Wet chemistry uses chemicals and heat to digest feed into the various nutrient components. Near Infrared Reflectance Spectroscopy (NIRS) analyzes the light spectrum which is reflected off of a feed when it is exposed to infrared light. Each nutrient has unique absorption and reflection characteristics.
Feed and forage analysis is only as good as the sample of feed. It must be representative.
Feeds and forages are analyzed for dry matter (DM), crude protein (CP), soluble protein (SIP), degradable intake protein (DIP), undegradable intake protein (UIP), unavailable protein (ADF-CP), the Cornell Net Carbohydrate/Protein System protein fractions (A, B1, B2, B3, and C), neutral detergent fiber (NDF), NDF digestibility (dNDF), acid detergent fiber (ADF), crude fiber (CF), lignin, crude fat (ether extract (EE)), and minerals. Non-structural carbohydrates (NSC or NFC), relative feed value (RFV), and energy (TDN, ENE, or NEl) are calculated.
It is recommended that concentrate feeds be analyzed as they arrive on the farm if they come from somewhere other than a reputable feed company. All forages should be analyzed on a regular basis based on field, harvesting, and storage variability.
How Are Feeds and Forages Analyzed?
A. Wet Chemistry
Wet chemistry is the traditional way of analyzing feeds. It is generally regarded as the most accurate. With wet chemistry, the various nutrients in the feed are isolated using chemicals and heat to digest the feed. Generally, samples are weighed before and after chemical extraction of nutrients to determine the amount (percentage) of each nutrient in the feed or forage. The problem with wet chemistry is that it is time-consuming, involves expensive chemicals, and needs a fair amount of skilled labor.
B. Near Infrared Reflectance Spectroscopy (NIRS) –
With NIRS a spectrophotometer analyzes the light spectrum which is reflected off of a sample when it is exposed to infrared light. Each of the components (DM, CP, ADF, ADF-CP and NDF) that we normally isolate chemically has unique absorption and reflection characteristics based on its molecular structure (carbon, nitrogen, and hydrogen bonds). NIRS is fast and inexpensive. For day-to-day purposes on the dairy farm, it is generally accurate enough.
It is important that the NIRS system be calibrated with determined wet chemistry values for feeds and forages. Generally, NIRS is fairly accurate within the particular region from where its calibration feeds and forages came from. But, a sample from Mexico, for example, should not be run by NIRS in a lab in New York unless the New York lab has specifically calibrated its NIRS system with samples from Mexico.
Minerals such as calcium and phosphorus cannot be directly estimated using the NIRS system because they don’t reflect infrared light. Labs do try to estimate the mineral content of feeds indirectly based on their typical relationship with other nutrients in the particular feeds. But, generally mineral analysis reports from an NIRS analysis are not considered to be very accurate.
The feed or forage analysis will only be as good as the sample that is taken. For hay, sample multiple bales using a hay auger. For grain, sample multiple sites using a grain thief. For silage out of a bunker, it is best to get multiple bucket loads from various points on the bunk face, mix them in a mixer wagon, and then sample from the mixer wagon. If a mixer is not available, take multiple grab samples, mix in a bucket, and then take a sample.
The dry matter content of a feed or forage is one of the most important analyses to be done due to the fact that the cow obtains all of her nutrients (with the exception of water) from the dry matter. Rations are usually balanced on a dry matter basis with a predicted dry matter intake. Forages should always be compared on a Dry Matter Basis not on an As-Fed Basis.
As-Fed Basis analyses include the water content of the feed. This dilutes the percentage of nutrients in a feed. Commercial grains which are 90% DM, are normally purchased and compared on an As-Fed Basis. For example, an 18% Complete Feed contains 18% CP on an As-Fed Basis but 20% CP on a DM Basis (18/0.90 = 20).
Dry Matter (DM)
The dry matter of a feed is determined by placing a feed sample in a 221oF (105oC) oven until a constant weight is reached (usually overnight). The loss in weight equals the moisture content of the sample. For silage samples, normal variation in the DM analysis results in a possible error of up to +/- 3.7 units across forage labs.
On the farm, DM can be determined using a Koster Tester which dries feed samples with forced hot air or using a microwave oven. With either of these methods, it is important that the feed sample be thoroughly dried. Generally, if the sample weight is the same on three consecutive weighings with heat applied in between, the sample is dry.
The moisture content refers to the amount of water in a feed or forage.
Wet Weight – Dry Weight
% Moisture = Wet Weight x 100
% Dry Matter = 100 - % Moisture
Proteins are defined as chains of 50 or more amino acids. Protein is needed to maintain the cow, especially for muscle development and milk production. Historically, nutritionists have balanced for a specific amount of crude protein (CP) in a ration. Then, soluble protein, degradable intake protein, and undegradable intake protein were defined and rations were balanced for those protein fractions. The Cornell Net Carbohydrate/Protein System further fractionated feeds into A, B1, B2, B3, and C. Now, nutritionists are further breaking down protein into individual amino acids for even more fine-tuning and higher levels of milk production and production efficiency.
Crude Protein (CP)
The crude protein content of a feed is obtained by multiplying the nitrogen content by the factor 6.25. This assumes that protein is about 16% nitrogen (100/16 = the factor, 6.25). The nitrogen content of a sample is traditionally determined by the macro-Kjeldahl procedure. The sample is first digested with sulfuric acid and the nitrogen is converted to ammonium sulfate, then the ammonium sulfate is converted to ammonia gas and trapped in acid by a distillation procedure, and finally the amount of ammonia in the acid is determined by titration. Expected variation in CP values across labs is +/- 0.84 units for corn silage and +/- 1.52 units for haycrop silages.
Non-Protein Nitrogen (NPN) sources, such as urea and ammonia, are not actually proteins yet they are given very high CP values. Since urea contains 45% nitrogen, it contains 281%CPE (“Crude Protein Equivalent”) (45 x 6.25). This nitrogen is converted to protein by the rumen microbes. Microbial protein is a source of protein for the cow.
Soluble Protein (SIP)
The amount of soluble protein gives an indication of how rapidly crude protein in a feed will be used by the rumen microbes. Soluble protein is more likely to be readily used by the microbes than insoluble protein is. Insoluble protein is determined by incubating a feed sample in a buffer solution for an hour and determining the CP content of the residue that is left. The soluble protein is then determined by difference and expressed either as a proportion of the DM or the CP. Soluble protein is comprised of ammonia (NH3), nitrates (NO3), peptides, and some true proteins. It is present in large amounts in alfalfa silage. Urea is 100% soluble. SIP is part of the DIP fraction.
Degradable Intake Protein (DIP)
Degradable Intake Protein is the amount of protein that is digested in the rumen. It includes true proteins that are more slowly digested in the rumen, in addition to the soluble protein fraction. It is measured in laboratories by incubating the feed with an enzyme which is intended to digest feed protein at a similar rate as the rumen microbes, for the amount of time that the feed is predicted to normally remain inside the rumen of the cow. DIP can also be estimated using the in situ procedure. In this procedure, feed samples are placed in a dacron bag with a specific pore size and then hung in the rumen of a fistulated cow for the length of time that the feed is predicted to normally reside in the rumen (16 hours for concentrates and 30 hours for forages). The rumen microbes go into the bag and digest the feed and the insoluble (undigested) feed is left inside the bag.
Undegradable Intake Protein (UIP) (Bypass Protein) (Escape Protein)
Undegradable Intake Protein is the protein that is not used by the rumen microbes. UIP may either be absorbed in the small intestine or part of it may not be used at all by the cow and will show up in the manure. UIP is all of the protein that isn't rumen degradable.
UIP (%CP) + DIP (%CP) = 100%
Unavailable Protein (ADF-CP) (Bound Protein) (ADIN)
Unavailable protein is measured by boiling feed in an acid detergent solution and determining the protein content of the residue. Unavailable protein is assumed to be of no use to the cow. UIP includes unavailable protein. In forages and feeds, especially haylage and distillers grain, unavailable protein is often formed as a result of extensive heating.
Commercial laboratories usually report a value for “available crude protein” which is CP minus unavailable protein. “Effective crude protein” is calculated by multiplying CP by a discount factor according to the amount of ADF-CP.
The Cornell Net Carbohydrate/Protein System uses five protein fractions
(A, B1, B2, B3, and C) rather than SIP, DIP, and UIP:
Fraction A is the soluble non-protein nitrogen. It is assumed to be immediately available to the rumen microbes.
Fraction B1is the soluble peptides (short chains of amino acids) and soluble true protein. It is rapidly available to the rumen microbes.
Fraction B2 is the protein which is not soluble in buffer but is soluble after boiling the feed or forage in neutral detergent solution. It is intermediate in availability to the rumen microbes.
Fraction B3 is the protein which is not soluble after boiling the feed or forage in neutral detergent solution but is soluble after boiling the feed or forage in acid detergent solution. It is slowly available to the rumen microbes.
Fraction C is the protein which remains insoluble after boiling a feed or forage in acid detergent solution. This is the same as unavailable protein and passes through the cow into her manure.
Plant nutrients can be classified as either cell contents (starches and many plant proteins) which are generally rapidly digested in the rumen or cell walls (also known as fiber) which are slowly digested by the rumen microbes. Fiber is a complex carbohydrate which is bonded in such a way that the cow’s enzymes cannot break it down. Only microbes living in the cow’s rumen can break down the bonds and digest (or ferment) fiber.
The detergent system is used to analyze for fiber in feedstuffs. Fiber is important for maintaining normal rumen function. However, excess fiber in a ration will limit nutrient intake and ration digestibility.
Neutral Detergent Fiber (NDF)
Neutral detergent fiber is determined by boiling a feed sample in neutral detergent solution and weighing the residue that is left. Normal variability in NDF analyses across labs can be +/- 2.8 units for high quality forages, and up to +/- 3.3 units for higher fiber forages. NDF contains hemicellulose, cellulose, and lignin. Hemicellulose is fairly digestible, cellulose is less digestible, and lignin is indigestible in the rumen. Hemicellulose is a branched chain of different types of sugars. Its non-uniformity makes it relatively more digestible in the rumen. Cellulose is a linear chain of one type of sugar called glucose. Cellulose chains become tightly compacted in plants and are therefore, more difficult for the rumen microbes to digest. Lignin is actually a polymerized product of many non-carbohydrate compounds. Wood is highly lignified. Lignin can only be broken down by burning or by aerobic microorganisms.
NDF Digestibility (dNDF)
An estimate of NDF digestibility can be obtained by in vitro laboratory procedures using microbes taken from the rumen of a fistulated cow or by in situ procedures in which feed or forage samples are hung in a dacron bag inside the rumen of a fistulated cow. Samples are digested by the rumen microbes for the amount of time that they are expected to normally reside in the rumen (18 hours for concentrates and 30 hours for forages). NDF digestibility can be very helpful in predicting the energy value of a feed as well as the amount of NDF that the cow can be expected to eat in a day. NDF digestibility is not directly related to the amount of ADF or NDF in a feed or forage.
Acid Detergent Fiber (ADF)
Acid Detergent Fiber is determined by boiling a feed sample in acid detergent solution and weighing the residue that is left. For corn silage, expected variation in ADF values across labs is +/- 1.5. Expected ADF variability for haycrop silage is +/- 2.2 units. ADF contains cellulose and lignin. Because ADF is negatively related to digestibility, it has historically been used to predict the energy value of feeds. New energy equations are not based solely on ADF because of new understanding and measurement of NDF digestibility.
Crude Fiber (CF)
Crude Fiber is still sometimes reported on feed analysis reports and on commercial feed tags (guarantees). Most nutritionists no longer use crude fiber for the purpose of ration balancing because it doesn’t consistently relate to the digestibility and intake of feeds. ADF and NDF have replaced CF. Crude Fiber contains all of the cellulose in a feed or forage but only part of the hemicellulose and lignin, depending on the type of feed or forage and its maturity.
Lignin is determined by soaking the acid detergent residue from a feed or forage in concentrated sulfuric acid for 3 hours and then measuring the residue that is left. Lignin is undigestible. Lignin also binds with some of the cellulose in the plant, making the cellulose undigestible as well. Some nutritionists take this into account when balancing rations.
Fat is an energy dense nutrient containing 2.25 times the amount of energy generally found in carbohydrates. Most feeds and forages contain a low level of fat. Some concentrates like whole roasted soybeans, distillers’ grains, and whole cottonseed, contain significant amounts of fat. Fat is also fed to cows in the form of tallow and calcium salts of fatty acids, like Megalac®.
Crude Fat (Ether Extract)
Fat is generally determined by an ether extraction. Unfortunately, since ether may also solubilize plant pigments, esters, and aldehydes, the term “crude fat” is more appropriate.
Non-Structural Carbohydrates (NSC) or Non-Fiber Carbohydrates (NFC)
Non-Structural Carbohydrates or Non-Fiber Carbohydrates usually make up 35-40% of the dry matter in a ration designed for high production. NFC is calculated by difference [100-(%NDF + %CP + %Fat + Ash)]. NFC is generally more rapidly digested than fiber. It is a significant source of energy for the rumen microbes. The microbes also use NFC to make microbial protein.
Non-Fiber Carbohydrate is made up of different amounts of simple sugars, beta-glucans, galactans, pectins, and starch.
Simple sugars (like in maple syrup and molasses) are rapidly fermented in the rumen to the stronger acids, propionic acid and lactic acid. Simple sugars are not present in large amounts (4-6% of the ration DM) in traditional dairy cattle diets.
Beta-glucans and galactans (like in kidney beans – the musical fruit!) are present in beans and in small amounts in some grains fed to cows.
Pectins (like in jelly) are present in feeds like beet pulp, citrus pulp, and alfalfa. Beta-glucans, galactans, and pectins, also referred to as soluble fiber, must be fermented in the rumen since starch enzymes in the intestine cannot break them down. Soluble fiber is fermented rapidly but yields a weaker acid called acetic acid, as a by-product. Thus, soluble fiber generally does not lower rumen pH and cause acidosis as easily as starches and sugars.
Most of the NFC is made up of starch (28-32% of the total ration DM). Starch digestibility, both in the rumen and intestine, has a crucial impact on milk production. Cereal grains, such as corn and barley, provide the greatest proportion of the starch in a cow’s diet. Starch is made up of units of a sugar, called glucose, which are bonded together. Depending on the starch source and processing, the glucose units may be very tightly bonded and compacted together or they may be weakly linked together. For this reason, different starches may be either rapidly or slowly fermented in the rumen.
Notes on NFC:
Some laboratories distinguish between NFC and NSC. They define NFC by the above equation [100-(%NDF + %CP + %Fat + Ash)]. They define NSC as only the starches and sugars in the feed or forage. Starches and sugars can be determined directly by enzymatic analysis.
Also, The Cornell Net Carbohydrate / Protein System defines NFC as [100-((%NDF - %NDF-CP)+ %CP + %Fat + Ash)]. This equation is actually more correct because it doesn’t double-count the protein that is contained in NDF.
Relative Feed Value
Relative Feed Value (RFV) was designed to give one number from which forages could be compared. It is based on the amount of ADF and NDF in a forage. The lower the ADF and NDF is, the higher the RFV is, and the higher the quality. RFV of 100 is average, 150 is very good. Alfalfa hay containing 41% ADF and 53% NDF would have an RFV of 100. Most nutritionists do not rely on RFV values. The amount of CP, SIP, ADF, NDF and minerals in a forage is more descriptive and more useful for nutritionists to use.
Energy is required by the cow for maintenance, growth, reproduction, and lactation. Energy is not directly measured in feed. It is predicted based on the amount(s) of other nutrients, such as protein, fiber, nonstructural carbohydrates, and fat. Unfortunately, labs, feed industry competitors, and university researchers all use slightly different equations to predict energy. This can be confusing.
Total Digestible Nutrients (TDN) (%) is the sum of digestible crude fiber, digestible crude protein, digestible fat multiplied by the factor 2.25, and digestible nitrogen-free-extract (NFE). Theoretically, feeds or forages must be fed to a cow and analyzed by a complete digestion trial to determine TDN. If no part of the cow’s ration comes out the other end, the TDN value of the feed would be 100%. But, of course, that doesn’t happen! Many of the TDN values for feeds were assigned by Frank Morrison at Cornell University and were first published in the 1936 edition of his book “Feeds and Feeding”. Today, equations have been developed to predict TDN based on the amount of other nutrients in the feed or forage, most commonly ADF.
Estimated Net Energy (ENE) (Mcal/lb) accounts not only for the amount of digestible nutrients in a feed or forage but also for the amount of energy which is wasted by the cow and not used for productive purposes, such as in the form of heat. Morrison also developed tabular values for the ENE content of feeds and forages.
Net Energy for Lactation (NEl) (Mcal/lb)accounts for the amount of digestible nutrients in a feed or forage fed to a milking cow whose energy needs are 3 times what they would be if she were just in a “maintenance state”. It also accounts for the nutrients that are used for non-productive purposes (such as heat) and the efficiency with which the cow uses energy for milk production, pregnancy and maintenance.
The only way to actually measure NEl is to put a cow in an enclosed chamber and then measure everything (including heat and gasses) going into the cow and coming out. Of course, that’s expensive so it is rarely done. So, many tabular values are used. NEl values are calculated for forages and some grains based on their ADF content. Recently more sophisticated equations have been developed which predict NEl based on the energy derived from protein, digestible fiber, NSC, and fat.
For a lactating dairy cow, NEl is the only energy estimate needed for ration balancing. This is because the efficiency with which energy is used for milk production, pregnancy and maintenance is similar.
For beef cows and for growing dairy heifers, feed programs estimate energy requirements and supply in terms of Net Energy for Maintenance (NEm) (Mcal/lb)and Net Energy for Gain (NEg) (Mcal/lb). NEm is the energy value of a feed if it is used only for maintaining a cow in a steady state. NEg is the energy value of a feed if it is used for growth. Beef cows and heifers have requirements for both NEm and NEg.
The Ash content of a feed or forage is a measure of the total amount of all minerals in it. Samples are burned at 600oC (1112oF) for two hours to burn off all of the organic matter. What is left is just ash, the inorganic, non-burnable, mineral portion of the feed or forage.
Macrominerals Commonly Analyzed For:
Macrominerals are required by the cow in larger amounts and they are found in larger amounts in the cow’s tissues.
Trace Minerals Commonly Analyzed For:
Trace Minerals are required by the cow in smaller amounts and they are found in smaller amounts in the cow’s tissues.
Dairy One. Understanding and Significance of Forage Analysis Results.
Chase, L.E. 1987. NIRS-A New Technology for Feed Analysis. Proceedings of the Advanced Dairy Nutrition Seminar for Agribusiness.
Hoffman, K. Too Many Components, The Forage Analysis, Cornell Cooperative Extension Service.
Van Soest, P.J. 1982. Nutritional ecology of the ruminant. O&B Books, Inc., Corvallis, OR.
Testing Forages for Accurate Ration Formulation
C.R. Staples, Ph.D., University of Florida
Using a Microwave Oven to Determine Moisture of Forages
Explains the procedure for using a microwave oven to test forage moisture.
Sampling Feeds for Analysis
B. Anderson, T. Mader, and R. Grant, University of Nebraska - Lincoln
Provides practical guidelines for obtaining representative samples of hay and silage for feed analysis. Covers different types of storage systems.
Sampling Corn Silage for Analysis
Outlines a general procedure for sampling corn silage for feed analysis.
Feedstuffs In: Feeding the Dairy Herd North Central Regional Extension Publication
J.G. Linn et al.
Feed Nutrients In: Feeding the Dairy Herd North Central Regional Extension Publication
J.G. Linn et al.