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New Jersey Agricultural Experiment Station, New Brunswick
Abstract
Methods of partitioning the nutrients of feeds should be consistent with the present knowledge of the utilization and function of the various classes of nutrients. Ideally, protein ought to be represented by an analysis for true protein. Lipids should be analyzed by a method that determines truly metabolizable lipids. Carbohydrates ought to be separated from non-nutritive lignin and partitioned into at least two major fractions: (1) fibrous carbohydrate, digestible by symbiotic microorganisms in the digestive tracts of herbivorous animals; and (2) soluble carbohydrate, digested by enzymes secreted by the digestive system of all animals.
Critics of the proximate analysis have pointed out the failure of the crude fiber and NFE fractions to represent accurately carbohydrate materials. Lignin, the major contaminant of concern, is present in both crude fiber and NFE fractions in variable amounts. Van Soest (1964, 1967) emphasized the failure of the crude fiber and NFE to give a nutritional separation of fiber, and stressed the need for a clear nutritional definition of fiber, based upon nutritional availability, exclusive of chemical methods and physical properties. When the Lucas test was applied to various fractions used or proposed for partitioning forage nutrients, the results were consistent with other knowledge of their nutritional availability and uniformity.
The fibrous carbohydrates, cellulose and hemicellulose, were nearly identical in average digestibility, but there was greater variation in cellulose digestibility among forages. Holocellulose, by the combination of hemicellulose and cellulose, is a logical representation of fibrous carbohydrate and is recommended for future forage work with horses and other herbivorous animals. True digestibility of these fibrous carbohydrates is approximately equal to the apparent digestibility.
CWC contains lignin and part of the ash and protein along with the fibrous carbohydrates. CWC should be recommended in practical feed analysis to represent the fibrous fraction when the sacrifice of accuracy is not serious.
Cellular content is a good representation the total soluble fraction, but the content of soluble carbohydrate and protein are of greater interest for nutrition and the ash and ether extract contents must be determined to complete the analysis. Soluble carbohydrate, protein, ash and ether extract digestibilities studied by the Lucas test were all in reasonable agreement with previously reported estimations of true digestibility, and the deviation from regression (Sb) was small. The true digestibilities of the soluble fractions were 30 to 40% higher than average apparent digestibilities. Apparent digestibilites are dependent upon the nutrient content and have no value for comparing the availability of these nutrients among feeds.
Of the other fractions studied, ADF is recommended for separating cellulose, hemicellulose and lignin, but it does not possess nutritional uniformity because of its combination of cellulose and lignin and should not be used as a nutrient fraction. Continued use of crude fiber, NFE and other carbohydrate will only delay progress in determining the nutritive value of forage and feedstuffs for horses and other herbivorous animals.
1 Paper of the Journal Series, New Jersey Agricultural Experiment Station, Rutgers University, The State University of New Jersey, New Brunswick.
2 Present address: Department of Animal Science, Utah State University, Logan.
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