Nutritive Value of Feeds for Beef Cattle
Published on Wed, 04/05/2017 - 10:16am
Nutritive Value of Feeds for Beef Cattle
By David Lalman
Animals require chemical elements and compounds to sustain bodily functions, for skeletal and tissue growth, and to support the reproductive process. The necessary chemical elements and compounds are referred to as nutrients and can be classified into six categories: water, carbohydrates, lipids or fats, proteins, minerals, and vitamins. Requirements for these nutrients are discussed in ANSI-3010. The process of rumen fermentation allows ruminant animals to consume, digest, and acquire these necessary nutrients from a wide range of feed resources. Livestock producers must ensure that the animal’s diet includes the proper balance of the six essential nutrients in a physical form that maintains digestive system health and function. In order to accomplish this, knowledge of available feed nutrient composition, physical and digestive characteristics, and the animal’s nutrient requirements are necessary.
Nutritive value is determined by nutrient concentration and nutrient digestibility. Nutrient concentration and digestibility data can be determined using digestion trials or by measuring chemical composition and applying this information to estimate digestibility. Table 1 contains average composition values for numerous feeds that are common in Oklahoma. Values in the table represent averages from numerous different sources, such as the National Research Council’s Nutrient Requirements of Beef and Dairy Cattle publications, commercial laboratories, research trials, and other publications. Beef magazine also publishes a Feed Composition Guide that is updated annually. It is located at http://images.beef-mag.com/files/13/feedcomp03.pdf.
It is imperative that producers recognize that values published in any table are merely averages and that variation among feed commodities, oilseeds, and particularly forages and roughages can be extreme. For this reason, producers are advised to have their feeds and forages tested for nutrient composition by commercial laboratories. In an effort to improve quality control and standardization among commercial laboratories, the National Forage Testing Association (NFTA) provides a unique certification service. At http://www.foragetesting.org, one can also view NFTA’s recommendations for laboratory procedures and equations for use in predicting energy availability for different forage types.
Dry matter expresses the proportion of the feed that is not water. The moisture concentration is determined by weighing the feed sample soon after the sample has been collected. Next, the sample is placed in a drying oven until all of the water has been evaporated. Finally, the dried sample is weighed again and the dry matter content is calculated by difference.
The first method of determining carbohydrates in feeds separated carbohydrates into crude fiber and nitrogen free extract (NFE) fractions. The crude fiber portion of the feedstuff was intended to represent the indigestible fiber fraction and NFE was supposed to represent the more readily digestible carbohydrates, such as sugars and starches. However, it was soon discovered that this system had serious limitations, particularly for fibrous feeds like forages.
Because of the wide variation in chemical analyses for crude fiber and NFE, the detergent fiber system was developed, which better reflects true carbohydrate digestibility in ruminants. The neutral detergent solubles (NDS) fraction is comprised of cell contents and has digestibility of near 100%. The neutral detergent insoluble fiber (NDF) fraction is made up of primarily cell wall tissue, which consists of hemicellulose, cellulose, and lignin. The NDF fraction also contains small amounts of silica and fiber-bound or heat-damaged protein. The NDF fraction of feeds and forages is quite variable in digestibility. Using an acid solution, the NDF residue can be further separated into acid detergent solubles (primarily hemicellulose) and acid detergent insoluble fiber (ADF). The ADF fraction contains cellulose, which has variable digestibility, and lignin, which is nearly indigestible.
The digestibility of forage fiber is proportional to the amount of lignin in the plant material. This relationship explains why some forages and feeds contain high NDF concentrations, but remain high in digestibility, while others may contain moderate or low NDF concentrations yet are low in digestible energy.
The effective NDF (eNDF) value shown in Table 1 is a measure of the feed NDF that is effective in stimulating rumen motility or churning. The table expresses eNDF as a percentage of NDF. The layman term for eNDF is the “scratch” value of the feed. If the rumen stops churning, acidic gasses build up causing the pH to drop. The result is bloat, acidosis, and/or founder, as well as reduced diet digestibility. This value is determined by several factors including particle size, density, hydration, and degree of lignification. In order to maintain optimal forage digestion, the diet should contain a minimum of 20% eNDF on a dry matter basis.
Requirements for metabolizable, degradable, and crude protein (CP) were discussed in ANSI-3010. Protein values in the table reflect CP, which is simply nitrogen concentration multiplied by 6.25. The degradable intake protein (DIP) column is an estimate of the proportion of the crude protein that is actually degradable in the rumen and is expressed as a percentage of CP. Undegradable protein (% of CP) can be calculated by subtracting the DIP value from one.
Feed Energy Values
Feed energy values are expressed on a dry matter basis as percent total digestible nutrients (TDN), net energy for maintenance (NEm), and net energy for gain (NEg) units (mega calories per 100 lb of feed). TDN is determined by carrying out a digestion trial and summing the digestible protein and carbohydrates plus 2.25 times digestible ether extract. Ether extract (EE) is the fat or lipid portion of the feed. The net energy system is generally thought to be more precise in estimating the energy value of feeds, particularly roughages. The net energy of feed is the portion that is available to the animal for maintenance or various productive purposes. The portion used for maintenance (NEm) is used for muscular work, maintenance and repair of tissues, maintaining a stable body temperature, and other body functions. Most of this energy will leave the animal’s body as heat. The energy that is used for productive purposes (NEg) may be recovered as retained energy in the tissues or in some product, such as milk.
Minerals that are needed by animals in larger quantities are referred to as macro minerals. These minerals are shown in Table 1 and feed concentration is expressed on a percent of DM basis. Minerals that are needed by animals in much smaller quantities are referred to as micro minerals and feed concentration is expressed in parts per million (ppm) in the table. To convert ppm to percent, simply move the decimal place four places to the left. For example, if a feed contained 12 ppm copper, the copper concentration expressed as a percentage would be 0.0012%.
Producers have to ensure that their animals’ diets include the proper balance of the six essential nutrients in a physical form that maintains digestive system health and function. To accomplish this, producers must have good knowledge of available feed nutrient composition, physical and digestive characteristics, and the animal’s nutrient requirements.
Nutrient concentration and digestibility data can be determined by using digestion trials or measuring chemical composition and applying this information to estimate digestibility. It is imperative that producers recognize that values published in any table are merely averages and that variation among feed commodities, oilseeds, and particularly forages and roughages can be extreme. For this reason, producers are advised to have their feeds and forages tested for nutrient composition by commercial laboratories.
2003 Feed Composition Guide. (2003) Beef. Retrieved from http://images.beef-mag.com/files/13/feedcomp03.pdf.
NRC. (2000) Nutrient Requirements of Beef Cattle (7th Ed.). National Academy Press, Washington, DC.