It’s More Than Corn: Digestibility and Energy
This next section will try to help explain the components involved when formulating a ration.
What is the difference between a concentrate and roughage?
What is TDN?
TDN is a figure that indicates the relative energy value of feed to an animal and is expressed in pounds and in percent. There are four factors that affect TDN: % dry matter, digestibility of the Dry Matter, amount of mineral matter in the digestible dry matter and the amount of fat in digestible dry matter. Basically, if you take a feedstuff and subtract the water and all of the non-digestible components, what you are left with is the TDN.
Feeds are grouped based on their percentage of TDN for cattle on an as-fed-basis.
Group A – These are pure fats, high fat feeds, and high in digestibility: soybean oil, dried whole milk, cotton seed, soybean seed, and dried bakery product to mention a few.
Group B – These are mostly low fiber grains, grain byproducts, protein feeds and dried citrus pulp: dried brewers grains, corn gluten meal, soybean meal, wheat, corn, triticale, barley, rye, peanut meal and dried beet pulp.
Group C – These are largely more fibrous grains, byproduct concentrates and beet molasses: oats, cottonseed meal, fish meal, tankage, dried alfalfa leaves, rice bran and wheat middlings.
Group D – These are predominately hays, straws, hulls and corn cobs: oat hay, meadow hay, Kentucky bluegrass, alfalfa, timothy, bromegrass, fescue, orchard grass, ground corn cobs and wheat straw.
Group E – These are for the most part high moisture and high mineral feeds: corn silage, sweet potatoes, fescue pasture, orchard grass pasture, oat silage, fresh cow’s milk, wheat silage and rice hulls.
Why is energy important?
Energy values are essential in making certain the animal is receiving enough energy to meet their needs, based on their use. Energy is broken down into five areas.
- Gross energy (GE) is the total heat of combustion of a feedstuff as determined with a bomb calorimeter. The GE value of a feed has no relationship to the feed’s digestible, metabolizable or net energy values.
- Digestible energy (DE) is the portion of the energy of a feed which does not appear in the feces.
- Metabolizable energy (ME) is that portion of the GE consumed which is utilized by the animal for accomplishing work, growth, fattening, fetal development, milk production and heat production. It is the portion of GE that does not appear in the feces, urine and gases of fermentation. It is the DE minus the energy of the urine and methane.
- Net energy (NE) is the portion of the ME which may be used as needed by the animal for work, growth, fattening, fetal development, milk production and heat production. It differs from ME in that NE does not include the heat of fermentation and nutrient metabolism. No energy is used for heat production unless heat over and above that from other sources is required to keep the animal warm.
- Heat increment (HI) is the difference between ME and NE. It represents the heat produced by an animal incidental with nutrient digestion and metabolism. It is also called the work of digestion. This value is useful for keeping an animal warm during cold weather and can actually interfere with production by causing an animal to be too warm.
How do energy values compare to TDN?
Since the beginning of ration formulation TDN, DE, ME and NE have all been used for expressing the energy value of feeds. They all serve an important role in ration formulation. DE is probably the least precise because it does not allow for energy losses during digestion and utilization. TDN is superior to DE in this regard. ME figures represent a more precise measure of a feeds energy value to an animal than either TDN or DE. In final ration analysis, to the extent possible, NE values are the most precise measure of an animal’s energy needs. However, NE values have only been determined for a limited number of feeds.
How does protein figure in to the ration?
Every living animal, humans included, have a need for protein. It is the basic structural material from which all body tissues are formed. Also, all living tissue are constantly undergoing degeneration and protein is necessary for maintenance. Additionally, protein is required for most body enzymes and hormones. No other nutrient can replace protein in a ration. Every class of animal has a minimum level of dietary protein needed. Protein is also often the most expensive nutrient to feed. While high protein feeds may serve as a source of energy for livestock, they are usually too expensive to be a practical source of feed energy.
Stay tuned for minerals and vitamins.