Near Infrared Reflectance Spectroscopy (NIRS) is based on the assumption that the spectrum of radiation absorbed and emitted by the organic components of feed is similar between feeds of the same chemical and biochemical composition. It is calibrated on the basis of complex mathematical equations and computer prediction models which compare NIRS spectra of feeds to the chemical analysis. NIRS is useful in the evaluation and formulation of rations, but is of limited value when non-traditional feeds or forages from different regions than those used to calibrate the equipment are used. The technique is more rapid than traditional assays and relatively inexpensive. NIRS reports usually include dry matter, crude protein, acid detergent fiber (ADF), neutral detergent fiber (NDF), total digestible nutrients (TDN), net energy for maintenance (NEm) and lactation (NEl) of dairy cattle, Ca, P, Mg, and K.
 

These laboratory assays involve a variety of chemical and biochemical techniques to accurately measure mineral and biological materials. They are considered to be the most accurate (and traditional) methods of analysis. However, there are problems even with these standard techniques.
 

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.

Results of detergent system analysis usually include values for dry matter, soluble carbohydrates, neutral detergent (NDF), and acid detergent fiber (ADF), digestible protein and ash. Many laboratories routinely include it in their "basic“ feed analyses. This system does not estimate individual minerals or vitamins.

3. "Wet“ Mineral Chemistries
Laboratories conducting chemical determination of minerals most commonly use atomic absorption, calorimetric, or spectrophotometric techniques. Some techniques are more adequate for a given mineral than others, but in general most wet chemistries for the macrominerals are fairly accurate. Problems arise in trace mineral analyses, which are extremely susceptible to errors due to contamination and improper sampling either in the laboratory or at the farm. In a survey of commercial laboratories it was found that there were significant differences in the reported trace mineral content of duplicate samples of sweet feed (Ralston, 1992). If an analysis contains values that are higher or lower than anticipated, resubmit a sample of the feed to another laboratory before assuming that the feed is imbalanced.

Availability of the minerals to the animal, however, can only be determined accurately by digestion trials. For example, a ration suspected to be deficient in selenium, based on clinical signs exhibited by animals consuming it, is submitted for selenium analysis. The results come back as low normal selenium. This does not necessarily mean the animals are not suffering from selenium deficiency. The ration should be examined for substances that reduce the absorption or enhance the excretion of selenium, such as arsenic, aluminum, copper, and molybdenum. Unfortunately the interactions of minerals are quite complex and not entirely understood at this time.
 

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 laboratories.
 

The quality of hay is determined in large part by the moisture at which it was baled. All commercial laboratories will do determinations of moisture content. It is, however, sometimes advantageous to be able to dry a forage before submitting it for nutrient analysis. If this is done, the moisture content should be determined at the same time. Freeze drying is the most accurate method which results minimal alterations in the nutrient composition of the feed. If freeze drying equipment is not available, moisture determination can be performed with a commercially available moisture probe which is used directly on the bales or in the silos or by the microwave technique described below.

Determining moisture content of a forage in a microwave oven requires minimal equipment: a microwave oven, a postal or gram scale for weighing the samples, paper plates, and a glass of water. Weigh a paper plate for each sample to be analyzed and write the weight on the plate. Weigh approximately 3 ounces (100 gms) of chopped forage onto the plate (samples taken by a forage sampler will not have to be chopped). Spread the material evenly over the plate and place it in the microwave. Put a half-full glass of water in the microwave at the same time to reduce the chance of burning the sample. Pasture samples assumed to contain over 50% moisture should be heated at medium for four minutes initially. Hay samples (usually less than 30% moisture) should be heated for only three minutes initially. After heating, remove the sample from the oven and weigh it. Stir the forage on the plate and replace it in the oven for another minute. Weigh, stir, and reheat for 30 seconds. Continue drying and weighing until weight becomes constant. If the sample gets charred, you must redo the analysis on a fresh sample, using shorter drying times. The final weight divided by the original weight multiplied by 100 then subtracted from 100 will give you the percent water in the feed.