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Comparison of the National Research Council-2001 Model with the Dutch System (DVE/OEB) in the Prediction of Nutrient Supply to Dairy Cows from Forages

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.

The objective of this study was to compare the DVE/OEB system (DVE = truly absorbed protein in the small intestine; OEB = degraded protein balance) and the NRC-2001 model in the prediction of supply of protein to dairy cows from selected forages: alfalfa (Medicago sativa L. cv. Pioneer and Beaver) and timothy (Phleum pratense L. cv. Climax and Joliette). Comparisons were made in terms of 1) ruminally synthesized microbial CP, 2) truly absorbed protein in the small intestine, and 3) degraded protein balance. In addition, the effects of variety and cutting stage of the selected forages on the potential nutrient supply to dairy cows were also studied. The results showed that the predicted values from the DVE/OEB system and the NRC-2001 model had significant correlations with high R (>0.96) values. However, using the DVE/OEB system, the overall average microbial protein supply based on available energy was 12% higher, and the truly absorbed protein in the small intestine was 15% lower than that predicted by the NRC-2001 model. The difference was also found in the prediction of the degraded protein balances, which was 11% higher based on data from the NRC-2001 model. These differences are due to considerably different factors used in calculations in the two models, although both are based on similar principles. This indicates that a further refinement is needed for a modern protein evaluation and prediction system. In addition, this study showed that the two alfalfa varieties studied (Pioneer vs. Beaver) had no effect, but cutting stage had a profound influence on ruminally synthesized microbial CP (93, 96, 86 g/kg DM at stage of early bud, late bud, and early bloom, respectively) and truly absorbed intestinal protein predicted by the DVE/OEB system (80, 79, 67 g/kg DM at stage of early bud, late bud, and early bloom, respectively). With timothy, both variety (Climax vs. Joliette) and cutting stage had significant impacts on the potential protein supply predicted by both models. The potential protein supply (DVE or MP) to the dairy cow from Climax timothy was higher than that from Joliette timothy (DVE: 46 vs. 32 g/kg DM; MP: 61 vs. 38 g/kg DM). With increasing stage of cutting, the potential protein supply (DVE or MP) was reduced (DVE: 53, 39, 25 g/kg DM; MP: 62, 51, 36 g/kg DM at stage of joint, prebloom head, and full head, respectively).

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), AMCP^DVE = Truly absorbed rumen synthesized microbial protein in the small intestine (g/kg DM) (DVE/OEB), AMCP^NRC = Truly absorbed rumen synthesized microbial protein in the small intestine (g/kg DM) (NRC), ARUP^DVE = Truly absorbed bypass feed protein in the small intestine (g/kg DM; DVE/OEB), ARUP^NRC = Truly absorbed rumen undegraded feed protein in the small intestine (g/kg DM; NRC), Degraded Protein Balance^OEB = 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: MCP_RDP^DVE - MCP_FOM (DVE/OEB), Degraded Protein Balance^NRC = Based on the data from NRC 2001 model, the Degraded Protein Balance^NRC (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: RDP^NRC - 1.18MCP_TDN, DVE = Truly absorbed protein in the small intestine (g/kg DM), contributed by 1) feed protein escaping rumen degradation (RUP^DVE), 2) microbial protein synthesized in the rumen (MCP_FOM), 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), MCP_FOM = Microbial protein synthesized in the rumen based on available energy (g/kg DM; DVE/OEB), MCP^NRC = Ruminally synthesized microbial CP (g/kg DM), calculated as 0.130 x TDN, when RDP^NRC exceeds 1.18 x TDN-predicted MCP (MCP_TDN). When RDP^NRC is less than 1.18 x TDN-predicted MCP (MCP_TDN), then MCP is calculated as 0.85 of RDP^NRC (MCP_RDP; NRC), MCP_RDP^DVE = 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), MCP_RDP^NRC = Microbial protein synthesized in the rumen based on available protein, calculated as 0.85 of RDP^NRC (g/kg DM; NRC), MCP_TDN = 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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