title: Evaluation of the National Research Council (2001) dairy model and derivation of new prediction equations. 2. Rumen degradable and undegradable protein.
abstract: This work evaluated the National Research Council (NRC) dairy model (2001) predictions of rumen undegradable (RUP) and degradable (RDP) protein compared with measured postruminal non-ammonia, nonmicrobial (NANMN) and microbial N flows. Models were evaluated using the root mean squared prediction error (RMSPE) as a percent of the observed mean, mean and slope biases as percentages of mean squared prediction error (MSPE), and concordance correlation coefficient (CCC). The NRC (2001) over-estimated NANMN by 18% and under-estimated microbial N by 14%. Both responses had large mean biases (19% and 20% of MSPE, respectively), and NANMN had a slope bias (22% of MSPE). The NRC NANMN estimate had high RMSPE (46% of observed mean) and low CCC (0.37); updating feed library A, B, and C protein fractions and degradation rate (Kd) estimates with newer literature only marginally improved fit. The re-fit NRC models for NANMN and microbial N had CCC of 0.89 and 0.94, respectively. When compared with a prediction of NANMN as a static mean fraction of N intake, the re-derived NRC approach did not have improved fit. A protein system of intermediate complexity was derived in an attempt to estimate NANMN with improved fit compared with the static mean NANMN model. In this system, postruminal appearance of A, B, and C protein fractions were predicted in a feed-type specific manner rather than from estimated passage and degradation rates. In a comparison to independent data achieved through cross-validation, the new protein system improved RMSPE (34 vs. 36% of observed mean) and CCC (0.42 vs. 0.30) compared with the static mean NANMN model. When the NRC microbial N equation was re-derived, the RDP term dropped from the model. Consequently, 2 new microbial protein equations were formulated, both used a saturating (increasing at a decreasing rate) form: one saturated with respect to TDN and the other saturated over increasing intakes of rumen degraded starch and NDF. Both equations expressed maximal microbial N production as a linear function of RDP intake. The function relating microbial N to intake of rumen degradable carbohydrate improved RMSPE (24 vs. 28% of the observed mean) and CCC (0.63 vs 0.30) compared with the re-derived NRC model. The newly derived equations showed modest improvements in model fit and improved capacity to account for known biological effects; however, substantial variability in NANMN and microbial N estimates remained unexplained.
authors.affiliation: National Animal Nutrition Program, National Research Support Project 9, Virginia Tech, Blacksburg 24060; Department of Dairy Science, Virginia Tech, Blacksburg 24060. Electronic address: rrwhite@exchange.vt.edu.
authors.forename: R R
authors.initials: RR
authors.lastname: White
authors.affiliation: Department of Animal Sciences, The Ohio State University, Columbus 43210.
authors.forename: Y
authors.initials: Y
authors.lastname: Roman-Garcia
authors.affiliation: Department of Animal Sciences, The Ohio State University, Columbus 43210.
authors.forename: J L
authors.initials: JL
authors.lastname: Firkins
authors.affiliation: Department of Animal Science, University of Nebraska, Lincoln 68585.
authors.forename: P
authors.initials: P
authors.lastname: Kononoff
authors.affiliation: Department of Animal Science, Michigan State University, East Lansing 48824.
authors.forename: M J
authors.initials: MJ
authors.lastname: VandeHaar
authors.affiliation: Department of Animal Science, University of Nebraska, Lincoln 68585.
authors.forename: H
authors.initials: H
authors.lastname: Tran
authors.affiliation: Department of Dairy Science, Virginia Tech, Blacksburg 24060.
authors.forename: T
authors.initials: T
authors.lastname: McGill
authors.affiliation: Department of Dairy Science, Virginia Tech, Blacksburg 24060.
authors.forename: R
authors.initials: R
authors.lastname: Garnett
authors.affiliation: Department of Dairy Science, Virginia Tech, Blacksburg 24060.
authors.forename: M D
authors.initials: MD
authors.lastname: Hanigan
mesh_terms: D000821:Animal Feed; D000818:Animals; D004032:Diet; D004044:Dietary Proteins / Q000378:metabolism; D008962:Models, Theoretical; D009307:National Academy of Sciences, U.S.; D012417:Rumen / Q000378:metabolism*; D014481:United States
chemical_list: D004044:Dietary Proteins
keywords: National Research Council (2001) dairy model; duodenal flow; model evaluation