Analysis of Feeds and Forages for Horses
Sarah L. Ralston, VMD, Ph.D., dACVN, Department of Animal Science, Cook College, Rutgers University
Fact Sheet #714 - Reviewed 2004
However, protein, soluble fiber
and minerals in forages vary with climate, maturity, soil type, and
fertilization, and may differ significantly from the average values.
Regional averages based on common soil types and growing season may
be available from local extension agents or commercial analysis
laboratories, but even these may be inadequate if dealing with
horses with special needs. Forages fed to lactating, pregnant, or
growing horses should be analyzed rather than relying on published
averages if at all possible.
Other situations in which feed analyses are recommended include: 1) use of non-traditional or commercial mixed feeds for which complete analyses are not available, and 2) suspected nutritional problems based on clinical signs of imbalances or poor performance.
How to Sample FeedsGetting a sample that truly represents the overall feed is not always easy. Techniques differ according to the type of feed sampled and equipment available.
Ideally, a forage sampler (see Table 1 for vendors) should be used to drill core samples from 20 small bales or 20 sites on large, round bales of hay.
Samples (2 to 3 ounces each) should be taken from at least 20 sites and at a variety of depths in binned or bulk feeds. If sacks or bags are used to store the feeds, samples should come from at least two sites from ten bags. Commercially mixed grains which contain supplemental protein and/or mineral in powder form should be sampled only after thorough mixing or from both top and bottom of the bags to avoid bias due to settling of "fines". The samples should be mixed and a representative subsample submitted (at least 3 ounces) for analysis. Ideally, the scoop normally used to deliver feed to the animals should be used to obtain the samples.
To obtain representative samples from pastures, ten sites should be selected in areas utilized by the animals. Do not take samples from overgrown areas which are obviously not being grazed. Though easier to sample, these would not represent what the animals were consuming. If the entire pasture is being utilized in a fairly uniform fashion, you should mark sampling sites at regular intervals in an X pattern from each corner to insure a truly representative sample. Ideally, at each site a 1-foot-square area should be marked out and all forage within the square clipped to 0.75-inch height with stainless steel scissors. Do not, however, take plants which are obviously avoided by the animals. Samples from all sites should be mixed and 1/ 5 of the total subsampled for analysis.
It is important to ascertain if the
laboratory to which you are submitting samples is equipped to do the
requested analysis. It is also necessary to specify which analyses
you want performed, rather than submitting a feed sample with the
request to "analyze the nutrient content". Without specific
instructions, many laboratories will give more or less information
Ask local extension agents or feed stores if there are feed analysis laboratories in the region. Large feed companies will often perform forage analyses for their clients. Complete listings of certified laboratories are available from:
P.O. Box 371115
The most common nutrient concerns when balancing rations are the water, energy, fiber, protein, calcium, and phosphorus content of the feed. Virtually all feed analysis laboratories are equipped to provide estimates of these in one form or another. Most laboratories also provide analyses of sodium (Na), magnesium (Mg), chloride (Cl), potassium (K), copper (Cu), iron (Fe), zinc (Zn), manganese (Mn), sulfur (S), and cobalt (Co). Certain trace mineral analyses such as selenium (Se), iodine (I), aluminum (Al), and molybdenum (Mo) require special analytical methods and are not commonly available.
A. Near Infrared Reflectance Spectroscopy
B. "Wet Chemical" Analyses
1. Proximate Analysis
In a series of chemical extractions, the crude fiber, crude protein (Kjeldahl analysis of nitrogen content x 6.25), crude fat (ether extract), and ash (total mineral) content of feeds are determined on either a dry matter or as-fed basis. From these values the nitrogen free extract (NFE), which theoretically reflects the soluble carbohydrate content of the feed, is calculated. Total digestible nutrient (TDN) content of the feed is then calculated based on proximate analysis. Until recently it was considered to be the standard feed analysis, and virtually all feed analysis laboratories are equipped to perform it.
Despite its widespread use, there are many inaccuracies inherent in proximate analysis which limit its usefulness. Ether extraction removes not only fats but also waxes and other fat soluble materials, which may result in erroneously high estimates of fat content. Use of Kjeldahl analysis of feed nitrogen to derive the estimated total crude protein is based on several assumptions.
A. All proteins contain 16% nitrogen.
B. All nitrogen in a feed is in the form of protein.
C. The protein in the feed is totally digestible.
These assumptions frequently are not true, and protein availability may be grossly overestimated.
The Weende method of fiber determination commonly used in proximate analysis is considered to be inadequate for herbivorous animals in that it does not distinguish between the fermentable (cellulose and hemicellulose) and nonfermentable (lignin) components of fiber.
Calculation of nitrogen free extract compounds the above potential errors. Digestibility of protein and energy sources are not estimated by this technique. Proximate analysis does not measure individual minerals or vitamins.
Proximate analysis is still useful to obtain a rough, inexpensive estimate of a feed’s value. However, proximate analysis of fiber and protein is being rapidly replaced by the detergent fiber system in most laboratories.
2. Detergent Fiber (Van Soest) Analysis
This system is a modification of proximate analysis which uses improved methods for estimating the value of fiber and protein.
It has recently been accepted as the standard technique for analysis of forages for food animals. It uses a series of neutral and acid detergent extraction for analysis of fiber quality. Used in conjunction with Kjedahl analysis of nitrogen and a variety of other chemical treatments, detergent system analysis provides an estimate of the digestible versus indigestible portions of both fiber and protein in addition to soluble carbohydrates and ash. It is important to note that the “soluble carbohydrates” in this system include fibrous materials that are only available as energy sources to herbivorous animals such as horses and cows. Ether extraction is still used to give an estimate of fat content.
4. Vitamin Assays
Vitamin assays are much less available than
are mineral assays. The only vitamin routinely measured at
commercial laboratories is vitamin A. Usually assays for vitamin
A measure total carotenoids, which will overestimate the actual
amount of vitamin A activity, but at least gives an idea of the
vitamin value of the feed. With increasing use of high
performance liquid chromatography, assays for vitamin C,
B-vitamins, and vitamin E may become more available. Biologic
assays, using either animal or bacterial response to extracts of
the feed or substance in question, also are used to determine
concentrations of most vitamins. These assays are lengthy and
expensive. Vitamins which serve as co-enzymes in specific
biochemical reactions (ie: niacin, riboflavin, thiamine) may be
determined by measuring byproducts of the vitamin mediated
reaction by either calorimetric reactions or high performance
liquid chromatography (HPLC). These assays, however, require a
level of sophistication usually found only in research
C. Forage Moisture
When formulating or evaluating rations,
it is necessary to know the nutrient content of the ingredients.
While published average values for concentrates may be used, it
is strongly recommended that, when economically feasible,
forages be submitted for chemical analysis. To sample a feed, at
least 20 individual samples should be taken from a variety of
sites in the lot, thoroughly mixed and a subsample submitted for
nutrient analysis. To insure getting a truly representative
sample of hay, it is strongly recommended that a hay corer be
used rather than taking "grab samples". If grab samples are
taken or samples are mixed by hand, latex gloves should be worn,
especially if concerned about trace mineral content. It is
important that the type of analyses desired be specified. NIRS
is the most rapid and inexpensive analysis. It may be
inaccurate, however, if forages from outside of the region or
nontraditional feeds are submitted. Proximate analysis provides
a reasonable estimate of forage quality, but is not as accurate
as the newer detergent system of analysis. Vitamin A is the only
vitamin for which commercial laboratories commonly have assays.
National Research Council. 1982. US-Canadian Tables of Feed Analysis. National Academy Press, 2101 Constitution Avenue, NW; Washington, DC 20418.
Ralston, S.L. 1991. Principles of ration analysis. In: Naylor,
J.M.; Ralston, S.L. (eds). Large Animal Clinical Nutrition. Mosby
Yearbook Inc., St. Louis, MO.
Ralston, S.L.; Breuer, L. 1993. Variability in commercial
laboratory techniques and reliability of mineral analysis. Proc
13th. Equine Nutrition and Physiology Society Meetings. Gainesville,
Florida, 1992, p. 103.