify rates for each fraction. The amount of carbohydrate or N that is digested in the rumen is determined by the relative rates of degradation and passage. Ruminal passage rates are a function of DMI, particle size, bulk density, and the type of feed that is consumed (e.g., forage vs cereal grain).

Key Words: Cattle, Nutrition, Models, Feed Composition Tables, Metabolizable Energy, Metabolizable Rotein

J. Anim. Sci. 1992. 70:3562-3577

Introduction The Weende system for proximate analysis and the TDN system have been used for about a century as the basis for predicting the energy and protein available from feedstuffs (Morrison, 1956). Net energy systems were developed to adjust for methane, urinary, and heat increment losses (NRC, 1976, 19781. The NE systems have worked well under standard feeding conditions, but the

'Present address: W. H. Miner Agric. Res. Inst., Chezy, NY 12921.

'Present address: P. 0. Box 2077, Cowallis, OR 97339. Received May 24, 1991. Accepted July 13, 1992.

tabular value of NE for a particular feed is typically computed from TDN and represents the average expected value based on a group of feeds rather than the NE that will be derived by a particular group of cattle eating that feed. Because the feeding conditions of cattle are variable and often unique, accurate NE values usually are not available N a n Soest et al., 1984). Crude and digestible protein determinations do not completely account for the dynamics of ruminal fermentation and the potential loss of nitrogen as ammonia. The Cornell Net Carbohydrate and Protein System (CNCPS) has equations that estimate the fermentation and passage of feed carbohydrate and protein fractibns. This information can be used as a basis for predicting ME and protein absorption.

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PREDICTING FEED ENERGY AND PROTEIN VALUES

Fermentation, Passage, and Absorption Van Soest et al. (1984)summarized the effects of variations in intake. feed composition, ruminal digestion rates, and passage rates on the NE and protein values of feeds and developed discount factors to adjust for these effects. The Van Soest discount system works well under a variety of feeding conditions. Static adjustments, however, cannot fully accommodate the dynamics of ruminal fermentation (Russell et al., 1992). New protein systems (NRC, 1985,1989)have attempted to define the amount of protein escaping ruminal degradation, but these systems have 1) a single N pool; 2) used TDN or OM rather than ruminally degraded carbohydrates to control microbial protein synthesis; 3) not partitioned ruminal microorganisms according to carbohydrate and N utilization; 4)not accommodated the effect of growth rate, amino acid availability, or pH on the efficiency of microbial growth; and 5)not integrated the inverse relationship between rates of carbohydrate and protein fermentation. In a companion paper (Russell et al., 1992) we have demonstrated that the CNCPS equations could be used to predict the growth of ruminal bacteria accurately so long as 1) carbohydrates were partitioned into structural (SC) and nonstructural (NSC) components, 2) degradation rates of these components could be estimated, 3) N availability could be described in terms of ammonia and peptides, and 4) changes in ruminal pH could be accommodated by variations in diet composition. In this paper we provide 1)tables of feed composition that partition feedstuffs into carbohydrate and protein fractions, 2) methods for calculating the amount of N and carbohydrate that can be used to drive ruminal microbial growth, and 3) mechanistic equations for predicting metabolizable energy and protein values that are based on ruminal rates of digestion and passage, microbial growth kinetics, and postruminal digestibilities.

Feed Fractions The CNCPS assumes that feedstuffs are composed of protein, carbohydrate, fat, ash, and water. Protein and carbohydrate DM are further subdivided by chemical composition, physical characteristics, ruminal degradation, and postruminal digestibility characteristics (Table 1). The values presented in Table 1 are from NRC (1982, 1984,19891,Krishnamoorthy et al. (19821,Van Soest et al. (19841,Abdalla et al. (1988a,b),and estimates by the authors based on the above and on our unpublished data. Fractions of protein, carbohydrate, ash, fat, and water in a feedstuff can be Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

3563

derived from the following standard laboratory chemical analyses: 1. 2. 3. 4. 5.

6. 7. 8.

DM of the feed (AOAC, 1980); NDF and lignin (Van Soest et al., 1991); Total nitrogen as assayed by macro- or microKjeldahl (AOAC, 1980); Soluble protein as determined by the procedure of Krishnamoorthy et al. (1983); Nitrogen that is insoluble in neutral detergent (without sodium sulfite) and acid detergent (Van Soest et al., 1991); Ash (AOAC, 1980); Solvent-soluble fat (AOAC, 1980); and Computation of NSC from the determinations of NDF, protein, fat, and ash or directly (Van Soest et al., 1991): 100 - NNDF - NDF protein) + fat + ashl.

Feed Protein Fractions Each feed is further described by the proportion of NDF that is effective in meeting fiber requirements because of particle size, based on the values of Mertens (1985). Feed protein is partitioned into three fractions: nonprotein nitrogen (NPN), true protein, and unavailable nitrogen (Van Soest et al., 1981). These have been described as Fractions A (NPN),B (true protein), and C (bound true protein), respectively (Pichard and Van Soest, 1977). True protein is further fractionated into three subfractions (Bl, B2, and B3) based on their inherent rates of ruminal degradation (Van Soest et al., 1981; Krishnamoorthy et al., 1983). Roe et al. (1990) presented a summary of recommended procedures to determine protein fractions. Fractions A and B1 are soluble in buffer (Roe et al., 1990) and B1 is determined as the TCA-precipitable fraction (Van Soest et al., 1981; Krishnamoorthy et al., 1983). Nonprotein N (ammonia, peptides, amino acids) is rapidly converted to ammonia in the rumen. Essentially all the soluble protein in silages and cut forages is in the form of NPN (Pichard, 1977; Pichard and Van Soest, 1977). The NPN content of the soluble protein in common feedstuffs is presented in Table 1. Fraction B is subdivided to estimate rates of ruminal degradation. Fraction B1 is rapidly degraded in the rumen (Van Soest et al., 1981). In harvested forages Fraction Bl is a small fraction of the total soluble protein (approximately 5%) and concentrates can contain twice as much Fraction B1 as forages do (Pichard, 1977;Krishnamoorthy et al., 1982). Most of the soluble protein in fresh pastures is Fraction B1 (Van Soest, 1982). In the CNCPS Fraction B1 is degraded in the rumen.

SNIFFEN ET AL.

3564

Table 1. Carbohydrate and protein fractions, fat, and ash in common feedsa

Feedstuff Grains Barley heavy Beet pulp Canola meal Corn dry grain Corn HM grain Corn flaked grain Corn dry ear Corn HM Ear Corn hominy Milo ground Milo HM or flake Molasses beet Molasses cane Oats 489 g/L Oats 412 g/L Soybean hulls Wheat ground Wheat middlings Protein concentrates Alfalfa meal Bloodmeal Brewers grain dry Brewers grain wet Corn distill. dry Corn distill. wet Corn gluten feed Corn gluten meal Cottonseed whole Cottonseed meal Feathermeal Fishmeal Linseed meal Soybean meal 44 Soybean meal 49 Soybeans raw Soybeans heated Sunflower meal Urea Hay crop forages, northb Alfalfa hay prebl Hay earlybl Si1 earlybl Hay midbl Si1 midbl Hay mature Pasture spg Pasture sum Grass hay late veg Hay midbl Hay mature Pasture spg Pasture s u m Pasture fall Hay crop forages, southb Alfalfa hay prebl Hay earlybl Hay midbl Hay mature

NDF, Yo of DM 19.0 54.0 18.2

Lignin, NSP, of % of NDF DM O h

Starch, CP, of % of NSC DM

Soluble, ADFIP, NPN, Yo of Yo of o/o of CP CP SP

Yo of

NDFIP, Fat, Yo of CP DM

Oh

13.0 9.7 42.3 10.1 10.1 10.1 9.0 9.0 11.5 12.5 12.5 8.7 5.8 13.3 13.1 12.1 16.0 18.4

17.0 26.5 32.4 11.0 40.0 8.0 16.0 30.0 18.0 12.0 30.0 100.0 100.0 53.1 53.1 17.9 30.0 40.0

6.4 11.0 6.4 5.0 5.3 5.0 7.8 8.3 3.0 4.8 5.1 0 0 5 .O 5.0 14.0 2.0 2.6

29.4 97.4 85.0 70.0 100.0 80 70.0 100.0 80.0 30.0 100.0 100.0 100.0 18.5 18.5 60.8 25.0 30.0

8.0 52.6 10.6 15.0 15.9 15.0 17.8 18.7 7.8 10.0 10.1 0 0 11.0 11.0 20.0 4.0 4.0

2.6 4.4 7.8 1.6 1.6 1.6 1.9 1.9 3.1 2.1 2.2 11.3 13.1 3.4 4.6 5.1 1.9 5.2

28.0 4.9 4.1 8.0 22.0 25.0 49.0 4.2 40.0 20.0 3.8 12.0 20.0 20.0 20.0 44.2 5.7 30.0 100.0

17.1 1.2 12.0 10.0 20.0 12.0 2.1 2.0 6.0 7.6 2.4 .9 2.4 2.0 2.0 2.9 7.3 4.8

0

100.0 4.9 70.7 50.0 77.3 65.6 100.0 71.4 2.0 40.0 4.9 0 50.0 55.0 55.0 22.6 100.0 36.7 100.0

25.0 0 40.4 38.0 63.1 54.8 7.8 11.0

90 0 0 0 0 0 0 0

18.9 91.7 25.4 29.2 29.5 29.5 25.6 65.9 23.0 44.8 88.6 66.6 38.3 49.0 55.1 42.8 42.8 25.9 281.0

%

Ash, of DM

10.5 3.7 19.4 11.0 11.0 11.0 7.1 7.1 3.6 5.0 5.0 0 0 9.4 9.4 3.0 6.3 5.9

4 30

4 4 14 2 2

90 90 0 90 90

7

90 0 100 100 100 100 100

0

24.4 0 13.0 11.9 9.1 9.1 2.1 7.1 22.7 25.0 10.0 0 24.0 3.0 3.0 10.0 10.0 30.0 0

39.0 42.0 42.0 46.0 46.0 55.0 33.0 38.0

16.6 16.9 16.9 18.9 18.9 22.2 8.0 8.5

7 7 7 7 7 7

10 10 10 10 10 10 8 8

21.7 19.0 10.0 17.0 17.0 12.0 28.0 24.0

30.0 30.0 50.0 28.0 45.0 25.0 46.0 46.0

10.0 10.0 15.0 14.0 18.0 20.0 2.2 3.0

98.0 96.0 100.0 98.0 100.0 92.0 2.2 2.2

15.0 17.8 26.7 25.2 32.0 35.6 10.0 12.0

3.0 3.2 3.2 2.6 2.6 1.6 2.7 2.7

10.0 9.0

55.0 67.0 72.0 50.0 55.0 48.0

5.5 7.5 12.5 6.0 7.0 6.5

1 1 1 1 1

6 6

10

25.0 25.0 25.0 41.0 42.0 43.0

5.7 6.1 6.5 2.0 2.2 2.0

96.0 96.0

6 5 5 5

16.0 9.1 7.0 24.0 15.0 22.0

31.0 31.0 31.0 14.5 24.0 18.4

2.6 2.6 2.6 3.7 3.7 3.7

7.2 6.3 6.0 10.7 9.0 10.0

37.0 40.0 44.0 58.0

18.9 20.0 22.7 24.8

7 7 7 7

10 10 10 10

27.0 25.0 22.0 14.0

30.0 30.0 28.0 25.0

10.0 10.0 14.0 20.0

96.0 96.0 96.0

15.0 17.8 25.2 35.0

4.0 3.0 2.6 2.0

10.2 9.6 9.1 8.9

9.0 9.0 9.0 28.0 28.0 55.0 23.0 23.0 0 0 32.0 42.0 67.0 16.0 37.0 45.0 0 46.0 42.0 44.0 40.0 45.0 14.0 44.0 29.0 10.0 2 .o 25.0 14.0 8.0 13.4 13.4 40.0

2 2 2 2 2

90 90 90 90 90 90 90 90 90 90 100 0 0

100 90 90 90

25 29 29 19 19

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96.0 2.4 4.8 2.3

92.0

6.0 10.0 50.0 1.o 10.0 5.0 5.0 4.4 23.6 7.7 0

2.1

.e 1.2 4.3 4.3 4.3 3.7 3.7 7.7 3.1 3.3 .2 .1

5.4 4.9 2.1 2.0 4.9 3.0 2.4 6.5 6.5 10.3 9.9 2.4 2.4 20.0 1.3 3.2 5.1 1.5 1.5 1.0 18.8 18.8 1.2

0

10.6 10.2 4.8 4.8 5.1 4.8 7.5 1.8 4.8 6.3 3.8 25.4 8.5 7.3 6.5 5.8 5.8 6.3 0

9.0 9.0 9.0 8.0 10.0 9.8

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PREDICTING FEED ENERGY AND PROTEIN VALUES

Table 1 (continued). Carbohydrate and protein fractions, fat, and ash in common feedsa

Feedstuff

NDF, % of DM

Bermuda hay late veg 70.0 Fescue, K31, hay 63.0 Hay full bloom 67.0 Hay mature 70.0 Grain crop forages, northb Corn silage 45% gr. 41.0 Corn silage 35Y0 gr. 46.0 Corn silage 259'0 gr. 52.0 Grain crop forages, southb Corn silage 40% gr. 45.0 Corn silage 25% gr. 55.0

Lignin, NSP, of % of NDF DM %

8.e 6.3 7.5

Starch, CP, of % of NSC DM

%

Soluble, ADFIP, NPN, % of Yo of CP CP SP

96 of

6 6 6 6

10.0 16.4 12.1 9.2

25.9 25.9 25.9 25.9

7.3 8.7 9.6

100 100 100

9.0 8.6 8.3

45.0 50.0 55.0

7.9

9.0 10.9

100 100

9.2 8.1

45.0 50.0

7.9

10.0

1 1 1 1

8.9

8.9 8.9 8.9

8.0

8.5

8.0

NDFIP, Fat, of % of CP DM

%

Ash, Yo of DM 8.0

25.4 25.4 25.4 25.4

34.2 34.2 34.2 34.2

2.5 8.1 5.3 4.3

100.0 100.0 100.0

16.4 16.0 16.0

3.2 2.8 2.1

5.0 7.0

100.0 100.0

16.4 16.0

3.1 2.1

4.0 7.0

9.0 8.0

7.0

8.0

'NSP is nonstructural polysaccharides (pectin, galactins, fructans, betaglucans, etc.), NSC is nonstructural carbohydrates, ADFIP is acid detergent insoluble protein, SP is soluble protein, NDFIP is neutral detergent insoluble protein. See Van Soest and Fox (1992)for a more complete feed library containing these chemical composition values. bDifferences between north and south are primarily a function of degree days and growing season temperatures. The north values are typical of northwestern, corn belt, lake and northeastern states in the United States, and values for north grasses are representative of cool-season grasses.

Unavailable or bound protein, Fraction C, is the protein that is insoluble in the acid detergent (acid detergent insoluble protein, ADIP) fraction (Pichard and Van Soest, 1977).Fraction C contains protein associated with lignin, tannin-protein complexes, and Maillard products that are highly resistant to microbial and mammalian enzymes (Krishnamoorthy et al., 1982, 1983). Fraction C cannot be degraded by ruminal bacteria and does not provide amino acids postruminally (Krishnamoorthy et al., 1982). The Fraction C content of feedstuffs has been measured by several researchers (Goering and Adams, 1973; Pichard and Van Soest, 1977; Waldo and Goering, 1979; Krishnamoorthy et al., 1982; Muscato et al., 1983; Van Soest and Sniffen, 1984). At least five common feeds may contain important amounts of protein in the bound or indigestible form: 1) haycrop silages, 2) dehydrated alfalfa, 3) citrus pulp, 41 corn distillers grains, and 5 ) brewers dried grains (Waldo and Goering, 1979). Table 1 contains estimates of the Fraction C content of common feedstuffs. Fraction B3 is insoluble in neutral detergent but soluble in acid detergent (neutral detergent insoluble protein [NDIPI minus ADIP; Goering and Van Soest, 1970; Krishnamoorthy et al., 1982). Fraction B3 is slowly degraded in the rumen because it is associated with the cell wall (Pichard, 1977; Van Soest et al., 1981; Krishnamoorthy et al., 1983). The NDIP and ADIP contents of common feedstuffs have been measured (Krishnamoorthy et al., 1982; Muscato et al., 1983). Protein supplements contain a small amount of Fraction B3, but forages, fermented grains, and byproduct feeds contain significant amounts of Fraction B3 (KrishnamoorDownloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

thy et al., 1982). Prolamin proteins, such as zein protein in corn, are found in Fraction B3 (Van Soest et al., 1981). In the CNCPS, a high percentage of Fraction B3 escapes degradation in the rumen. Buffer insoluble protein minus the protein insoluble in neutral detergent is used to estimate Fraction B2. Some Fraction B2 is fermented in the rumen and some escapes to the lower gut. The fate of Fraction B2 depends on the relative rates of digestion and passage. Fraction B2 is typified by the glutelin protein found in small grains (Van Soest et al., 19811. The following equations can be used to calculate the five protein fractions contained in the jth feedstuff from the values given in Table 1:

where CPj(%DM)= percentage of crude protein of the jth feedstuff; NPNj(%CP) = percentage of crude protein of the jth feedstuff that is non-protein nitrogen x 0.25; SOLPj(%CP)= percentage of the crude protein of the jth feedstuff that is soluble protein; NDIPj(%DMl = percentage of the jth feedstuff that is neutral detergent insoluble protein; ADIPj(%DM)= percentage of the jth feedstuff that is acid detergent insoluble protein; PAj(%CP) = percentage of crude protein in the jth feedstuff that is non-protein nitrogen; PBIj(%CP) = percentage of crude protein in the jth feedstuff that is

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SNIFFEN ET AL.

rapidly degraded protein; PB2 (%CPl = percentage of crude protein in the jt feedstuff that is intermediately degraded protein; PB3j(%CPl = percentage of crude protein in the jth feedstuff that is slowly degraded protein; and PCj(%CP) = percentage of crude protein in the jth feedstuff that is bound protein.

h

Feed Carbohydrate Fractions Given the crude protein, fat, and ash content of a feedstuff, expressed as a percentage of DM, the total carbohydrate (CHO)content in the feedstuff can be estimated, calculated by difference (100 CP - fat - ash). Carbohydrates can then be classified according to degradation rate, as with protein (Fraction A is fast and is sugars; Fraction B1 is intermediate and is starch; Fraction B2 is slow and is available cell wall; and Fraction C is unavailable cell wall). These fractions are computed from feed content of NSC, SC, and indigestible fiber (C). The fraction CHO C is lignin x 2.4 (Smith et al., 1972; Mertens, 19731, the material remaining after 72 h of digestion in vitro (Mertens, 1973). The lignin content of forages ranges from 5 to 25% of the plant cell wall; legumes have a higher content than do grasses (Van Soest, 1982). The NDF component includes cellulose, hemicellulose, and lignin (Goering and Van Soest, 1970). Available SC (Fraction B2) can be determined by subtracting the fraction CHO C from ash-free NDF that has been corrected for associated protein. Fraction B2 is slowly fermented in the rumen by bacteria that require ammonia as their sole nitrogen source (Russell et al., 1992). Nonstructural carbohydrates contain sugars (Fraction A) and starch and pectin (Fraction Bl). The NSC fraction represents the carbohydrates that are soluble in neutral detergents and it can be estimated as 100 minus protein, NDF corrected for protein, lipids, and ash in a feedstuff. The NSC can also be measured directly (MacGregor et al., 1983). The calculated NSC is usually in close agreement with direct measurements, but feeds containing large amounts of pectin will have a somewhat lower true NSC. Given the proportion of starch and pectin in the NSC, the Fraction A (sugars and organic acids) can be determined by difference. The sugar content of most ruminant diets is normally quite low unless sugar byproduct feeds or fresh, lush grasses are fed (Van Soest, 1982; Sniffen et al., 1983). Starch is a major component of cereal grains and is a major component of forages. Sugars are fermented rapidly by ruminal microorganisms. Starches from ensiled and processed grains are rapidly digested in the rumen, but dried grains can contain a significant Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

amount of insoluble starch, which is digested slowly. Pectins are unimportant in grasses and cereals, but legume forages, seed products, citrus pulp, and beet pulp contain significant amounts of pectin (Van Soest, 1982). Pectins are rapidly fermented in the rumen (Van Soest, 1982). All NSC fractions are fermented by ruminal bacteria that can utilize either ammonia or peptide as a nitrogen source (Russell et al., 1992). Using the chemical analyses described above, equations used to calculate carbohydrate composition (Table 1) of the jth feedstuff are listed below: CHOj(%DM)= 100 - CPj(%DM)- FATj(%DMl - ASHj(YoDM1 . O 1 +LIGCCj(YoCH0) = 1OO*(NDFj(%DMl* NINj(%NDFl*2.4)/CHOj(O/oDM) CB2j(%CHOl = 1OO*((NDFj(%DMl NDIPj(%CP)* .O1 * CPj(%DMl - NDFj(%DM).O 1 * LIGNINj(% NDFI 2.4l/CHOj(%DM)) CNSCj(%CH-= 100 - B2j(%CHO)- Cj(%CHO) 0) CBj(%CHOl = STARCHj(%NSC)* (100 - B2j(%CHO)- Cj(%CH0))/1OO CAj(%CHO)= (100 - STARCHj(%NSC)l~(lOO - B2j(%CHO)- Cj(%CH0))/1OO where CPj(%DMl = percentage of crude protein of the jth feedstuff; CHOj(%DM) = percentage of carbohydrate of the jth feedstuff; FATj(%DM) = percentage of fat of the jth feedstuff; ASHj(%DM) = percentage of ash of the jth feedstuff; NDFj(%DMl = percentage of the jth feedstuff that is neutral detergent fiber; NDIPj(%DM)= percentage of neutral detergent insoluble protein of the jth feedstuff; LIGNIN~WONDF) = percentage of lignin of the jth feedstuffs NDF; STARCHj(YoNSC1 = percentage of starch in the nonstructural carbohydrate of the jth feedstuff; SUGARj(%NSC) = percentage of sugar in the nonstructural carbohydrate of the jth feedstuff; CA(%CHO)= percentage of carbohydrate of the feedstuff that is sugar; CBlj(%CHO)= percentage of carbohydrate of the jth feedstuff that is starch + NSP; CB2j(%CHO)= percentage of carbohydrate of the jth feedstuff that is available fiber; and CCj(%CHO) = percentage of carbohydrate in the jth feedstuff that is unavailable fiber. Carbohydrate and protein fractions in feeds are then summed to determine the intake of each fraction.

jh

Ruminal Kinetics Sophisticated, dynamic models of the rumen that incorporate many of the mechanistic principles of rumen function have been developed

PREDICTING FEED ENERGY AND PROTEIN VALUES

(Baldwin et al., 1977; Mertens and Ely, 1979; France et al., 1982; Gill et al., 1984) and some of the principles used in these earlier models have been incorporated into this system. Most models of ruminant digestion assume that nutrient ingestion occurs on a steady-state, continuous basis and that digestion rates follow fist-order kinetics (Waldo et al., 1972; Mertens, 1973; Mertens and Ely, 1979; Van Soest et al., 1981; Krishnamoorthy et al., 1983; Ewing and Johnson, 1987). Based on the model of Waldo et al. (19721, rumen digestibility (RD) is defined as the specific rate of rumen digestion (Kdl divided by the specific rate of disappearance due to digestion and passage (Kd + Kpl: RD = Kd/(Kd + Kp). Ruminal escape (RE) is defined as RE = Kp/(Kd + Kp).

Ruminal Degradation and Passage Rates Ruminal passage rate constants (Kp) are shown in Tables 2 and 3, based on the work of Hartnell and Satter, 1979; Colucci et al., 1982; and Erdman et al., 1987. Recent research (Welch, 1986) has shown that particle size, density, and hydration rate can affect the passage rates of feeds, and these effects were incorporated into the passage rate estimates, based on judgments made by the authors. Passage rates are also influenced by the level of intake, and adjustments for intakes above maintenance are presented in Tables 2 and 3. These rates assume additivity across feeds in a particular diet. The use of variable ruminal passage rates for each ingredient in the diet provides a method for estimating variations in their ruminal digestibility. As rates of passage increase, the extent of ruminal digestion and energy availability are reduced (Van Soest et al., 1984). The model computes Kp for each feed ingredient as described by Chalupa et al. (1991): Kp [forages] = .388 + (.002.DMI/BW.75) + (.00024forage O!O in DM21 Kp [concentratesl = -.424 + (1.45 Kp[foragel1 e

where DMI is in grams and BW.75 is in kilograms. The Kp is adjusted (AD for particle size using the effective NDF (ENDF) values in Tables 2 and 3:

+ 701 lOO/(ENDF + 90).

fif[forages] = lOO/[ENDF

Af [concentratesl

=

Protein Degradation and Passage Rates Potentially degradable true protein, the protein B fractions, range in degradability from .04 to 26Oo/o/h across most common feeds (Tables 4 to 61. Residual protein corrected for bacterial protein Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

3567

can be measured by enzymatic or in situ digestion (Nocek, 1985; Nocek and Russell, 1988). After the residual N is plotted as a natural logarithm vs time (Pichard and Van Soest, 19771, curve-peeling techniques can be used to estimate the digestion rate of each fraction. However, peptides arising from protein degradation are only utilized by ruminal microorganisms at a rate of .07 g of peptide per gram of microorganism per hour (Russell et al., 19921. When the degradation of the protein is rapid, peptides accumulate and small amounts of peptides as well as protein can escape. The following equations calculate the amounts of protein fractions that are ruminally degraded: RDPAj = DIET PAj RDPBlj = DIET PBIj.(Kdlj/(Kdlj + KPjN RDPB2j = DIET PB2j * (Kdzj/&dzj + Kpj1) RDPB3j = DIET PB3j.(Kdaj/(Kd3j + KpjN RDPEPj = RDPBlj + RDPB2j + RDPB3j where Kdlj = rate of ruminal digestion of the rapidly degraded protein fraction of the jthfeedstuff, h-l; Kd2j = rate of ruminal digestion of the intermediately degraded protein fraction of the jth feedstuff, h-l; Kd3j = rate of ruminal digestion of the slowly degraded protein fraction of the jth feedstuff, h-1; Kpj = rate of passage from the rumen of the jth feedstuff, h-l; RDPAj = amount of ruminally degraded NPN in the jth feedstuff, g/d; RDPBIj = amount of ruminally degraded B1 true protein, in the jth feedstuff, g/d; RDPB2j = amount of ruminally degraded B2 true protein, in the jth feedstuff, g/d; RDPB3i = amount of ruminally degraded B3 true protein in the jth feedstuff, g/d; and RDPEPj = amount of rumen degraded peptides from the jth feedstuff, g/d. The undegraded protein is passed to the small intestine. The following equations calculate the amount of each protein fraction that escapes ruminal degradation: REPBlj = DIET REPB2j = DIET REPB3j = DIET REPCj = DIET

PBIj*(Kpj/(KdIj+ KPj)) PB2j*(Kpj/(Kdzj+ KPj1) PB3j*(Kpj/(Kd3j+ Kpjl1 PCj

where REPBlj = anount of ruminally escaped B1 true protein, in the jth feedstuff, g/d; REPB2j = amount of ruminally escaped B2 true protein, in the jth feedstuff, g/d; REPB3j = amount of ruminally escaped B3 true protein in the jth feedstuff, g/d; and REPCj = amount of rumen escaped bound C protein from the jth feedstuff, g/d.

SNIFFEN ET AL.

3568

Carbohydrate Degradation and Passage Rates As with protein, the model assumes that all feed carbohydrate must disappear through passage or digestion, and that the kinetics are first-order. Because sugars, the Fraction CHO A, are fermented very rapidly in the rumen, 300%/h (Sniffen et al., 19831, virtually none of the sugar escapes ruminal degradation. More slowly fermented but potentially available carbohydrates, the Fractions CHO B1 and B2, are digested a t rates varying from 2 to 50%/h, and some of this material may escape ruminal degradation. Tables 4, 5, and 6 list logical ranges for rates of ruminal fermentation of the carbohydrate fractions for common grains, protein supplements, and forages. Carbohydrate rates, like protein rates, can be obtained using in situ procedures (Nocek, 1985; Nocek and Russell, 1988). It is not possible at this time to obtain reliable estimates of digestion rates for carbohydrates by enzymatic procedures. Digestion rates for the Fraction CHO B1 are typically 3 to 8%/h. Pectins are rapidly digested, but certain types of starch can be degraded slowly. Uncooked starches ranked in the order of decreasing rate of digestion are wheat, barley, oats, corn, sorghum, and legume (Van Soest, 1982). The Fraction CHO B2 of legumes is generally degraded at a faster rate than in grasses (Van Soest, 1982; Varga and Hoover, 1983). The Fraction CHO B2 of mature grains has a mean digestion rate of 5.l%/h (Smith et al., 19721, and feeds with 50 to 60% CP (corn gluten meal, soybean meal, and peanut meal) had rate constants of 4.8 to 5.4%/h (Varga and Hoover, 1983). Protein sources with 25 to 30% CP (distillers grains, corn gluten feed, and brewers grains) had more rapid rates, 6.5 to 7.2%/ h (Varga and Hoover, 19831. Corn and corn byproducts (hominy, corn gluten feed, and corn gluten meal) all had lower rates than wheat products [middlings and bran) or barley feeds (barley and brewers grains) (Varga and Hoover, 1983).

The following equations are used to calculate the amounts of each of the carbohydrate fractions of the jth feedstuff that are ruminally digested: RDCAj RDCBlj RDCBPj

= =

=

DIET CAj'(Kd4j/(Kd4j + KPjN DIET CBlj.(Kd5j/KdSj + Kpj)) DIET CBSj.(Kdsj/(Kdej+ Kpj))

where K&j = rate of ruminal sugar digestion of the jth feedstuff, h-l; Kdsj = rate of ruminal starch digestion of the jth feedstuff, h-l; K&j = rate of ruminal available fiber digestion of the jth feedstuff, h-l; RDCAj = amount of ruminally degraded sugar from the jth feedstuff, g/d; RDCBlj = amount of ruminally degraded starch from the jth Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

feedstuff, g/d; and RDCB2j = amount of ruminally degraded available fiber from the jth feedstuff, g/d. The following equations are used to calculate the amounts of each of the carbohydrate fractions of the jth feedstuff that escape the rumen: RECAj = DIET CAj.(Kpj/(Kd4j + KPjN RECBlj = DIET CBlj*(Kpj/(Kd5j+ KpjN RECB2j = DIET CBzj.(Kpj/(Kdej + Kpj)) RECCj = DIET CCj where RECAj = amount of ruminally escaped sugar from the jth feedstuff, g/d; RECBIj = amount of ruminally escaped starch from the jth feedstuff, g/d; RECB2j = amount of ruminally escaped available fiber from the jth feedstuff, g/d; and RECCj = amount of ruminally escaped unavailable fiber from the jth feedstuff, g/d.

Microbial Flow to the Small Intestine All microorganisms that pass from the rumen are assumed to be bacteria, and microbial turnover (starvation and predation) is accounted by variations in the yield coefficient (Russell et al., 1992). Based on the microbial composition given in a companion paper, bacterial fractions escaping the rumen are computed as follows: REBTPj REBCWj REBNAj REBCHOj REBFATj REBASHj

= .60..625. BACTj = .25 .625. BACTj = .15 * .625 *BACTj = .21.BACTj = .07*BACTj = .044*BACTj

where BACTj 7 amount of bacterial total protein passed to the intestines by the jth feedstuff, g/d; REBTPj = amount of bacterial true protein passed to the intestines by the jth feedstuff, g/d; REBCWi = amount of bacterial cell wall protein passed to the intestines by the jth feedstuff, g/d; REBNAj = amount of bacterial nucleic acids passed to the intestines by the jth feedstuff, g/d; REBCHOj = amount of bacterial carbohydrate passed to the intestines by the jth feedstuff, g/d; REBFATj = amount of bacterial fat passed to the intestines by the jth feedstuff, g/d; and REBASHj = amount of bacterial ash passed to the intestines by the jth feedstuff, g/d.

Intestinal Protein Absorption The absorption of bacterial N (true protein, nucleic acid, and cell wall protein) and escaped protein (B1, B2, B3, and Cl is calculated by

PREDICTING FEED

ENERGY AND PROTEIN VALUES

multiplying each fraction by its respective digestibility. The model uses intestinal true digestibilities of 100, 100, 100, and 8O%, respectively, for the peptide, B1, B2, and B3 protein fractions, based on data summarized by Van Soest (1982).The ADIP or C protein fraction is completely unavailable for digestion and does not contribute to absorbable amino acids (NRC, 1985). Because feed protein appearing in the feces (insoluble NDIP and ADIP protein that is either keratin, Maillard products, or bound to lignin) is resistant to peptic digestion, it seems that feces contain little true protein (Van Soest, 1982). The availability of microbial N has not been directly determined, but Storm et al. (1983) and Tas et al. (1981) estimated that bacterial true protein was 85 and 87% digestible, respectively. The

3569

CNCPS assumes that the total tract digestibility of bacterial true protein is 100Y0. The literature is inconsistent relative to the digestibility and pathways of nucleic acid utilization by cattle. The true digestibilities of ruminal bacterial RNA and DNA have been reported to be 89 and 8O%, respectively (Storm et al., 19831 and 87 and 819'0, respectively (Smith and McAllan, 1971). In the CNCPS,nucleic acid is entirely digested postruminally, but the absorbed nucleic acid is excreted in the urine (Smith, 1969). Bacterial cell wall protein is not released by proteolytic enzymes in the abomasum or small intestine and seems to have little nutritional value (Mason and White, 1971; Mason and Palmer, 1971).Therefore, bacterial cell wall protein is completely unavailable for digestion and does not contribute to the absorbed amino acid pool.

Table 2. Ruminal passage rate constants for concentrates (%/hJa Level of maintenanceb Ingredient Lightweight concentrates Dried brewers grains Wheat middlings Soybean mill feed Citrus pulp Beet pulp Wheat bran Whole cottonseed Whole soybeans Dehy alfalfa Corn cobs, ground Intermediate-weight concentrates Ground barley Ground wheat Ground oats Fish meal Hominy feed Distillers, w/sol Corn and cobmeal Blood meal Heavyweight concentrates Whole dry corn Corn meal Cracked corn High-moisture corn Whole Coarsely rolled Intermediately rolled Finely rolled Soybean meal Cottonseed meal Corn gluten meal Corn gluten feed Peanut meal Meat and bonemeal

Effective NDFC of NDF

lx

2x

3x

2.0 2.0 1.0 1 .o 1 .o 2.0 1.5 1.5 2.0 2.0

2.5 2.5 2.0 2.0 2.0 2.5 2.0 2.0 2.5 3.0

3.0 3.0 3.0 2.5 2.5 3.0 2.5 2.5 3.0 3.0

18 2 33 33 33 33 100 100 6 56

2.5 2.5 2.5 2.5 2.5 3.0 2.5 2.5

3.0 3.0 3.0 3.5 3.0 3.5 3.0 3.5

3.5 3.5 4.0 4.0 3.5 4.0 3.5 4.0

34 34 34 9 9 4 56 9

2.5

6.5

3.5

4.0 4.0 4.O

5.0

100 48 60

3.5 2.5 2.0 3.0

4.0 3.0 2.5 4.0

5.0 4.0 3.0 5.0

100 70 60 48

3.5 3.5 3.0 3.0 3.5 3.0

4.0 4.0 4.0 4.0 4.0 4.0

5.0 5.0

23 36 36 36 36 8

3.0

6.0

6.0 6.0

5.0 6.0

%

&Valuesare based on data of Hartnell and Satter (19791,Colucci et al. (19821,and Erdman et al. (1987). Assumed NDF intake is > 1 YO of body weight with 75% of NDF intake coming from forage NDF. If NDF intake, as a percentage of body weight, is e 1%, then increase passage rates by approximately 20%. bComputed as total ration DM consumed/ration DM needed for maintenance. CProportionof NDF that is effective in meeting fiber requirements. Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

3570

SNIFFEN ET AL.

Table 3. Ruminal passage rate constants for forages (%/hIa Effective NDFC

Level of maintenanceb Ingredient Legumes High-quality, 18 to 21% CP Long 20% > 2.54 cm length .635 cm length Average quality, c 18% CP Long 20% > 2.54 cm length 3 3 5 cm length Grasses Long 20% > 2.54 cm length .635 cm length Corn silage Mature, > 50% grain Normal chop Fine chop Intermediate, 30 to 50% grain Normal chop Fine chop Immature, c 30% grain Normal chop Fine chop

Yo of NDF

1x

2x

3x

2.5 3.0

4.0

3.0 3.5 5.0

4.0 4.5 6.0

92 82 67

2.0 2.5 3.0

2.5 3.0 3.5

3.0 3.5 4.0

92 82 67

2.0 2.0 3.0

2.5 3.0 3.5

3.0 4.0 4.5

98

2.0

4.0

2.5 5.0

3.0 6.0

61

1.5 3.0

2.0

4.0

2.5 5.0

81 71

1.o 2.0

1.5 3.0

2.0 4.0

81 71

88 73

71

&Valuesare based on data of Hartnell and Satter (19791, Colucci et al. (19821, and Erdman et al. NDF intake is > 1% of body weight with 75% of NDF intake coming from forage NDF. If NDF intake, as a percentage of body weight, is < 146, then increase passage rates by ap roximately 20%. %Computed as total ration DM consumedhation DM needed for maintenance. Choportion of N D F that is effective in meeting fiber requirements.

(91871. Assumed

Equations for calculating digested protein from feed and bacterial sources are listed below: DIGPBlj DIGPBBj DIGPB3j DIGFPj DIGBTPj DIGBNAj DIGPj

= REPBlj = REPB2j = .80 * REPB3j = DIGPBlj DIGPB2j = REBTPj =

=

+

+ DIGPBSj

REBNAj DIGFPj + DIGBTPj

+ DIGBNAj

where DIGPBlj

=

digestible B1 protein from the

jth feedstuff, g/d; DIGPB2i = digestible B2 protein

from the jth feedstuff, g/d; DIGPB3j = digestible B3 protein from the jth feedstuff, g/d; DIGFPj = digestible feed protein from the jth feedstuff, g/d; DIGBTPj = digestible bacterial true protein produced from the jth feedstuff, g/d; DIGBNAj = digestible bacterial nucleic acids produced from the jth feedstuff, g/d; and DIGPj = digestible protein from the jth feedstuff, g/d.

Intestinal Carbohydrate Absorption Postruminal starch digestibility seems to be in the range of 60 to 100% (Karr et al., 1966; Tucker et al., 1968; Waldo, 1973; Hoover, 1978; Russell et al., Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

1981; Siciliano-Jones and Murphy, 1989; Swingle et al., 1990; Zinn, 1990). Only small amounts of starch are recovered in the feces (Van Soest, 1982)if grain is adequately processed. Postruminal true digestibilities are given in Table 7. The small intestine lacks the enzymes to digest cellulose and hemicellulose (MacRae and Armstrong, 1969; Beaver et al., 1972; Thompson et al., 19721, but cellulose and hemicellulose can be fermented by bacteria in the large intestines. Lower tract digestion of cellulose and hemicellulose ranged from 18.5 to 49.5% and 2.5 to 46%, respectively (Hoover, 1978). Therefore, a n average postruminal true digestibility of 20% is used. The model assumes a postruminal true digestibility of 95% for bacterial carbohydrate (Van Soest, 1982). The equations for calculating digested carbohydrate due to the jth feedstuff are listed below:

VFAj DIGFG DIGBG DIGCj

= = = =

RDCAj + RDCBlj + RDCB2j RECAj + std&*RECBLj+ .20*RECB2j .95*REBCHOj VFAj + DIGFCj + DIGBCj

where stdig = postruminal starch digestibility, g / g; DIGFCj = intestinally digested feed carbohydrate from the jth feedstuff, g/d; VFAj = ruminally digested carbohydrate from the jth feedstuff, g/d;

3571

PREDICTING FEED ENERGY AND PROTEIN VALUES

DIGBCj = digested bacterial carbohydrate produced from the jth feedstuff, g/d; and DIGCj = digestible carbohydrate from the jn feedstuff, g/d.

Intestinal Fat Absorption Ruminal bacteria hydrogenate fat, but they do not ferment it. Therefore, the model assumes that all dietary fat passes to the small intestine. The following equation is used to calculate ruminally escaped fat from the jth feedstuff: REFATj

DIET FATj

where REFATj = amount of ruminally escaped fat from the jth feedstuff, g/d.

Ruminants readily hydrolyze and absorb triglycerides, and pos truminal true digestibility approaches 100% (Van Soest, 1982). The CNCPS assumes that bacterial fat has a postruminal true digestibility of 95% Wan Soest, 19821. Equations for calculating digestible fat from feed and bacterial sources are listed below: DIGFFj = .95*REFATj DIGBFj = .95 REBFATj DIGFj = DIGFFj + DIGBFj

where DIGFFj = digestible feed fat from the jth feedstuff, g/d; DIGBFj = digestible bacteria1 fat from the jth feedstuff, g/d; and DIGFj = digestible fat from the jth feedstuff, g/d.

Table 4. Digestion rate constants [%AI) for grainsa Carbohydrate Ingredient

Protein

A

B1

B2

B1

B2

B3

75-150 100-200 200-300

5-10 10-20 20-30

3-5 5-7

120-150 140-160 150-175

3-5 6-9

.06-.07 .08-. 10 .09-.12

150-200 200-300 300-400 300-400

10-15 15-20 20-30 30-40

5-7

140-160 200-250 200-250 200-250

9-10 10-11 11-12

.09-.12 .lo-.20 .15-.25 .20-.30

100-150 150-250 250-350 250-350

10-15 15-20 20-30 30-40

4-6 6-8 6-8

125-150 125-150 125-250 125-250

4-7 8-9 9-10 10-11

.08-.09 .OO-.15 .lo-.20 .15-.25

75-125 125-175 250-350 250-350

10-15 15-20 20-30 30-40

4-6 6-8 6-8

120-150 120-150 120-150 120-150

3-5 6-7 8-9 9-10

.07-.08 .09-.10 .lo-.15 .lo-.20

75-125 150-200 200-300 250-350 150-200

10-15 15-20 20-30 30-40 20-30

3-5

120-140 120-150 120-150 200-300 120-150

3-5 5-6 7-8 8-9 5-6

.06-.07 .07-.08 .08-.10 .09-.15 .07-.08

100-200 200-300

5-15 15-20

120-150 150-170

6-8

6-8

8-10

.09-. 15 .10-.20

250-350

30-40

4-6

300-350

12-15

.20-.50

250-350 250-350

20-30 30-40

4-6 4-6

250-350 250-350

12-15 13-16

.20-.50 .25-.55

250-350 250-350

35-45 40-50

8-10 10-14

250-350 300-400

12-15 14-16

.20-.50 .25-.55

Corn

Dry, whole shell corn Whole Cracked corn Cornmeal High-moisture corn > 35% moisture Whole Coarsely rolled Intermediate rolled Finely rolled 30 to 35% Moisture Whole Coarsely rolled Intermediate rolled Finely rolled 25 to 30% Moisture Whole Coarsely rolled Intermediate rolled Finely rolled .e 25% Moisture Whole Coarsely rolled Intermediate rolled Finely rolled Steam-flaked corn Sorghum Dry,rolled Steam-flaked Oats Ground Barley Ground, dry Ground, wet Wheat Dry,rolled Steam-flaked

7-0

6-8

6-8

8-10

8-10

8-10 6-8

6-8 6-8 6-8

4-5

4-6

4-6

'Based on data of Waldo et al. (10721, Mertens (19731, Mertens and Ely (19791, Ewing and Johnson (19891, Van Soest et al. (10811, Krishnamoorthy et al. (19831, Hoover (19831, and Smith et al. (1972). Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

SNIFFEN ET AI,.

3572

Table 5 . Digestion rate constants (%h) for proteinaceous feedsa Carbohydrate Ingredient

A

B1

Protein B2

Soybean 250-350 25-35 2 4 Whole, raw Whole, heated 250-350 3545 4-6 Meal, solvent 250-350 40-50 4-8 Meal, expeller 250-350 35-45 4-8 Canola, solvent 250-350 4050 4-8 Peanut, solvent 4-8 250-350 40-50 Cottonseed Solvent 4-8 250-350 30-40 Expeller 4-8 250-350 25-35 Whole, linted 250-350 20-30 3-5 Whole, delinted 250-350 20-30 1-2 Corn gluten meal 250-350 40-60 4-6 Corn gluten feed 250-350 40-60 6-8 Corn distillers w/sol 250-350 15-20 6-8 Wheat middlings 250-350 60-85 10-15 Animal meals Fishmeal 0 0 0 Meat and bonemeal 0 0 0 Bloodmeal 0 0 0 Feathermeal 0 0 0 Brewers grain 4-8 3540 250-350 Alfalfa meal, dehy 25-50 3540 8-10 250-350 0 0 Whey &Based on Waldo et al. (19721, Mertens (19731, Mertens and Ely (1979), Ewing and Krishnamoorthy et al. (19831, Hoover (19831, and Smith et al. (19721.

B1

B2

B3

150-250 100-200 200-260 150-250

6-10 5-6 9-12 6-8

.10-.30 .15-.20 .lo-.30 .15-.20

200-260 200-260

11-13 12-14

.10-.30 .lo-.30

120-200 100-150 150-200 100-200

6-10 6-8 10-12 6-10

.lo-.20 .lo-.15 .20-.30 .20-.30

100-200 100-200 100-200 200-300

2 4 2 4 3-4 5-6

.05-. 10 .05-. 10 .05-.15 .08-.15

100-200 100-200 50-100 100-1 50

5-6 5-6 2 4 34

.08-. 15 .08-.15 .05-.08 .05-.10

100-200 100-200 300400

8-8 7-9 0

.lo-.20 .lo-.20 0

Johnson (19891, Van Soest et al. (19811,

Table 6. Digestion rate constants (Wh) for foragesa Carbohydrate Ingredient Corn silage > 40% DM Coarsely chopped Finely chopped 3 0 4 0 % DM Coarsely chopped Finely chopped < 30% DM Coarsely chopped Finely chopped Legumes Hay Silage Coarsely chopped Finely chopped Grasses Hay Silage Coarsely chopped Finely chopped

Protein

A

B1

B2

B1

B2

B3

200-300 250-350

10-20 20-30

3-6 4-8

150-250 250-350

8-9

10-12

.08-.10 .lo-.20

200-300 250-350

15-25 25-30

4-8 8-10

200-300 250-350

9-10 10-1 1

.lo-.20 .15-.25

200-300 250-350

25-35 3540

4-8 8-10

250-350 250-350

10-1 1 10-12

.15-.25 .20-.30

200-300

25-35

3-0

100-200

8-10

1.0-1.5

200-300 250-350

3040 3545

4-7 5-9

100-200 100-200

10-12 12-14

1.5-2.0 1.5-2.0

200-300

25-35

2-4

120-150

10-12

200-300 200-300

3540 4045

3-5

200-250 250-300

12-14 13-15

4-6

.08-.10 1.0-1.2 1.1-1.3

'Based on Waldo et al. (19721, Mertens (19731, Mertens and Ely (19791, Ewing and Johnson (19891, Van Soest et al. (19811, Krishnamoorthy et al. (19831, Hoover (19831, and Smith et al. (19721. Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

3573

PREDICTING FEED ENERGY AND PROTEIN VALUES

Fecal Losses Direct chemical analysis shows the absence of plant cell contents in ruminant feces; the only exceptions are starch and heat-damaged proteins, both of which can be measured (Van Soest, 1982). There is little evidence of potentially digestible feed protein in feces; feed protein appearing in the feces is insoluble and is either NDIP or ADIP protein (Van Soest, 1982). The following equations calculate undigested feed residues appearing in the feces from NDIP, ADIP, starch, fiber, fat, and ash fractions, based on data summarized by Van Soest (19821: FEPBSj = (1 - .80l*REPB3j FEPCj = REPCj FEFPj = FEPB3j + FEPCj FECBlj = (1 - stdig1.RECBlj FECBZj = (1 - .201*RECBOj FECCj = RECCj FEFCj = FECBlj + FECB2j + FECCj FEFAj = DIET ASHj where FEPBBj = amount of feed B3 protein fraction in feces from the jth feedstuff, g/d; FEPCj = amount of feed C protein fraction in feces from the jth feedstuff, g/d; FEFPj = amount of feed protein in feces from the jth feedstuff, g/d; FECBlj = amount of feed starch in feces from the jth feedstuff, g/d; FECB2j 7 amount of feed available fiber in feces from the jth feedstuff, g/d; FECCj = amount of feed unavailable fiber in feces from the jth feedstuff, g/d; FEFCj = amount of feed carbohydrate in feces from the jth feedstuff, g/d; and FEFAj = amount of feed ash in feces from the jth feedstuff, g/d. A variety of metabolic materials are excreted in the feces, including microbial matter and endogenous substances. Microbial matter in ruminant feces is largely indigestible cell walls of ruminal bacteria and bacterial cells produced in the lower tract (Van Soest, 1982). Microbial matter appearing in the feces is composed of indigestible bacterial cell walls, bacterial carbohydrate, fat, and ash (Van Soest, 19821: FEBCWj FEBCPj FEBCj FEBFj FEBASHj FEBACTj

= =

= =

REBCWj FEBCWj (1 - .951*REBCHOj (1 - .95).REBFATj REBASHj FEBCPj + FEBCj + FEBFj + FEBASHj

where FEBCWj = amount of fecal bacterial cell wall protein from the jth feedstuf'f, g/d; FEBCPj = amount of fecal bacterial protein from the jth feedstuff, g/d; FEBq = amount of bacterial carbohydrate in feces from the jth feedstuff, g/d; Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

Table 7. Postruminal starch digestibilities

(%)a

Entering intestines

%

Feed corn Whole corn Dry, rolled Cracked Cum meal High moisture, whole High moisture, ground Steam-flaked Sorghum Dry, rolled Dry, ground Steam-flaked

50-80 65-75 70-80 80-90 80-90 85-95 92-97

60-70 70-80 90-95

*Based on Swingle et al. (1990) and Zinn (1990), using dairy and beef cattle.

FEBFj = amount of bacterial fat in feces from the jth feedstuff, g/d; FEBASHj = amount of bacterial ash in feces from the jth feedstuff, g/d; and FEBACTj = amount of bacteria in feces from the jth feedstuff, g/d. Endogenous substances consist of calcium and magnesium salts of fatty acids, bile salts, sloughed animal cells, mucus, and keratinized tissue Wan Soest, 1982). According to Lucas et al. (19611, endogenous protein, carbohydrate, and ash are: FEENGPj FEENGFj FEENGAj

= = =

.0387.DIET PROT; .017*DIETFAT; .OIIQ*DIETASH

where FDj = feed DM consumed; FEENGPj = amount of endogenous protein in feces from the jth feedstuff, g/d; FEENGFj = amount of endogenous fat in feces from the jth feedstuff, g/d; and FEENGAj = amount of endogenous ash in feces from the jth feedstuff, g/d. Total fecal DM is calculated by summing p r o tein, carbohydrate, fat, and ash DM contributions from undigested feed residues, microbial matter, and endogenous matter: FEPROTj FECHOj FEFATj FEASHj IDMj

= = = =

FEFPj + FEBCPj + FEENGPj FEFq + FEBCj FEBFj + FEENGFj FEFAj + FEBASHj + FEENGAj FEPROTj + FECHOj + FEFATj FEASHj

+

where FEPROTj = amount of fecal protein from the jth feedstuff, g/d; FECHOj = amount of carbohydrate in feces from the jth feedstuff, g/d; FEFATi = amount of fat in feces from the jth feedstuff, g/d; FEASHj = amount of ash in feces from the jth feedstuff, g/d; and IDMj = amount of indigestible DM in feces from the jth feedstuff, g/d.

SNIFFEN ET AL..

3574

Total Digestible Nutrients True TDN can be calculated by subtracting fecal losses from the potentially digestible nutrient intake. Apparent TDN is potentially digestible nutrient intake minus indigestible bacterial and feed components appearing in the feces: TDNAPPj

=

=

1.65) =

(1.37.MECj - .138*MECj]+ .0105*MEC?-

apparent TDN from the jth

NElaj NEga

= =

MECje.65 (1.42.MEC - .174.MEC2 + .0122.MEC3-

=

1.65)

= .001 .TDNAPPj*4.409*.82

ME,i/(FDi

1.12)

NEma = (1.37-MEC- .138*MEC2+ .0105.MEC3-

The CNCPS computes ME and NE values from TDN rather than from the absorbed end products of total tract digestion. It is aggregated at the level that is the first critical step beyond present NRC systems in improving accuracy of feed energy and microbial yield values used to evaluate and formulate diets. That step is to compute carbohydrate fraction pool sizes and microbial yield sensitive to intake level, ruminal fermentation and passage rates, ruminal microbial growth, bacterial composition, postruminal digestibilities, and endogenous nutrient contributions. The determination of NE values directly from individual absorbed VFA, starch, and fat requires a much more complex submodel of the rumen. Based on our preliminary unpublished observations, the more complex submodels of the rumen we have evaluated did not seem to improve the prediction of feed energy and microbial yield compared to the CNCPS. However, we are currently evaluating the addition of determinations for pH and pool sizes of end products of ruminal fermentation to the present CNCPS structure. The ME values for each feed are based on the assumption that 1 kg of TDN is equal to 4.409 Mcal of DE and 1 Mcal of DE is equal to .82 Mcal of ME (NRC, 1976):

MEC

1.42.MECj - .174.MECf + .0122.MECB -

=

NEmaj

Metabolizable and Net Energy Values of Feeds

=

NEgaj

(DIET PROTj - FEPROTj) + (DIET CHOj - FECHOj)+ 2.25 * DIET FATj FEFATj)

where TDNAPPj feedstuff, g/d.

MEaj MECi

NE, and NE, values are computed from ME, based on NRC (1984) equations to adjust for differences between forages and concentrates in efficiency of use of ME. The term NE1 is calculated based on NRC (1989).

*

.001)

i-1

MEIADMI * .OO1)

where MEaj = metabolizable energy available from the jth feedstuff, Mcal/d; MECj = metabolizable energy concentration of the jth feedstuff, Mcal/ kg; ME1 = metabolizable energy supplied by the diet, Mcal/d; and MEC = metabolizable energy concentration of the diet, Mcal/kg. Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

1.121

NEla

=

MECje.65

where NEgaj = net energy for gain content of the jth feedstuff, Mcal/kg; NEmd = net energy for maintenance content of the ith feedstuff, Mcal/kg; NE1,j = net energy for lactation content of the jth feedstuff, Mcal/kg; NEga = net energy for gain content of the diet, Mcal/kg; NEma = net energy for maintenance content of the diet, Mcallkg; and NE1a = net energy for lactation content of the diet, Mcal/kg.

Metabolizable Protein Metabolizable protein (MP) is digested feed and bacterial protein minus bacterial nucleic acids. The model generates a variable MP estimate for each ingredient based on protein composition, ruminal protein digestion rates, passage rates, bacterial yield, bacterial composition, and postruminal digestibilities of feed and bacterial protein fractions. Total feed MP is the s u m of each feed MP: MPai

=

DIGPi - DIGBNAi i-1

where MPaj = metabolizable protein from the jth feedstuff, g/d, and MP, = metabolizable protein available in the diet, g/d.

Differences Between the Cornell Net Carbohydrate and Protein System and the National Research Council System The CNCPS differs from the NRC (1985, 1989) in many respects. In the NRC system, protein is divided into two fractions, degraded intake protein (DIP) and undegraded intake protein (UIP),

PREDICTING FEED ENERGY AND PROTEIN VALUES

whereas the CNCPS divides protein into five fractions L4, B1, B2, B3, and C). Average, static tabular values for DIP and UIP are used by the NRC (1985, 19891, whereas the CNCPS generates MP values that are based on the various protein fractions, ruminal digestion, and passage rates. The NRC uses a constant, intestinal true digestibility of 80% for UIP, whereas the CNCPS applies true digestibilities of 100, 100, 80, and 0% for B1, B2, B3, and C protein fractions. In the NRC, bound protein contributes to absorbed protein, but the CNCPS assumes that bound protein does not contribute to absorbed protein. The use of a single DIP value in the NRC does not accommodate differences in ruminal bacterial N utilization (ammonia vs amino N). The CNCPS accounts ammonia, peptides, and protein as separate pools and the ruminal bacteria are partitioned according to N utilization. The NRC protein systems (1985, 1989) use empirical regression equations that relate NE values to static TDN and microbial crude protein production to daily TDN intake. The amount of TDN is a poor indicator of ruminal fermentation when rations are high in fat or oil (Satter and Roffler, 1975). Ruminal bacteria only grow on ruminally degraded carbohydrate and do not use fat as an energy source (Nocek and Russell, 19881; therefore, NRC systems may overestimate microbial growth. Another disadvantage of empirically based TDN is the large negative intercept that may underestimate microbial protein contributions at low TDN intakes. The CNCPS uses mechanistic equations that predict a variable TDN and microbial protein yield from fermentable SC and NSC carbohydrate intake, rates of fermentation, the availability of amino N, and pH.

Associative Effects of Feeds Validations with our unpublished experimental and case study data indicate that the digestion and passage rates presented in Tables 2 through 6 are appropriate for use with typical mixed diets. However, we have found that users may need to adjust these rates, as well as the microbial growth rates, based on their knowledge of factors that may result in unusually low or high digestion and passage rates. The ruminal digestion rates assume a normal pH, and the submodel of the rumen adjusts microbial protein yield in high grain rations in which pH can be expected to be 6.0 to 6.2. However, it does not automatically adjust microbial growth on the SC fraction, which is very pH-sensitive. In an evaluation of data from experiments with 0 to 10% forage, we concluded that the use of a growth rate of 0 for the SC bacteria was Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

3575

appropriate for this type of diet. The associate effects of N sources are accounted for primarily by determining the ammonia and peptide requirements of the SC and NSC pools. By adding a calculation to estimate the branched-chain VFA requirement for SC bacteria as described in the submodel of the rumen (Russell et al., 19921, the positive effects of using true protein to provide ammonia for SC bacteria in high-forage diets can be accounted for.

Implications The Cornel1 Net Carbohydrate and Protein System uses standard chemical analyses to estimate the pool sizes of biologically significant carbohydrate and protein fractions and microbial growth on these fractions. Because ruminal and lower gut digestion is a function of fermentation and passage rates, total digestible nutrients and metabolizable protein can be predicted in a more mechanistic fashion for field application in specific feeding situations. This system provides the beginning structure necessary to predict more accurately ruminally degraded carbohydrate and nitrogen fractions and absorbed amino acids from microbial and feed protein. Further research should result in field-usable models of the rumen with lower levels of aggregation.

Literature Cited Abdalla, H. O., D. C. Fox, and R. R. Seaney. 1988a. Protein distribution in four cool-season grass varieties alone or in combination with trefoil. J. Anim. Sci. 66:2325. Abdalla, H. O., D. C. Fox,and R. R. Seaney. 1988b. Variation in protein and fiber fractions in pasture during the grazing season. J. Anim.Sci. 68:2883. AOAC. 1980. Official Methods of Analysis (13th Ed.). Association of Official Analytical Chemists, Washington, DC. Baldwin, R. L., L. J. Koong, and M. J. Ulyatt. 1977. A dynamic model of ruminant digestion for evaluation of factors affecting nutritive value. Agric. Systems 2:255. Beaver, D. E.,J. F. Coelho da Silva, J.D.H. Prescott, and D. G. Armstrong. 1972. The effect in sheep of physical form and stage of growth on the sites of digestion of a dried grass. 1. Sites of digestion of organic matter, energy and carbohydrate. Br. J. Nutr. 28:347. Chalupa, W., C. J. Sniffen, D. G. Fox, and P. J. Van Soest. 1991. Model generated protein degradation nutrition information. In: Proc. Cornel1 Nutr. Conf. p 44. Ithaca, NY. Colucci, P. E., L. E. Chase, and P. J. Van Soest. 1982. Feed intake, apparent diet digestibility, and rate of particulate passage in dairy cattle. J. Dairy Sci. 65:1445. Erdman, R. A,, J. H. Vandersall, E. Russek-Cohen, and G. Switalski. 1987. Simultaneous measures of rates of ruminal digestion and passage of feeds for prediction of ruminal nitrogen and dry matter digestion in lactating dairy cows. J. Anim. Sci. 04:585. Ewing, D. L., and D. E. Johnson. 1987. Corn particle starch digestion, passage and size reduction in beef steers: A

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Goering, H. K., and R. S. Adams. 1973. Frequency of heatdamaged protein in hay, haycrop silage and corn silage. J. Anim. Sci. 37:295 (Abstr.). Goering, H. K., and P. J. Van Soest. 1970. Forage fiber analyses (apparatus, reagents, procedures, and some applications). Agric. Handbook 379. ARS, USDA, Washington, DC. Hartnell, G. F., and L. D. Satter. 1970. Determination of rumen fill, retention time and ruminal turnover rates of ingesta at different stages of lactation in dairy cows. J. Anim. Sci. 48: 381.

Hoover, W. H. 1978. Digestion and absorption in the hindgut of ruminants. J. Anim. Sci. 46:1789. Karr, M. R.,C. 0.Little, and G. E. Mitchell, Jr. 1966. Starch disappearance from different segments of the digestive tract of steers. J. Anim. Sci. 25:652. Krishnamoorthy, U.C., T. V. Muscato, C. J. Sniffen. and P. J. Van Soest. 1082. Nitrogen fractions in selected feedstuffs. J. Dairy Sci. 65:217. Krishnamoorthy, U. C., C. J. Sniffen, M. D. Stem, and P. J. Van Soest. 1983. Evaluation of a mathematical model of digesta and in-vitro simulation of rumen proteolysis to estimate the rumen undegraded nitrogen content of feedstuffs. Br. J. Nutr. 50:555. Lucas, H. L., Jr., W.W.G. Smart, Jr., M. A. Cipolloni, and H. D. Gross. 1061. Relations between digestibility and composition of feeds and foods. S-45 Report, North Carolina State College. MacGregor, C. A,, M. R.Stokes, W. H. Hoover, H. A. Leonard, L. L. Junkins, Jr., C. J. Sniffen, and R. W. Mailman. 1983. Effect of dietary concentration of total nonstructural carbohydrates on energy and nitrogen metabolism and milk production of dairy cows. J. Dairy Sci. 66:39. MacRae, J. C., and D. G. Armstrong. 1969. Studies on intestinal digestion in the sheep. 2. Digestion of some carbohydrate constituents in cereal and hay-cereal. Br. J. Nutr. 23:377. Mason, V. C., and R.Palmer. 1071. Studies on the digestibility and utilization of the nitrogen of irradiated rumen bacteria by rats. J. Agric. Sci. (Camb.) 76:567. Mason, V. C., and F. White. 1071. The digestion of bacterial mucopeptide constituents in sheep. I. The metabolism of 2,6 diaminopimelic acid. J. Agric. Sci. (CambJ 7791. Mertens, D. R. 1073. Application of theoretical mathematical models to cell wall digestion and forage intake in ruminants. Ph.D. Dissertation. Cornell Univ., Ithaca. NY. Mertens, D. R. 1985. Effect of fiber on feed quality for dairy cows. 46th Minnesota Nutr. Cod. p 209. St. Paul, MN. Mertens, D. R.,and L. 0. Ely. 1979. A dynamic model of fiber digestion and passage in the ruminant for evaluating forage quality. J. Anim. Sci. 49:1085. Morrison, F. B. 1956. Feeds and Feeding Wnd EdJ. Morrison Publishing Co., Clinton, I k Muscato, T.V., C. J. Sniffen, U. C. Krishnamoorthy, and P. J. Van Soest. 1083. Amino acid content of noncell and cell wall fractions in feedstuffs. J. Dairy Sci. 66:2108. Nocek, J. E. 1985. Evaluation of specific variable affecting in situ estimates of ruminal dry matter and protein digestion. J. Anim. Sci. 60:1347. Nocek, J. E., and J. B. Russell. 1088. Protein and energy as an integrated system. Relationship of ruminal protein and carbohydrate availability to microbial protein synthesis and milk production. J. Dairy Sci. 71:2070. NFlC. 1076. Nutrient Requirements of Beef Cattle (5th Ed.). National Academy Press, Washington, DC. Downloaded from https://academic.oup.com/jas/article-abstract/70/11/3562/4705805 by guest on 07 March 2018

-NRC. 1978. Nutrient Requirements of Dairy Cattle (5th Ed.). National Academy Press, Washington, DC. NRC. 1981. Nutritional Energetics of Domestic Animals and Glossary of Energy Terms (2nd Ed.). National Academy Press, Washington, DC. NRC. 1982. United States-Canadian Tables of Feed Composition. National Academy Press, Washington, DC. NRC. 1984. Nutrient Requirements of Beef Cattle (6th Ed.). National Academy Press, Washington, DC. NRC. 1985. Ruminant Nitrogen Usage. National Academy Press, Washington, DC. NRC. 1989. Nutrient Requirements of Dairy Cattle (6th Ed.). National Academy Press, Washington, DC. Pichard, D. G. 1977. Forage nutritive value. Continuous and batch in vitro fermentations and nitrogen solubility. Ph.D. Dissertation. Cornell Univ., Ithaca, NY. Pichard, D. G., and P. J. Van Soest. 1977. Protein solubility of ruminant feeds. Proc. Cornell Nutr. Conf. p 91. Ithaca, NY. Roe, M. B., C. J. Sniffen, and L. E. Chase. 1990. Techniques for measuring protein fractions in feedstuffs. Proc. Cornell Nutr. Conf. p 81. Ithaca, NY. Russell, J. B., J. D. O’Connor, D. G. Fox, P. J. Van Soest, and C. J. Sniffen. 1092. A net carbohydrate and protein system for evaluating cattle diets I. Ruminal fermentation. J. Anim. sci. 70:355 1. Russell, J. R.,A. W. Young,and N. A. Jorgensen. 1981. Effect of dietary corn starch intake on ruminal, small intestinal and large intestinal starch digestion in cattle. J. Anim. Sci. 52: 1170.

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Res. 36:313. Smith, R. H., and .4. B. McAllan. 1971. Nucleic acid metabolism in the ruminant. 3. Amounts of nucleic acids and total and ammonia nitrogen in digesta from the rumen, duodenum and ileum of calves. Br. J. Nutr. 25:181. Sniffen, C. J., J. B. Russell, and P. J. Van Soest. 1983. The influence of carbon source, nitrogen source and growth factors in rumen microbial growth. Proc. Cornell Nutr. Conf. p 26. Ithaca, NY. Storm, E., D. S. Brown, and E. R. Orskov. 1983. The nutritive value of rumenmicroorganisms in ruminants. 3. The digestion of microbial and nucleic acids in, and losses of e n dogenous nitrogen from, the small intestine of sheep. Br. J. Nutr. 50:470. Swingle, R.S., J. Moore, M. Moore, and T.Eck. 1990. Utilization of starch from processed sorghum grain. I n Proc. Southwest Nutr. and Management Conf. p 52. Tempe, AZ. Tas,M. V., R.A. Evans, and R.F.E. Axford. 1981. The digestion of amino acids in the small intestine of sheep. Br. J. Nutr. 45: 167.

Thompson, D. J., D. E. Beever, J. F. Coelho da Silva, and D. G. Armstrong. 1972. The effect in sheep of physical form on the sites of digestion of a dried lucerne diet. Br. J. Nutr. 28: 31.

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Varga, G. A., and W. H. Hoover. 1983. Rate and extent of neutral detergent fiber degradation of feedstuffs in situ. J. Dairy Sci. 66:2109. Waldo, D. R. 1973. Extent and partition of cereal grain starch digestion in ruminants. J. h i m . Sci. 37:1082. Waldo, D. R.,and H. K. Goering. 1979. Insolubility of proteins in ruminant feeds by four methods. J. Anim. Sci. 49:1560. Waldo, D. R.,L. W. Smith, and E. L. Cox. 1972. Model of cellulose disappearance from the rumen. J. Dairy Sci. 55: 125.

Welch, J. G. 1986. Physical parameters of fiber affecting passage from the rumen. J. Dairy Sci. 692750. Zinn, R. A. 1990. Optimizing the value of steamflaked corn in diets for feedlot cattle. In: Proc. Southwest Nutr. and Management Conf. p 36. Tempe, AZ.