title: Evaluation of the National Research Council (2001) dairy model and derivation of new prediction equations. 1. Digestibility of fiber, fat, protein, and nonfiber carbohydrate.
abstract: Evaluation of ration balancing systems such as the National Research Council (NRC) Nutrient Requirements series is important for improving predictions of animal nutrient requirements and advancing feeding strategies. This work used a literature data set (n = 550) to evaluate predictions of total-tract digested neutral detergent fiber (NDF), fatty acid (FA), crude protein (CP), and nonfiber carbohydrate (NFC) estimated by the NRC (2001) dairy model. Mean biases suggested that the NRC (2001) lactating cow model overestimated true FA and CP digestibility by 26 and 7%, respectively, and under-predicted NDF digestibility by 16%. All NRC (2001) estimates had notable mean and slope biases and large root mean squared prediction error (RMSPE), and concordance (CCC) ranged from poor to good. Predicting NDF digestibility with independent equations for legumes, corn silage, other forages, and nonforage feeds improved CCC (0.85 vs. 0.76) compared with the re-derived NRC (2001) equation form (NRC equation with parameter estimates re-derived against this data set). Separate FA digestion coefficients were derived for different fat supplements (animal fats, oils, and other fat types) and for the basal diet. This equation returned improved (from 0.76 to 0.94) CCC compared with the re-derived NRC (2001) equation form. Unique CP digestibility equations were derived for forages, animal protein feeds, plant protein feeds, and other feeds, which improved CCC compared with the re-derived NRC (2001) equation form (0.74 to 0.85). New NFC digestibility coefficients were derived for grain-specific starch digestibilities, with residual organic matter assumed to be 98% digestible. A Monte Carlo cross-validation was performed to evaluate repeatability of model fit. In this procedure, data were randomly subsetted 500 times into derivation (60%) and evaluation (40%) data sets, and equations were derived using the derivation data and then evaluated against the independent evaluation data. Models derived with random study effects demonstrated poor repeatability of fit in independent evaluation. Similar equations derived without random study effects showed improved fit against independent data and little evidence of biased parameter estimates associated with failure to include study effects. The equations derived in this analysis provide interesting insight into how NDF, starch, FA, and CP digestibilities are affected by intake, feed type, and diet composition.
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, Michigan State University, East Lansing 48824.
authors.forename: M J
authors.initials: MJ
authors.lastname: VandeHaar
authors.affiliation: Department of Animal Science, University of Wisconsin, Madison 53706.
authors.forename: L E
authors.initials: LE
authors.lastname: Armentano
authors.affiliation: Department of Animal Sciences, The Ohio State University, Columbus 43210.
authors.forename: W P
authors.initials: WP
authors.lastname: Weiss
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; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D002417:Cattle; D004032:Diet; D004043:Dietary Fiber / Q000378:metabolism; D004063:Digestion; D005260:Female; D007774:Lactation; D009307:National Academy of Sciences, U.S.; D012417:Rumen / Q000378:metabolism; D012820:Silage; D014481:United States
keywords: National Research Council (2001) dairy model; model evaluation; total-tract digestibility