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ARS Home » Midwest Area » Madison, Wisconsin » U.S. Dairy Forage Research Center » Cell Wall Biology and Utilization Research » Research » Research Project #440182

Research Project: Accurate Evaluation of the Nutritional Value of Alfalfa

Location: Cell Wall Biology and Utilization Research

Project Number: 5090-31000-026-048-S
Project Type: Non-Assistance Cooperative Agreement

Start Date: Apr 16, 2021
End Date: Jan 11, 2022

Feed evaluation analyses for composition and digestibility are crucial to describing the value of alfalfa and other feeds for meeting the nutrient requirements of animals. The precision and accuracy of those measures impact the perceived value of individual feeds and how nutritionists apply the values in ration formulation. The actual variability in compositional analyses measured chemically or with near infrared reflectance spectroscopy (NIRS), separate from variability associated with subsampling, is not common knowledge for common analyses (crude protein, neutral detergent fiber [NDF], dry matter, ash, etc.). One objective will be to compile and publish information on actual analytical variability of compositional analyses. The utility of the determination of NDF digestion rates and extents of digestion using a three-pool (indigestible, fast and slowly digestible) versus current two-pool (indigestible and digestible) models is not known; the importance of this distinction is that the three-pool approach requires substantially greater time and analytical work and is more expensive. Determination of which approach is warranted is important for assuring justified use of resources in these evaluations. Thus, the second objective will be to determine whether NDF digestibility kinetics are accurately/ sufficiently described using two or three pools. It is important particularly for alfalfa because this forage appears to behave differently than grasses. The importance of evaluating these factors on alfalfa as well as on other feedstuffs is that all are used together in ration formulation and the variability/ utility of the values affects how nutritionists perceive the feeds and use them for formulation. Additionally, solid information on the variability and utility of the measures will inform commercial feed analysis laboratories and researchers of the need to further refine the assays to be more accurate and useful if such is possible.

For the objective addressing analytical variability, information on inherent variability of feedstuff analyses for loss on drying/ dry matter (DM; 100% minus loss on drying), crude protein (CP), ash, neutral detergent fiber (NDF), and ether extract will be derived from publicly available data sets from a laboratory testing program run by the Association of American Feed Control Officials and from published peer-reviewed ring test and collaborative studies. Analytical variability is defined as the standard deviation from the mean and coefficients of variation for different feedstuffs and methods for a given analyte. Cooperator will contribute their time to work with the ARS principal investigator to analyze the data and provide summary data in tables on the analytical variation by sample and provide a write-up to describe the impact and application of the values. For the objective addressing NDF digestibility described with two pools or three pools, 12 samples (six diverse alfalfa samples, three each warm season and cool season grasses) will be obtained, dried at 55°C, and ground through a 1 mm screen of a Wiley mill. Three researchers have agreed to provide diverse alfalfa samples. If these researchers cannot supply the forages as planned, other samples will be substituted. Forages will be sent out for professional subsampling into individual sample packets, this to help assure comparability of the replicates of materials. Samples will be evaluated using near infrared reflectance spectroscopy (NIRS) to evaluate subsample comparability. Samples will be sent to two separate commercial feed analysis labs for in vitro fermentation with rumen microbes in duplicate, in two fermentation runs, with fermentation blanks, at 11 time points (0, 3, 6, 12, 18, 24, 30, 48, 72, 120, 240 hours) followed by analysis for residual NDF; these results reflect what would be obtained by users in the field, and to determine variability within and between labs. Comprehensive compositional analyses including but beyond NDF (e.g., CP, water-soluble carbohydrates, acid detergent fiber, lignin, etc.) will be performed by a commercial lab to characterize the samples. Raw data on residual NDF from all sample and fermentation blank analyses from all hours of all fermentations will be sent to Cooperator who will process the data and fit two-pool versus three-pool non-linear curves and parameterize those curves (parameters: lag time, two- and three-pool sizes, kinetic rates associated with pools). Parameters will be integrated with physiologically likely passage rates to determine predicted ruminal NDF digestibility. Comparison of results will determine the deviation in predicted NDF digestibilities of each kinetic approach and the number of fermentation time points required to accurately determine the parameters. Magnitudes of differences between the approaches will be used to assess the comparability and practical utility of both approaches. Cooperator will provide a report of the findings. As their contribution, Cooperator will collaborate with the ARS principal investigator on interpretation of the findings and writing of the related peer-reviewed manuscript.