ABCs of Forage Analysis – What is Energy?
Adapted by Dr. Ed DePeters, UC Davis & Jennifer Heguy UCCE Merced, Stanislaus & San Joaquin
Energy content of feedstuffs is an important attribute because energy intake of dairy cattle impacts milk production, growth, reproduction, and health. High quality forage is high in energy, but what exactly is energy?
Energy is a difficult entity to describe. You can isolate protein, for example casein from milk, and you can hold casein in your hand and you can see the casein protein. The same is true for fiber; we can extract the fiber from alfalfa hay using the NDF (neutral detergent fiber) method and see the fiber. However, we cannot isolate energy, we cannot see it, and we cannot hold it. But we can measure the energy content of feedstuffs.
To measure energy, we first must be able to define what energy is. By physical definition, energy is the ability to do work – it is the movement of a force through a given distance. For our purposes, energy is used to drive the various metabolic pathways in the body of dairy cattle to synthesize milk and meat for productive purposes.
Energy is often inaccurately referred to as a nutrient. There are six nutrients: water, proteins, carbohydrates, lipids, minerals, and vitamins. Energy is not a nutrient, but a property that some nutrients possess. For example, proteins, carbohydrates, and lipids can be metabolized in the body of animals to yield energy. The highest energy yielding nutrient is lipids. The energy content of nutrients is the basis for our dairy feeding systems as well as the system used for humans.
The NRC (National Research Council) for dairy cattle uses the Net Energy (NE) system to determine the energy content of feedstuffs and the energy requirements of animals. The NE system attempts to measure the sources of energy loss in the dairy cow. Basically, the NE system is an energy balance system. Let’s take a look at the different fractions of the NE system. Refer to Table 1 to see the differences between energy values of high quality versus low quality alfalfa hay in our example.
Gross Energy – this is the total potential energy in a feedstuff. Gross energy is also called Intake Energy. You’ll notice that the gross energy contents for the high and low quality alfalfa hays are similar. This is because they are quite similar in content of proteins, lipids, and carbohydrates. Gross energy doesn’t tell us the complete story of how the animal will utilize the feedstuff. The availability of the Gross Energy to the animal is what is important.
Digestible Energy – this is the energy remaining after some energy is lost in feces. The amount of energy lost in the feces of the high quality alfalfa hay is less than the low quality alfalfa hay. The high quality alfalfa hay has less lignin and less fiber so it is more digestible than the lower quality alfalfa hay.
Metabolizable Energy – this measure takes into account the amount of energy lost in the urine and gas of the animal. Basically, Metabolizable Energy is the amount of energy in a feedstuff that is available for productive purposes (e.g. synthesis of milk and meat).
Net Energy – this is the amount of energy in the feedstuff that is retained in the animal in the form of maintenance of body tissue and synthesis of product (e.g. milk and meat). The energy lost as heat associated with fermentation and nutrient metabolism are accounted for. Net Energy is our most accurate estimate of energy content of feeds.
The largest loss of energy occurs in the loss of fecal energy. So feedstuffs with high digestibility, for example high quality corn silage, have high energy compared with lower quality forages such as straws. Another example of digestibility would be the energy content of BMR corn silage, which is generally higher than conventional corn silage because BMR corn silage is more digestible (lower lignin in the stalk and higher fiber digestibility).
Table 1. Energy values of high and low quality alfalfa hay.
Data are from the 1989 NRC for Dairy Cattle
Predicting the energy content of feedstuffs is an essential step in preparing diets for dairy cattle. But just as important is how the skilled nutritionist on your dairy uses the energy values of each individual feedstuff to formulate a diet. This requires knowledge of many factors, including the rate of digestion in the rumen as well as the rate of passage from the rumen, that impact fermentation balance and the digestibility of nutrients. The diet must provide enough available energy to the lactating dairy cow to support maintenance of the animal as well as productive functions including the synthesis of milk and meat, the support of the immune system, and the development of the fetus.
Link to PDF Version: ABCs of Forage Analysis- What is Energy