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Title: Advances in the Assessment of the Nutritive Value of Forages

item Burns, Joseph

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2010
Publication Date: 1/20/2011
Citation: Burns, J.C. 2011. Advances in the Assessment of the Nutritive Value of Forages. Crop Science. 51:390-402.

Interpretive Summary: An assessment was made of the early methodologies used in the estimation of the nutritive value of forages. Consideration was directed toward the method of proximate analysis, its partial replacement by the detergent fractionation of forages and the resulting fractions. The concept of ‘nutrient entity’ was addressed and integrated into the chemical fractions relative to nutrient value. The use of in vitro bioassays involving microbial assessment of nutritive value, which moves the methodology closer to the animal’s system, were discussed including the use of individual in vitro test tubes vs. batch and continuous culture systems. Limitations of some continuous culture systems were addressed. Gaps were noted in the transfer of nutritive value entities between the forage-animal interface and confinement feeding systems. Generally, forages are grouped as ‘roughage’ and ignored as unique sources of nutrients in ration formulation. Reassessment of traditional ‘contrived fractions’, relative to the ‘bugs’ view, is proposed and expanded upon. Contrived fractions reflect man’s view and are generated mainly from chemical degradation of the cell wall matrix. Proposed is an innovative and nondestructive approach using the information contained in the near-infrared spectra from natural particles of forage. The spectra is then related directly to in vivo animal responses and hence an estimate of forage quality. Finally, the challenge for the future is given to replace the traditional contrived fractions by spectral analysis.

Technical Abstract: The consumption of forages, which are primary producers, results in ‘value added’ animal products as meat, milk and fiber, and in some cases, recreation. How a forage might support the value added product of interest has been a concern since the mid-1800’s. The characteristic of a forage that composes its nutritive value (NV) and its consumption and conversion by the animal determines the quality of the nutrient entity and provides an estimate of its forage quality. The first system that emerged in the mid-1880’s to address forage NV was the Weende proximate analysis which separated forage into crude protein, crude fiber, ether extract, nitrogen free extract, ash and water with the concept of nutrient entity defined in the 1950’s. By the mid 1960’s, the neutral detergent fiber system partially replaced proximate analysis and the two, along with continued innovations in solubility, serve as methodologies for estimating NV. An assessment of NV from the microbial view point was introduced in the early 1960’s (development of the two-stage in vitro dry matter disappearance method) followed in the 1970’s by continuous cultures and both are addressed. Innovation in spectral technology introduced near infrared spectrophotometry in the mid 1980’s providing a rapid analysis of the NV fractions. A reassessment of the approach to estimating forage quality, based on the bug’s view of ‘particle value’, using near infrared spectral scans of the ‘as fed’ forage has been proposed.