Feed analysis: proximate and van Soest
How feeds are analysed: Weende proximate analysis, Kjeldahl protein, the van Soest detergent fibre system, dry matter conversions, and a hay quality sum.
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A dairy cow's ration, a broiler's feed and a sheep's hay all have to be balanced for energy, protein, fibre and minerals. To do that, nutritionists need to know what is in each feed. Two laboratory systems have dominated feed analysis for more than a century and a half: the Weende proximate analysis, devised in Germany in the 1860s, and the van Soest detergent system, developed in the United States in the 1960s to describe fibre better. Both are still in daily use, alongside faster near-infrared methods.
Feed analysis is a core topic in animal nutrition courses and exams.
Dry matter first
Feeds contain very different amounts of water: fresh grass about 75 to 85 per cent, silage 60 to 70 per cent, hay and grain about 10 to 15 per cent. Nutrient contents are therefore usually reported on a dry matter (DM) basis, and converted to an as-fed basis for practical rations:
- DM is found by drying a sample, for example at 100 to 105 °C, until its weight is constant.
- Nutrient (as fed) = nutrient (DM basis) × DM fraction.
Weende proximate analysis
Developed by W. Henneberg and F. Stohmann at the Weende experiment station near Göttingen around 1860, it splits dry matter into five fractions:
- Crude protein (CP): nitrogen measured by the Kjeldahl method (J. Kjeldahl, 1883) or by combustion (Dumas), multiplied by 6.25, on the assumption that protein contains 16 per cent nitrogen. It includes non-protein nitrogen such as urea.
- Ether extract (EE): material dissolved by ether, mainly fats and oils, plus some pigments and waxes.
- Crude fibre (CF): the residue after boiling in dilute acid and then dilute alkali, minus ash. It recovers cellulose and part of the lignin, but loses much of the hemicellulose and some lignin.
- Ash: residue after burning at about 550 to 600 °C, the total mineral content.
- Nitrogen-free extract (NFE): calculated by difference: 100 − (CP + EE + CF + ash). It is meant to represent sugars and starch, but also contains the hemicellulose and lignin lost from crude fibre, and all analytical errors.
The weakness of crude fibre, and hence NFE, for forages led to the detergent system.
The van Soest detergent system
Peter van Soest and colleagues at the US Department of Agriculture and later Cornell University developed it in the 1960s:
- Neutral detergent fibre (NDF): the residue after boiling in a neutral detergent solution: hemicellulose, cellulose and lignin, the total cell wall. NDF is closely related to feed intake, since cell walls fill the rumen.
- Acid detergent fibre (ADF): the residue after acid detergent: cellulose and lignin. ADF is related to digestibility.
- Acid detergent lignin (ADL): lignin left after strong sulphuric acid, indigestible and limiting digestion of other fibre.
By difference: hemicellulose = NDF − ADF, cellulose = ADF − ADL. The material dissolved by neutral detergent is the cell contents: sugars, starch, protein, fats and organic acids, almost completely digestible.
Taking a good sample
An analysis is only as good as the sample. Hay bales are sampled with a core probe from 15 to 20 bales of the same lot; silage from several points across the face or with a probe; grain from a moving stream or several depths of a bin. Subsamples are mixed, sealed quickly to prevent moisture loss, and sent to the laboratory promptly, chilled or frozen for wet feeds.
Other analyses
- Minerals: calcium, phosphorus, magnesium, trace elements, by atomic absorption or ICP spectroscopy.
- Starch, sugars and non-fibre carbohydrates.
- Gross energy by bomb calorimetry.
- In vitro digestibility, using rumen fluid (Tilley and Terry, 1963) or enzymes.
- Near-infrared reflectance spectroscopy (NIRS): fast prediction of many nutrients, calibrated against wet chemistry.
- Mycotoxins and anti-nutritional factors where relevant.
Analysing a hay sample
A grass hay sample contains 88 per cent dry matter. On a DM basis it has 12 per cent CP, 2.5 per cent EE, 30 per cent CF and 8 per cent ash. Kjeldahl analysis gave 1.92 per cent nitrogen.
- CP = 1.92 × 6.25 = 12.0 per cent (DM basis).
- NFE = 100 − (12 + 2.5 + 30 + 8) = 47.5 per cent.
- CP as fed = 12.0 × 0.88 = 10.6 per cent.
Detergent fibre: NDF 55 per cent, ADF 35 per cent, ADL 5 per cent (DM basis).
- Hemicellulose = 55 − 35 = 20 per cent; cellulose = 35 − 5 = 30 per cent.
Relative feed value (RFV), an index used in North American hay markets:
- Digestible dry matter (DDM, %) = 88.9 − 0.779 × ADF = 88.9 − 27.3 = 61.6.
- Dry matter intake (DMI, % of body weight) = 120 ÷ NDF = 120 ÷ 55 = 2.18.
- RFV = DDM × DMI ÷ 1.29 = 61.6 × 2.18 ÷ 1.29 ≈ 104.
An RFV of 100 corresponds to full-bloom alfalfa; higher values mean better hay.
Key points
- Report nutrients on a dry matter basis; convert to as-fed using the DM fraction.
- Weende proximate analysis: CP (N × 6.25), EE, CF, ash, and NFE by difference.
- Crude fibre underestimates fibre; NFE includes hemicellulose and lignin lost from CF.
- Van Soest: NDF (cell wall; relates to intake), ADF (cellulose plus lignin; relates to digestibility), ADL (lignin).
- Hemicellulose = NDF − ADF; cellulose = ADF − ADL.
- NIRS and in vitro digestibility give fast, practical feed evaluation.
What the exams ask
- School-leaving and entrance papers: feed nutrients and their functions, and dry matter.
- Degree and research-fellowship entrances: steps of proximate analysis, Kjeldahl method, limitations of crude fibre and NFE, the van Soest system, and calculations.
- Professional examinations: interpreting feed reports, forage quality indices, and choosing analysis methods.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why multiply nitrogen by 6.25?
Proteins average about 16 per cent nitrogen, and 100 ÷ 16 = 6.25. Some feeds differ: milk proteins are closer to 6.38 and many cereals to 5.7, so specific factors are used in food analysis.
Why is NDF important for cows?
It measures the bulky cell wall that fills the rumen, so it limits how much a cow can eat. Enough NDF of the right physical form is also needed for rumination and a healthy rumen.
Is crude fibre still used?
Yes, in feed labelling laws in many countries and for monogastric feeds, but for ruminant forages, NDF and ADF are far more useful.
Sources
- McDonald, P. et al. Animal Nutrition, 7th edition. Pearson, 2011. Chapter 1.
- Van Soest, P. J., Robertson, J. B. and Lewis, B. A. "Methods for dietary fiber, neutral detergent fiber, and nonstarch polysaccharides in relation to animal nutrition." Journal of Dairy Science 74, 3583–3597 (1991).
- AOAC International. Official Methods of Analysis, 21st edition. AOAC, 2019.
- Tilley, J. M. A. and Terry, R. A. "A two-stage technique for the in vitro digestion of forage crops." Journal of the British Grassland Society 18, 104–111 (1963).
- Rohweder, D. A., Barnes, R. F. and Jorgensen, N. "Proposed hay grading standards based on laboratory analyses for evaluating quality." Journal of Animal Science 47, 747–759 (1978).
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