Since the quality of forage directly impacts the production success, there are many ways to test forage. The first term to be understood is dry matter (DM). When forage is harvested by machine, the amount above the leftover stubble can be dried, since 70-90% of the herbage is water, and weighed to determine the amount of dry matter, usually referred to per acre. Testing can be done on representative samples. But if the forage is consumed by grazing animals there are still other ways to test for quality. Forage quality can be determined by many ways but the three main methods are discussed below:

  • Organoleptic Observation
  • Chemical Composition
  • Feed Trial Evaluations

Organoleptic observations means using the sense organs (eyes, nose, taste, ears, touch) to evaluate the quality of the forage. This method is practical because it can be easily done, requires no special equipment, and is readily applied. It is the simplest method but it provides the least information. Organoleptic observation can be useful in some ways but cannot determine chemical composition. Visually, high-quality forage should show leafiness, vivid color, little foreign material, an appropriate stage of maturity, and no molds. The smell should be fresh and in the case of silage sweet; a specific odor that you can learn to recognize. Livestock use their sense of smell in selection. Some producers can even taste the right flavors, especially in good silage. Touch can also be used to check for the right consistency. Good hay should not be too brittle. Good silage will not be slimy.

The stage of maturity at cutting or grazing influences quality more than the species, variety, production location, soil fertility, or seasonal influences. Early harvested alfalfa hay (cut at pre-bud or early bud stage) or grass harvested at boot or head emergence will have the highest nutritive value for livestock. The right time for harvest also makes the forage more palatable and digestible. Both decrease as the crop grows older. But yield continues to improve as the plants mature so there is a constant balancing act for producers between getting the best quality and the best yield. However, on a yearly basis, early cut hay yields as much feed (digestible dry matter) per acre as later-cut hay. Livestock will eat more of it because it is so palatable and digestible. Harvesting at the best times also wisely utilizes the regrowth potential of grasses and legumes.

Two-thirds of the protein (the desired portion of food) is found in leaves of forages. Livestock will naturally select the leafy portions when grazing but retaining leaves is more difficult in hay and silage making. Leaf shatter during raking and baling greatly reduces hay quality, especially of legumes. Look for lots of leaves in a bale to indicate good quality.

A laboratory evaluation of the chemical composition of forage allows the livestock manager to more accurately determine how much forage and supplement are needed for a particular animal and production goal. It can answer how nutritious a forage is and allows for better rationing. But the results are only valid to the extent that the sample represents the forage being considered. Good hay evaluation requires 20 core samples taken horizontally , 12-18 inches (30.5-45.7 centimeters) deep from random bales. Cubed hay needs 40 cubes chosen randomly from a lot of 200 tons or less, placed in air-tight containers for shipment. Freeze ten spots of exposed silage or several clipped, equally-sized samples from various locations from the pasture for a good forage evaluation. The samples are weighed in the testing lab, dried at 140 degrees F (60 degrees C) for 24 hours, allowed to air equilibrate for 24 hours, and weighed again. Subsamples are used for various tests. One subsample is used for drying at 221 degrees F (105 degrees C) for 24 hours. This sample is weighed while hot and determines the dry matter content (DM). Forage quality often refers to the DM

Historically from about the mid-1800's, forage quality has been measured in the laboratory by proximate analysis which looks at the crude fiber (CF), ether extract, nitrogen-free extract, crude protein (CP), and ash. The ash is the residue after burning or combustion and indicates the mineral content. The CP is determined by analyzing for nitrogen (N) by a method invested by Kjeldahl. True protein is made up of amino acids with approximately 16% nitrogen so 100% divided by 16% = 6.25 which is used to multiply the nitrogen in a sample to estimate the amount of protein within. Ether extract is the total compounds that can be extracted with hot diethyl ether. Crude fiber is the organic matter insoluble in weak acid and weak alkali. Nitrogen-free extract is the amount left when subtracting all the four components from 100 or the total sample.

The quality of forages has mainly utilized crude protein and crude fiber. Protein is critical for animal health but crude protein does not indicate the protein available to the animal. Other testing methods help determine this.

Crude fiber is calculated by treating a sample with ether to remove fats, then boiling it alternately in a weak acid and a weak alkali or base. The remains will be crude fiber and ash which is removed. Crude fiber contains cellulose, himicellulose, and lignin. Cellulose and hemicellulose are both partially digestible but lignin is not digestible. Forages that have more than 18% dry matter crude fiber are called roughages. Concentrates have less than 18% dry matter crude fiber.

An alternative analysis was developed by Peter J. Van Soest in the 1960's. This alternative more accurately predicts what nutrients the animals can use by distinguishing between cell walls and cell contents. Cell walls include cellulose and hemicellulose which are less digestible and lignin and silica, largely undigestible. Cell contents, include sugars, starches, vitamins, and minerals which are almost all digestible. This methods results in a neutral detergent fiber (NDF) and an acid detergent fiber (ADF). These can then lead to calorie or megacalorie quantities.

Neutral detergent fiber (NDF) is determined when a sample is extracted with a neutral detergent solution. The cell contents are largely soluble and the cell wall components are insoluble. The NDF score can predict dry matter intake (DMI) because a high NDF score means the animal feels full longer because certain components are taking longer to be digested. If the animal feels full, it eats less. Corn grain is roughly 10% NDF which means it is nearly 90% digestible. Straw is about 20 - 50% digestible with a NDF of up to 80%.

A detergent solution containing sulfuric acid is used in acid detergent fiber (ADF) analysis. Those cell components, like lignin, that are not soluble in this acid are even more useless to the animal so ADF scores predict digestible dry matter (DDM). High ADF scores indicate that little is being digested. ADF ranges from 3% in corn grain to 40% in mature forages and 50% in straws. Using the NDF and ADF scores an estimated digestible dry matter (EDDM) score can be obtained. EDDM = 88.9 - (o.779 x ADF%) which is comparable to the total digestible nutrients (TDN).

Near infrared reflective spectroscopy (NIRS) is another method of testing the quality of forage. After start-up costs, it is advantageous because it is relatively inexpensive, very fast, and samples are easily prepared. It can feasibly test for DM, CP, ADF, and NDF by performing multiple analyses. The process is based on different chemical constituents reflecting differently under infrared light. The accuracy does depend on accurate calibrations and technician competency.

Another way of determining forage quality is with feed trials and animal performance. Are animals gaining weight, producing more milk, successfully reproducing, and living longer? All of these take time to determine but can be measurements of successful feeding systems. The laboratory methods can be costly and require proper sampling but are faster and more specific. Sometimes cows are fistulated to determine exactly what is being consumed or excreted. Fistulated cows have an opening into their stomach. Comparing what is within the cow and later what is excreted can lead to better understanding of nutrition. But this type of evaluation is unpleasant and not as easily extrapolated to other animals.

Vitamins and minerals are not always tested for in all analyses. Often deficiencies or toxicities are determine by observations or symptoms.