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Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, SK, S7N 5A8, Canada
Corresponding author:
P. Yu; e-mail:
yupe{at}sask.usask.ca.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: DVE/OEB system • dairy cows • forages • NRC-2001 model • protein evaluation
Abbreviation key: AECP = Truly absorbed endogenous protein in the small intestine (g/kg DM) (NRC), AMCPDVE = Truly absorbed rumen synthesized microbial protein in the small intestine (g/kg DM) (DVE/OEB), AMCPNRC = Truly absorbed rumen synthesized microbial protein in the small intestine (g/kg DM) (NRC), ARUPDVE = Truly absorbed bypass feed protein in the small intestine (g/kg DM; DVE/OEB), ARUPNRC = Truly absorbed rumen undegraded feed protein in the small intestine (g/kg DM; NRC), Degraded Protein BalanceOEB = Reflects the difference between the potential microbial protein synthesis based on rumen degraded feed CP and that based on energy (rumen fermented OM) available for microbial fermentation in the rumen, calculated as: MCPRDPDVE - MCPFOM (DVE/OEB), Degraded Protein BalanceNRC = Based on the data from NRC 2001 model, the Degraded Protein BalanceNRC (g/kg DM) reflects the difference between the potential microbial protein synthesis based on ruminally degraded feed CP and that based on energy - TDN available for microbial fermentation in the rumen, calculated as: RDPNRC - 1.18MCPTDN, DVE = Truly absorbed protein in the small intestine (g/kg DM), contributed by 1) feed protein escaping rumen degradation (RUPDVE), 2) microbial protein synthesized in the rumen (MCPFOM), and 3) a correction for endogenous protein losses in the digestive tract (ENDP; DVE/OEB), ECP = Rumen endogenous crude protein (g/kg DM; NRC), ENDP = Endogenous protein losses in the digestive tract (g/kg DM; DVE/OEB), MCPFOM = Microbial protein synthesized in the rumen based on available energy (g/kg DM; DVE/OEB), MCPNRC = Ruminally synthesized microbial CP (g/kg DM), calculated as 0.130 x TDN, when RDPNRC exceeds 1.18 x TDN-predicted MCP (MCPTDN). When RDPNRC is less than 1.18 x TDN-predicted MCP (MCPTDN), then MCP is calculated as 0.85 of RDPNRC (MCPRDP; NRC), MCPRDPDVE = Microbial protein synthesized in the rumen based on rumen degraded feed crude protein, calculated as: CP x (1 - 1.11%RUP/100) (g/kg DM; DVE/OEB), MCPRDPNRC = Microbial protein synthesized in the rumen based on available protein, calculated as 0.85 of RDPNRC (g/kg DM; NRC), MCPTDN = Microbial protein synthesized in the rumen based on available energy (discounted TDN; NRC), MP = Metabolizable protein (g/kg DM) (defined as the true protein that is digested postruminally and the component amino acid absorbed by the intestine) contributed by 1) ruminally undegraded feed CP, 2) ruminally synthesized microbial CP, and 3) endogenous CP (NRC), TPSI = True protein supplied to the small intestine (g/kg DM)
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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In the DVE/OEB system, the protein value for feeds and the requirements for dairy cows are both expressed as the amount of protein (microbial and feed source) truly digested in and absorbed from the small intestine of the animal. This system can give information on the quantitative aspects of both ruminal and postruminal protein digestion in ruminants. In the DVE/OEB system, each feed has a DVE value, which stands for true absorbable protein in the small intestine, composed of: 1) digestible feed true protein escaping rumen degradation; 2) digestible true microbial protein synthesized in the rumen; and 3) a correction for endogenous protein losses in the digestive tract. Each feed also has a rumen Degraded Protein BalanceOEB value, called OEB in Dutch, which shows the (im)balance between microbial protein synthesis potentially possible from available rumen degradable protein and that potentially possible from the energy extracted during anaerobic fermentation in the rumen. When OEB is positive, it indicates the potential loss of N from the rumen. When negative, microbial protein synthesis may be impaired because of a shortage of N in the rumen. The optimum OEB value in a ration is, therefore, zero or slightly above (Tamminga et al., 1994).
The NRC dairy model (2001) introduced the concepts of metabolizable protein (MP), defined as true protein that is digested and absorbed by the intestine, and contributed by 1) ruminally undegraded feed CP, 2) ruminally synthesized microbial CP (MCPNRC), and 3) endogenous CP from rumen (ECP). Based on the data from NRC dairy model (NRC, 2001), the rumen Degraded Protein BalanceNRC can be calculated, and it reflects the difference between the potential microbial protein synthesis based on ruminally degraded feed CP and that based on TDN as energy available for microbial fermentation in the rumen.
Although the principles of these two models are similar, some of the factors used in quantifying calculations and some concepts differ. The objective of this study was to compare the DVE/OEB system and the NRC-2001 model in the prediction of potential nutrient supply to dairy cows from selected forages: alfalfa (Medicago sativa L.) and timothy (Phleum pratense L.). In addition, the effects of variety and cutting stage of the forages on potential nutrient supply to dairy cows were studied.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Rumen In Vitro Assay
Rumen in vitro digestibility of neutral detergent fiber (IVNDFD) of timothy and alfalfa (2 mm) after a 48-h incubation was determined using the Tilley and Terry procedure (Marten and Barnes, 1980).
Estimation of Total Digestible Nutrients
A summative approach was used to derive the total digestible nutrient (TDN1x; NRC, 2001). In this approach, the concentrations (% DM) of truly digestible nonfiber carbohydrate, crude protein (CP), and ether extract (EE) for each feed were estimated (Weiss et al., 1992) according to the equations in the NRC dairy model (2001). Truly digestible NDF (tdNDF) was obtained using a 48-h rumen in vitro assay (NRC, 2001), calculated as: tdNDF (% DM) = IVNDFD (%) x NDF (% DM).
In Situ Rumen Incubation Assay
Animal and diet.
Two dry Holstein-Friesian cows fitted with large rumen cannulas with an internal diameter of 10 cm were housed in tie stalls and bedded with straw at the dairy experimental station of the University of Saskatchewan. Each cow received daily 15-kg TMR, consisting of 27.5% pelleted concentrate [containing 55.8% barley, 11.7% soybean meal, 10.3% canola meal, 5% oats, 4.1% wheat, 3.3% wheat distillers dried grains, 3% mineral-vitamin mix, 2.2% corn gluten meal, 2.1% molasses, 0.7% canola oil, 0.6% cobalt-iodized salt, 0.6% sodium bicarbonate, 0.3% dynamate (International Minerals and Chemicals Corp., Mundelein, IL, USA), 0.05% niacin-magnesium mix, and 0.05% ground limestone], 55% barley silage, 12.5% alfalfa hay, and 5% dehydrated alfalfa. Both cows were individually fed twice daily at 0800 and 1600 h. Water was always available. Cows had free access to exercise grounds. The animals used in the experiment were cared for in accordance with the guidelines of the Canadian Council on Animal Care (1993).
In situ rumen degradation procedure.
Rumen degradation characteristics of the forage samples were determined using the in situ method as modified by McKinnon et al. (1995). All treatments were incubated in number-coded, commercially-made polyester bags (10 x 20 cm; ANKOM Company, Fairport, NY) with pore size of 53 ± 10 µm. These bags were tied 2 cm below the top with a ratio of sample size-to-bag-surface-area of 27.8 mg/cm2. Incubations were carried out for 72, 48, 24, 12, 8, 4, and 2 h, bags were inserted at 2100 (d 1), 2100 (d 2), 2100 (d 3), 0900 (d 4), 1300 (d 4), 1700 (d 4), 1900 (d 4), and all were removed at 2100 h (d 4). All treatments were randomly allocated over cows. This is because an initial experiment (Markert and Christensen, 2002) showed no significant animal effect on rumen degradation characteristics of the alfalfa and timothy forages. After incubation, the bags were removed from rumen and rinsed under a cold stream of tap water to remove excess ruminal contents and to stop microbial activity. Then the bags were washed without detergent by hand and subsequently dried at 55°C for 24 h. The 0-h incubation samples were washed under the same conditions. The residues were pooled according to forage species, variety, stage, and incubation time and then ground through a 1-mm screen (Retsch ZM-1, Brinkmann Instruments, Ltd., Ontario, Canada) for chemical analysis.
Rumen degradation model.
The rumen degradation characteristics described by the first order kinetics degradation model (Ørskov and McDonald, 1979), modified by Robinson et al. (1986), Tamminga et al. (1990), and Tamminga et al. (1994) were soluble fraction (A, %) which was determined from the 0-h incubations, undegradable fraction (C, %), insoluble fraction (B, %), which was degraded exponentially, the degradation rate (Kd, %/h), and lag time (T0, h). As each component has its own pattern of rumen degradation, to describe its degradation accurately, the models used for each component were different. The results were calculated using the NLIN (nonlinear) procedure of the statistical package of SAS (1991) using iterative least-squares regression (Gauess-Newton method) by the following first order kinetics equations:
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where, R(t) = residue (%) of the amount of incubated material after t h of rumen incubation.
For OM, rumen degradation was described by assuming three fractions (A, B, and C) with lag time T0. For CP, rumen degradation was described by assuming three fractions (A, B, and C) without lag time (T0 = 0) (Ørskov and McDonald, 1979).
Further calculations were:
For CP: Fraction of ruminally degraded feed CP was calculated as: 
; fraction of rumen undegraded feed CP was calculated as: 
, where, Kp of 4%/h was adopted for a forage (Tamminga et al., 1994).
For OM: fraction of ruminally degraded feed OM was calculated as: 
; fraction of rumen undegraded feed OM was calculated as: 
, where, Kp of 4%/h was adopted for a forage.
Chemical Analysis
The DM, ash, EE, and N contents were analyzed according to the procedure of AOAC (1990). The acid detergent fiber (ADF), NDF, and acid detergent lignin (ADL) values were analyzed according to the procedures of Van Soest et al. (1991).
The DVE/OEB System
The detailed concepts and formulas of the DVE/OEB system are provided by Tamminga et al. (1994). The following is a brief explanation to show how to calculate and predict protein supply to the small intestine of dairy cows as a result of feeding the two forage types as affected by variety and cutting stage.
Calculation of FOM and RUPDEV.
In the DVE/OEB system, bypassing rumen microbial degradation of feed protein (RUPDEV) was calculated as: RUPDEV = 1.11 x CP x %RUP, where, RUPDEV and CP in g/kg DM. The factor 1.11 is taken from the French PDI-system (Verite et al., 1987). This represents the regression coefficient of in vivo on in situ degradation data. The content of OM fermented in the rumen (FOM) is calculated as: FOM = OM x %RUOM, where, FOM and OM are in g/kg DM (Muia et al., 2001).
Microbial protein synthesis in the rumen.
Microbial protein synthesized in the rumen based on rumen fermented organic matter (MCPFOM) was estimated as: MCPFOM = 0.15 x FOM, where, MCPFOM in g/kg DM. The factor 0.15 means that per kg FOM, 150 g of microbial protein is assumed to be synthesized (Tamminga et al., 1994).
The content of true protein supplied to the small intestine (TPSI) was calculated as: TPSI = RUPDEV + 0.75 x MCPFOM, where, TPSI is in g/kg DM. The factor 0.75 means that 75% of microbial N is present in amino acids; the remaining is N in nucleic acids.
Intestinal digestion of feed and microbial protein.
The previously discussed RUPDEV and TPSI must be corrected for incomplete digestion and endogenous secretions (Tamminga et al., 1994). A correction is needed for protein losses due to incomplete digestion and from endogenous secretions. True digestibility of microbial protein is assumed to be 85% (Egan et al., 1985) and, therefore, the amount of truly absorbed rumen synthesized microbial protein in the small intestine (AMCPDVE) was estimated as: AMCPDVE = 0.85 x 0.75 x MCPFOM, where, AMCPDVE is in g/kg DM.
For feed ingredients, the content of truly absorbed bypass feed protein in the small intestine (ARUPDVE) was calculated as: ARUPDVE = RUPDVE - UCP, where, the amount of potential undigested CP (UCP) is calculated as: UCP = CP x %UCP, where, %UCP was protein residue remaining after an in situ incubation of 336 h. It was assumed that undegradable CP after a 336-h incubation is indigestible in the small intestine (Tamminga et al., 1994; Muia et al., 2001). In this experiment, %UCP was determined following a 72-h in situ incubation. This is because a previous study showed that further incubation from 72 to 336 h resulted in no further digestion in alfalfa (data not shown).
Endogenous protein losses in the small intestine.
In the DVE/OEB system, the endogenous protein losses in the digestive tract (ENDP) is related to the amount of undigested DM (UDM) excreted in the feaces. According to the DVE/OEB system, 75 g of absorbed protein per kg undigested DM in fecal excretion is required to compensate for the endogenous losses. Therefore, endogenous protein losses in the digestive tract are estimated as: ENDP = 75 x undigested DM, where, undigested DM and ENDP are in g/kg DM; UDM = undigested OM + undigested inorganic matter, where, undigested OM = OM x %Uom, %Uom was residue of forage OM following an incubation of 72 h, and undigested inorganic matter = ASH x 35% (CVB, 1996).
Truly digested and absorbed protein in the small intestine.
Truly digested and absorbed protein in the small intestine (DVE) are contributed by 1) feed protein escaping rumen degradation (RUPDVE), 2) microbial protein synthesized in the rumen (MCPFOM), and 3) a correction for endogenous protein losses in the digestive tract (ENDP). Therefore, the DVE value was estimated as: DVE = ARUPDVE + AMCPFOM - ENDP, where, DVE is in g/kg DM.
Degraded protein balance.
The Degraded Protein BalanceOEB is a balance between microbial protein synthesis from rumen degradable CP and that from the energy extracted during anaerobic fermentation in the rumen. Therefore, the Degraded Protein BalanceOEB value was estimated as: microbial protein synthesized in the rumen based on rumen degraded feed crude protein (MCPRDPDVE) - MCPFOM, where, all parameters are in g/kg DM, MCPRDPDVE = CP - 1.11 x %RUP. When the Degraded Protein Balance is positive, it indicates the potential loss of N from the rumen. When it is negative, microbial protein synthesis may be impaired, because of a shortage of N in the rumen. The optimum value of the Degraded Protein BalanceOEB in a ration is, therefore, zero or slightly above zero (Tamminga et al., 1994).
The NRC-2001 Model
The detailed concepts and formulas are provided by NRC (2001). A brief explanation is as follows:
Calculation of RDPNRC and RUPNRC.
Using the NRC-2001 model, the ruminally undegraded feed CP was calculated as: RUPNRC = CP x %RUP; the rumen degraded feed protein was calculated as: RDPNRC = CP x %RDP, where, a Kp was 4%/h; RDPNRC, RUPNRC and CP in g/kg DM.
Rumen microbial protein synthesis.
Ruminally synthesized microbial CP was calculated as: MCPNRC (g/kg DM) = 0.13 x TDN, when RDPNRC exceeded 1.18 x TDN-predicted MCP (MCPTDN). When RDPNRC was less than 1.18 x TDN-predicted MCP (MCPTDN), MCPNRC was calculated as 0.85 of RDPNRC (MCPRDP). The factor 0.13 means that per kg TDN, 130 g of microbial protein CP is assumed to be synthesized.
Intestinal digestion of feed and microbial protein.
In the NRC model (2001), digestibility and true protein of ruminally synthesized microbial CP are assumed to be 80%; therefore, the amount of truly absorbed MCPNRC (AMCPNRC) was estimated as: AMCPNRC = 0.80 x 0.80 x MCPNRC, where, AMCPNRC is in g/kg DM.
For feed ingredients, truly absorbed rumen undegraded feed protein in the small intestine (ARUPNRC) was calculated as: ARUPNRC = RUPNRC - UCP, where, UCP = the amount of potential undigested CP, calculated as: CP x %UCP.
Rumen endogenous protein in the small intestine.
Rumen endogenous CP (g/kg DM) according to the NRC model (2001) can be calculated as: ECP = 6.25 x 1.9 x DM (g/kg). Assuming that 50% of rumen endogenous CP passes to the duodenum and 80% of rumen endogenous CP is true protein (NRC, 2001), the truly absorbed endogenous protein in the small intestine (AECP) was estimated as: AECP = 0.50 x 0.80 x ECP, where, AECP and ECP are in g/kg DM.
Total metabolizable protein.
Metabolizable protein in the NRC dairy model (NRC, 2001) is contributed by 1) digestible RUPNRC, 2) digestible MCPNRC, and 3) ECP, calculated as: MP = ARUPNRC + AMCPNRC + AECP, where, MP is in g/kg DM.
Degraded protein balance.
Based on the data from the NRC dairy model (2001), the Degraded Protein BalanceNRC (g/kg DM) can be calculated. It reflects the difference between the potential microbial protein synthesis based on ruminally degraded feed crude protein (RDPNRC) and that based on energy (TDN) available for microbial fermentation in the rumen, calculated as: RDP - 1.18MCPTDN, where, the Degraded Protein BalanceNRC value is in g/kg DM.
Statistical Analysis
Statistical analyses were performed using the GLM procedure of SAS (1991). The model used for the analysis was:
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where, Ykl(i) is an observation of the dependent variable for the variety j at maturity k in the forage i; µ is the population mean for the variable; Fi is the forage type i, i = a, t; a is for alfalfa, and t is for timothy; Vj (a) is the effect of forage variety nested within alfalfa; Vj (t) is the effect of forage variety nested within timothy; Mk(a) is the effect of forage maturity nested within alfalfa; Mk(t) is the effect of forage variety nested within timothy; and eijk is the random error associated with the observation jk(i).
Treatments contrasts (SAS, 1991; alfalfa vs. timothy; Pioneer alfalfa vs. Beaver alfalfa; Climax timothy vs. Joliette timothy, etc.) were used to determine treatment differences. Significance was declared at P < 0.05.
The PROC CORR and PROC REG (SAS, 1991) was used for linear correlation and regression analysis between predicted values from the DVE/OEB system and the NRC-2001 model (MCPFOM vs. MCPTDN; MCPRDPDVE vs. MCPRDPNRC; DVE vs. MP; and Degraded Protein BalanceOEB vs. Degraded Protein BalanceNRC). Pearson correlation coefficients (R) and regression equation are presented.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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and 2. The detailed results and discussion on the effects of forage type, variety, and stage of cutting on chemical composition, and rumen degradation characteristics have been previously discussed (Yu et al., 2002a).
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and 4. The results showed that forage species (alfalfa vs. timothy) had a significant impact on the potential nutrient supply to dairy cows. Comparing alfalfa and timothy means, alfalfa species contained higher (P < 0.001) FOM, MCPRDPDVE and MCPFOM. As a result, potentially absorbed microbial protein in the small intestine (AMCPDEV) was also higher for alfalfa.
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For endogenous protein losses in the small intestine, alfalfa contained lower (P < 0.001) undigested DM; therefore, the ENDP value for alfalfa was lower.
The most important values are DVE and the Degraded Protein BalanceOEB values. The DVE value for alfalfa was nearly two times higher (P < 0.001) than that for timothy.
For the Degraded Protein BalanceOEB value, alfalfa had a positive value, but timothy had a negative value of Degraded Protein BalanceOEB, indicating potential rumen N loss in alfalfa and a shortage with timothy.
Comparing the two varieties of alfalfa (Pioneer vs. Beaver), there were no differences (P > 0.05) in predicted nutrient supply to dairy cows. However, comparing the two varieties of timothy (Climax vs. Joliette), Climax was better (P < 0.05) than Joliette in terms of MCPRDPDVE, ARUPDVE, DVE, and the Degraded Protein BalanceOEB values.
As alfalfa maturity advanced (stages 1 to 3), rumen fermented OM (FOM) was decreased (P < 0.05); therefore, AMCPDEV was decreased (P < 0.05); therefore, AMCPDEV was decreased (P < 0.05). RUPDEV was not affected (P > 0.05), undigested CP was increased (P < 0.05), and ARUPDEV tended to be affected (P < 0.10). Due to increasing undigested DM, ENDP (P < 0.05) was also increased. In general, the DVE value was decreased (P < 0.05) without affecting the Degraded Protein BalanceOEB value (P > 0.05). Similar trends were found for timothy. With increasing stage of cutting, FOM was decreased (P < 0.05), AMCPDEV decreased (P < 0.05), ARUPDEV decreased (P < 0.05), and ENDP (P < 0.05) increased. The total DVE value in timothy was decreased (P < 0.05) without affecting the Degraded Protein BalanceOEB value (P > 0.05) as maturity increased.
Using the NRC-2001 Model to Predict Nutrient Supply to Dairy Cows
Using the NRC-2001 model, the effects of variety and cutting stage on potential nutrient supply to dairy cows from alfalfa and timothy are presented in Tables 5 and 6. Comparing alfalfa and timothy means, alfalfa contained higher (P < 0.001) TDN1x, therefore, resulting in a higher (P < 0.001) MCPTDN. It also contained higher RDPNRC (P < 0.001), therefore, resulting in higher (P < 0.001) MCPRDPNRC. Alfalfa promoted increased synthesis of higher MCPNRC (P < 0.001), therefore, higher AMCPNRC (P < 0.001).
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Total MP, contributed from AMCPNRC, ARUPNRC, and AECP, in alfalfa was higher (P < 0.001) than that in timothy. Alfalfa also had a higher (P < 0.001) Degraded Protein BalanceNRC.
Comparing the two varieties of alfalfa (Pioneer vs. Beaver), no differences (P > 0.05) in predicted nutrient supply to dairy cows were found using NRC model (2001). However, comparing the two varieties of timothy (Climax vs. Joliette), Climax had a superior nutritive value than Joliette. It had higher AMCPNRC (P < 0.001) and ARUPNRC (P < 0.001) but similar AECP (P > 0.05). Therefore, Climax was higher (P < 0.001) in total MP and the Degraded Protein BalanceNRC value.
As alfalfa maturity advanced (stages 1 to 3), only AECP decreased (P < 0.05). For timothy, increasing stage of cutting decreased AMCPNRC (P < 0.10), ARUPNRC (P < 0.05), AECP (P < 0.05), MP (P < 0.05), and Degraded Protein BalanceNRC (P < 0.10).
Comparison of Predictions from the DVE/OEB System and the NRC-2001 Model
The mean, standard deviation, and range of the predicted values calculated according to the DVE/OEB system and the NRC-2001 model, and the correlations between two models are in Tables 7 and 8. The results show that the predicted values from the DVE/OEB system and the NRC-2001 model had significant correlations (P < 0.001) with high R values ranging from 0.96 to 0.99. The standard deviations and ranges of the predicted values for each treatment were of a similar magnitude. Using the DVE/OEB system, the overall mean for microbial protein supply based on energy, or on ruminally degraded protein were higher (+9.4 and +9.8 g/kg DM, respectively), but the total absorbed protein in the small intestine and the Degraded Protein Balance values were lower (-8.6 and -2.2 g/kg DM, respectively) in comparison to that predicted by the NRC-2001 dairy model.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The difference was also found in predicted ruminally undegraded feed protein between the two models. In the DVE/OEB system (Tamminga et al., 1994), the ruminally undegraded feed protein was calculated as: 1.11 x CP x %RUP. The factor 1.11 represents the regression coefficient of in vivo on in situ degradation data. However, using the NRC-2001 model, the ruminally undegraded feed CP was calculated as: CP x %RUP. No correction is made for the difference between in vivo and in situ results.
In the prediction of microbial protein supply based on ruminally degraded feed protein, the two models are also different. In the DVE/OEB system (Tamminga et al., 1994), it is assumed that 100% ruminally degraded feed protein could be potentially converted to microbial protein if enough energy is provided. However, in the NRC-2001 model, it is assumed that only 85% of ruminally degraded feed protein could be potentially converted to microbial protein.
To calculate truly absorbed protein in the small intestine contributed from the ruminally undegraded feed protein, the two models use different factor parameters. In the DVE/OEB system, the amount of truly absorbed rumen synthesized microbial protein in the small intestine was estimated as: 0.85 x 0.75 x MCPFOM. It assumes that true digestibility of microbial protein is 85% (Egan et al., 1985), and 75% of microbial N is present in amino acids; the remaining is N in nucleic acids. In the NRC-2001 model, digestibility and true protein of ruminally synthesized microbial CP are assumed to be 80%; therefore, the amount of truly absorbed rumen synthesized microbial protein in the small intestine was estimated as: 0.80 x 0.80 x MCPNRC. Although the individual coefficients differ, the net result is essentially the same between the two models (0.85 x 0.75) vs. (0.80 x 0.80).
Both models could predict the potential microbial protein synthesized in the rumen based on available energy. The DVE/OEB system uses rumen fermented OM as energy base to predict microbial protein. However, the NRC-2001 model uses available TDN as its energy base. This is a difference. Each model also uses different factor parameters to calculate microbial protein synthesized in the rumen. In the DVE/OEB system, 150 g of microbial protein CP is assumed to be synthesized per kg fermented OM. But in the NRC-2001 model, it is assumed that 130 g of microbial protein CP is assumed to be synthesized per kg TDN.
All above individual differences contribute to a difference in prediction of amount of total truly absorbed protein in the small intestine (DVE vs. MP) between the two models. In this study, using the DVE/OEB system, the overall average microbial protein supply based on available energy - rumen fermented OM was 12% higher, microbial protein supply based on ruminally degraded feed protein was 10% higher, and the truly absorbed protein in the small intestine was 15% lower than that predicted by the NRC-2001 model.
However, the correlation of the predicted values from the two models was high (R 0.96~0.99) (Tables 7
and 8). This is due to the fact that the standard deviations and the range of the predicted values from the two models were similar. Both are based on similar principles.
The difference was also found in the prediction of the degraded protein balance, which was 11% higher based on the data from the NRC-2001 model than in the DVE/OEB system (Tables 3 through 8). These differences are due to the factors used in quantifying calculations rather than the difference in principles upon which the two systems are based. The most important outputs (net results), the DVE in the DVE/OEB system vs. the MP in the NRC-2001 model, were a 8.6 g/kg DM difference, on average (Tables 7 and 8).
Both the DVE/OEB model and the NRC-2001 model are deterministic (not stochastic) and empirical and are based on several assumptions. For example, in the DVE/OEB model, 150 g of microbial protein is assumed to be synthesized per kg rumen fermented OM. In the NRC-2001 model, 130 g of microbial protein is assumed to be synthesized per kg TDN. Therefore, potential nutrient supply, such as microbial CP supply based on nutrient intake in each group, does not vary a great deal in either model. This is in contrast to microbial CP supply based on total purine derivative excretion (Yu et al., 2001) or based on in vivo measurement (De Boever et al., 1998). This is because in vivo or purine derivative methods are relative measures of the real microbial CP production. They are affected by several factors, such as ruminal pH and ruminal turnover rate and by individual animal variation (Hoover and Stokes, 1991; Moscardini et al., 1998), not only nutrient intake.
Protein evaluation results by both the DVE/OEB system and the NRC-2001 model indicated that the alfalfa varieties had no effect, but timothy varieties had a large impact on the potential nutrient supply to dairy cows. In this experiment, with increasing stage of cutting, the potential nutritive values were reduced in both forages. However, the sensitivity of the two models to detect the difference between stages was different. For example, using the DVE/OEB system, truly absorbed intestinal protein DVE values between stages in alfalfa were detected to be significantly different (Tables 3 and 4). However, using the NRC-2001 model, truly absorbed intestinal protein MP values between stages in alfalfa were not different (Tables 5 and 6).
The DVE/OEB system introduced a new concept — the role of energy balance in protein supply: the Degraded Protein Balance. It can also be calculated based on data from the NRC-2001 model. The Degraded Protein Balance (in the DVE/OEB system) shows the (im)balance between microbial protein synthesis from available rumen degradable CP and potential energy from anaerobic fermentation in the rumen. When the Degraded Protein Balance in a ration is positive, it indicates the potential N-loss from the rumen. When negative, microbial protein synthesis is predicted to be impaired because of a potential shortage of N in the rumen. The optimum degraded protein balance value in a ration is, therefore, zero or slightly above (Tamminga et al., 1994). Both models detected that alfalfa had a positive degraded protein balance, indicating a potential imbalance between feed N degradation and utilization if alfalfa is used alone. Timothy had a negative degraded protein balance, indicating a potential N shortage. However, the amount of predicted N losses in alfalfa or N shortages in timothy in terms of optimum rumen fermentation were different between the two models. Using the data from the NRC-2001 model resulted in higher degraded protein balances.
All of the results reported here were outputs from the models with inputs based on in vitro and in situ. The challenge is to apply the predictions and evaluate them in an animal experiment between the two models. The number of such studies is limited, particularly using the DVE/OEB system. Part of the reason is that information on DVE and the Degraded Protein Balance values of each feedstuff, or data from which these are derived is limited.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The DVE/OEB and the NRC-2001 model are systems, including both supply and requirement models. The requirement models between the DVE/OEB system and the NRC-2001 model are also different. The nutrient balance (model requirement minus model supply) between two models will be investigated.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication August 20, 2002. Accepted for publication November 22, 2002.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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