Chapter 5 Dietary energy.
THE CHALLENGE OF ENERGY IN NUTRITION
Energy is power used to perform work. In most instances, most of the work done by livestock involves the maintenance of life. If there is more energy available than that needed to maintain life, the extra energy will go toward the work of growth, production, and reproduction.
Livestock acquire energy from the organic compounds in feedstuffs. These compounds include carbohydrates, fats, proteins, and nucleic acids. The energy in these compounds is made available to the animal when they are oxidized during metabolism. As the electrons in these compounds are transferred to oxygen in the mitochondria of the cells, their energy is transferred to the chemical bonds of adenosine triphosphate (ATP). The energy stored in ATP is used as the immediate energy source for all the reactions of metabolism.
To address the challenge of assessing the energy value of feed, nutritionists use the net energy (NE) system, which identifies and quantifies the different energy-bearing fractions into which a feedstuff is degraded during digestion and metabolism.
THE NET ENERGY SYSTEM
Animals eat to acquire energy. In other words, appetite is driven by the animal's need for energy. This sets energy apart from the other feed nutrients required by livestock. Energy is the master nutrient. For the most part, the satisfaction of all other nutrient requirements will occur only because livestock seek energy in their feed. Feed energy value and animal energy requirement are usually described using the NE system shown in Figure 5-1.
[FIGURE 5-1 OMITTED]
Gross energy is the total energy contained in a feedstuff. It is determined by oxidizing (burning) the feedstuff and measuring the energy released as heat. It is not directly applicable to animal nutrition because animal digestive systems are not able to use the entire gross energy contained in a feedstuff.
Fecal energy is the energy contained in the fecal material resulting from ingestion of the feedstuff. The energy in the fecal material is determined by burning the fecal material and measuring its gross energy content.
Digestible energy (DE) is the difference between the gross energy and the fecal energy. It is the portion of the feedstuff energy assumed to be digestible. DE is approximately equivalent to total digestible nutrients (TDN) (Chapter 4). DE is the targeted energy used in horse, rabbit, and fish nutrition.
Urine and gas energy includes the energy contained in these wastes and produced during digestion of the feedstuff and subsequent metabolic activities. Energy-containing compounds in urine have been filtered out of the blood by the kidneys. These compounds may have originated in the feedstuff or may be of endogenous origin. The energy in gaseous waste (primarily methane) is important in ruminant animals, but is insignificant in other species.
Metabolizable energy (ME) is the difference between the DE and energy in urine and gas. In poultry nutrition, ME is used because the feces and waste from the urinary system are voided together. The ME for poultry is called MEn because it is corrected for nitrogen. The nitrogen correction involves accounting for feed protein (nitrogen) that was retained in the body rather than metabolized for energy. In swine and ruminant nutrition, ME is used for improved accuracy over DE. ME is also used in cat nutrition. In dog nutrition, ME is used, but because very few energy measurements have been conducted on dogs, the ME is calculated from gross energy rather than measured during a digestion trial (Figure 5-2).
Metabolic rate increases following the ingestion of feed. This results in increased energy losses as heat, which must be accounted for when determining the amount of feed energy available to the animal. The increased metabolic rate that follows the ingestion of feed has been described as the specific dynamic effect and the heat increment.
NE is calculated as the difference between the ME and the heat increment. The NE content of a feed represents the actual energy value of the feed to the animal. Animals use feed NE with varying efficiencies (Figure 5-3). The NEm (net energy for maintenance, dairy heifers) value for ground corn grain (IFN 4-02-854) is 0.98 Mcal/lb., dry matter basis. The NEl (net energy for lactation, dairy) value for ground corn grain is 0.91 Mcal/lb., dry matter basis. The NEg (net energy for gain, dairy heifers) value for the same feedstuff, however, is only 0.67 Mcal/lb., dry matter basis. NEm is only used for heifers, not for adult cows. For adult dairy cows, energy requirements and energy values of feed are expressed in NEl units when considering both maintenance and lactation functions because ME is used with similar efficiency for maintenance and lactation (Moe & Tyrrell, 1972). In the goat NRC (1981), the NE is reported without reference to function.
Figure 5-2 Calculating feedstuff ME value for the dog Protein has a gross energy of 2.00 Mcal/Ib. The crude protein content of a feedstuff is assumed to be 80% metabolizable. 2.00 x 0.80 - 1.60. The crude protein measured in the feedstuff is multiplied by 1.60. Nitrogen-free extract has a gross energy of 1.88 Mcal/Ib. The nitrogen-free extract content of a feedstuff is assumed to be 85% metabolizable. 1.88 x.85 - 1.60. The nitrogen-free extract measured in the feedstuff is multiplied by 1.60. Fat has a gross energy of 4.26 Mcal/Ib. The crude fat content of a feedstuff is assumed to be 90% metabolizable. 4.26 x 0.90 - 3.83. The crude fat measured in the feedstuff is multiplied by 3.83. Fiber is assumed to have no energy value for the dog. The ME value of a feedstuff for dogs, Mcal/Ib., dry matter basis, is calculated as follows: (crude protein x 1.60) + (nitrogen-free extract x 1.60) + (crude fat x 3.83) Figure 5-3 Net energy in the feed is the gross energy in the product produced Another way to look at net energy is from the standpoint of the energy contained in the product produced. The feed net energy required to make 80 lbs. of milk is the gross energy contained in that milk; the feed net energy required to make a quantity of gain is the gross energy contained in that gain. As an example, say we have a quantity of milk that contains 40 Mcal of gross energy. Given that corn grain contains 0.78 Mcal of NEI/Ib., as fed basis, it would take (40/0.78) = 51 lb. of ground corn grain to produce this milk. Say we have a quantity of meat that contains this same amount of gross energy--40 Mcal. Given that ground corn grain contains 0.57 Mcal of NEg/Ib., as fed basis, it would take (40/0.57) = 70 lb. of ground corn grain to produce this gain.
NE may be used for the function of reproduction. The priority that the body assigns to pregnancy and lactation is a close second behind that assigned to maintenance. For short periods of time, animals will still maintain the pregnancy and/or lactate, even if the dietary energy content is not enough to support the maintenance functions. It is important to realize that although the animal is not ingesting enough energy to meet her maintenance energy needs, these needs are still being met with reserves of energy stored as body fat (she will be losing weight).
In dairy nutrition, it is recognized that the energy value of feedstuffs must be discounted in animals in which feed is passing rapidly through the digestive tract. Such animals have high dry matter and energy intakes. Dry matter and energy intakes are often expressed as multiples of values typical of a cow at maintenance. A feedstuff consumed by a cow with an energy intake of 3 x maintenance would have a higher energy value than would the same feedstuff consumed by a cow with an energy intake of 4 x maintenance. In the companion CD-ROM for dairy cattle, a discount is applied to the energy value of each feedstuff in cases of high ration TDN% and/or high animal dry matter intake (DMI).
NE may be used for the functions of growth, wool production, and mohair production. Growth (or gain) will not be supported if there is not enough energy in the diet to supply the entire maintenance energy requirement. Recall (Chapter 1) that the energy expended in dealing with a stressful environment (the essence of animal strain) is part of the animal's maintenance energy requirement. Insufficient energy to support maintenance can disrupt the hair follicle cycle, resulting in reduced quality and quantity of wool and mohair production.
ENERGY AND APPETITE
For the most part, an animal's appetite for food is a reflection of its metabolic need for energy. This fact has important implications regarding feeding management. If, for example, fat is added to the diet to replace a portion of the grain, the energy density of the diet will be increased. This means that it will take fewer pounds of the fat-added ration to supply a given amount of energy. Appetite may be reduced. It may be necessary to increase the density of other nutrients when fat is added in order to prevent nutrient deficiencies and maintain performance. Conversely, if the diet energy density is reduced, increased intake may present an opportunity for cost savings by reducing the density of other nutrients. The relationship between energy density and feed intake holds over a limited range of energy densities. Feed intake will be limited by gut fill at very low energy densities. At very high energy densities, behavioral and digestive problems may result or animals may consume energy beyond their need in order to achieve satiety.
Following is a summary of how the relationship between energy density and feed intake is handled for the various species in the NRC publications and in the companion application to this text. The cat energy information comes from the AAFCO (2003).
* Poultry feed intake and nutrient requirement calculations are based on an assumed feed energy density of 1,462 kcal of MEn/lb., dry matter basis.
* Trout DMI and nutrient requirement calculations are based on an assumed feed energy density of 1,814 kcal of DE/lb., dry matter basis.
* Catfish DMI and nutrient requirement calculations are based on an assumed feed energy density of 1,512 kcal of DE/lb., dry matter basis.
* Goat DMI and nutrient requirement calculations are based on an assumed feed energy density of 1,090 kcal of ME/lb., dry matter basis.
* Dog DMI and nutrient requirement calculations are based on an assumed feed energy density of 1,665 kcal of ME/lb., dry matter basis for all dogs except those at maintenance. Requirements for dogs at maintenance are based only on body weight. To account for the effect of high dietary energy density on DMI, the author of this text has modified the prediction of DMI for dogs as follows: predicted DMI is reduced 5 percent for each increase of 45 units above 1,814 kcal of ME/lb., dry matter basis.
* Cat DMI is predicted based on the assumed energy content of the diet as associated with food type. Dry foods are assumed to have lower energy density than wet foods, and cats are predicted to eat more dry matter of a low-energy dense food than a high-energy dense food. With two exceptions, cat nutrient requirement calculations are based on an assumed feed energy concentration of 1,814 kcal of ME/lb., dry matter basis. The exceptions are taurine and copper. Taurine in canned foods has been reported to be less available to the cat (Douglass, Fern, & Brown, 1991). Likewise, copper in extruded foods has been reported to be less available (AAFCO, 2003). To meet the cat's requirement for these two nutrients, the targeted requirement is increased when the problem food forms are identified in the input section of the companion application to this text.
* Dairy DMI and nutrient requirement calculations are based on animal, environment, and production inputs, energy density of the inputted ration, and whether or not ionophore is used. There is also a unique formula used to predict the energy value for fatty acids, fats, and animal proteins, and a single formula to predict all others. These formulas are displayed in the companion application for dairy cattle by striking F9.
* Beef DMI and nutrient requirement calculations are based on animal and environment inputs, ionophore use, anabolic implant use, and the energy density of the inputted ration.
* For horses, sheep, and rabbits, the NRC formulas and the companion application to this text do not use ration energy density to predict DMI.
* For swine, the NRC predicts feed intake and nutrient requirements based on the DE concentration expected from a diet based on corn grain and soybean meal diet. Such a diet would contain a DE concentration of 1,714 Mcal/lb., dry matter basis.
METABOLIC BODY SIZE
An animal's energy requirement comprises the total energy needed to perform the functions of maintenance, growth, production, and reproduction. In most cases, the requirement to perform maintenance functions will be the single largest component of the energy requirement. The maintenance energy requirement is the energy needed to carry on the activities associated with the animal's basal metabolism as well as activities (strains) associated with combating environmental stressors. Activities that are part of basal metabolism include respiration, circulation, transport of ions and metabolites, and body constituent turnover. The energy used in basal metabolic activities can be determined by measuring the heat lost from the surface of a stress-free, fasting, homeothermic animal. The basal metabolic rate is therefore related to the surface area of an animal's body.
The relationship between body weight and body surface area is body weight raised to the 0.67 power. However, Kleiber (1932) demonstrated that the relationship between body weight and basal metabolic rate is body weight raised to the 0.75 power (Figure 5-4). [BW.sub.kg.sup.0.75] is referred to as metabolic body size. Determination of energy requirements in formulas such as this is referred to as allometric scaling.
In the application accompanying this text, the maintenance energy requirement is determined using [BW.sub.kg.sup.0.75] for most domestic animals, but not for all. Metabolic body size predicts energy requirement as ME. Metabolic body size can only be applied in ration formulation if the ME content of the feedstuffs is available for the species considered. In the NRC publications for the horse, rabbit, and fish, feedstuff energy values are expressed as DE rather than ME. For these livestock, the energy requirement presented cannot be based on metabolic body size. The authors of the horse NRC (1989) further explain that the "values for energy requirements are not based on metabolic body size, because Pagan and Hintz (1986) found no benefit from using metabolic body weight ([kg.sup.0.75]) over weight ([kg.sup.1.0]) in determining the energy requirements of horses ranging in size from 125 to 856 kg." Though the cat NRC (1986) does use ME as the measurement of feedstuff energy value, the authors subscribe to the conclusion of Kendall, Blaza, and Smith (1983) who "found no extra precision when energy requirements of adult domestic cats were scaled to mass exponents of body weight (kg) of either 0.75 or 0.67, compared with unity." Payne (1965) reported dog energy requirements based on [BW.sub.kg.sup.0.67]. These values are reported in the dog NRC (1985) and are used in the companion application to this text. With regard to fish, one would not expect that metabolic body size as predicted by Kleiber's (1932) formula would apply because fish muscles are not working against gravity and because fish are poikilotherms. Brett (1973) reports that the maintenance energy requirement of fish is 5% to 10% that of other livestock of similar size in a thermoneutral environment.
[FIGURE 5-4 OMITTED]
HOW ENERGY IS MEASURED
Energy is measured in calories. A calorie is the energy needed to raise the temperature of 1 gram of water 1[degrees]C, or more specifically, from 14.5[degrees]C to 15.5[degrees]C. This value is too small for practical use in animal nutrition. A kilocalorie, or Kcal, is equal to 1,000 calories, and this is the standard energy unit for all livestock except horses and ruminants. For horses and ruminants, the megacalorie (Mcal) is the standard. One Mcal is equal to 1,000,000 calories or 1,000 kcal. A joule is an alternative to the calorie. The joule is defined in mechanical terms and can be converted to the calorie system: 1 joule = 0.239 calories. The joule has replaced the calorie in many countries and scientific journals, but is not in widespread use in the United States. Table 5-1 gives the energy requirements for selected animals.
Chapter 5 defines the terminology used in describing feedstuff energy content and animal energy requirements. Metabolic body size is defined and its application in animal nutrition is discussed.
1. From what four types of compounds in feed can animals derive energy for metabolism?
2. Starting with the gross energy of a feedstuff, describe how one arrives at that feedstuff's NE content.
3. In canine nutrition, the ME of a feedstuff is calculated from the gross energy in that feedstuff's protein, nitrogen-free extract, and fat. Give the gross energy for each of these compounds and the percentage of each compound's gross energy that is assumed to be metabolizable.
4. Explain why metabolic processes use more pounds of corn grain to produce a quantity of meat containing 40 Mcal of gross energy than to produce a quantity of milk also containing 40 Mcal of gross energy.
5. Explain why the dairy NRC committee developed formulas that discount feed energy value in animals with high DMIs.
6. Under what conditions is animal appetite limited by dietary energy density? Under what conditions is animal appetite limited by gut fill? Describe the relationship between appetite, energy density of the diet, and DMI.
7. For which species of domestic animals are DMI and nutrient requirements predicted by the NRC committees, based on an assumed feed energy density?
8. What is metabolic body size? Explain how metabolic body size is related to body surface area.
9. Which NRC committees do not use metabolic size in predicting maintenance energy requirement?
10. How is calorie defined, and how is it used in animal nutrition? How is joule defined, and how is it used in animal nutrition?
Association of American Feed Control Officials. (2003). Official publication. West Lafayette, IN.
Brett, J. R. (1973). Energy expenditure of sockeye salmon during sustained performance. J. Fish. Res. Board Can. 30, 1799-1809.
Douglass, G. M., Fern, E. B., & Brown, R. C. (1991). Feline plasma and whole blood taurine levels as influenced by commercial dry and canned diets. J. Nutr. 121(suppl.), S179-S180.
Johan, (2000). Retrieved August, 2004 from http://www. anaesthetist.com/physiol/basics/scaling/Kleiber.htm
Kendall, P. T., Blaza, S. E., & Smith, P.M. (1983). Comparative digestible energy requirements of adult beagles and domestic cats for bodyweight maintenance. J. Nutr. 113, 1946.
Kleiber, M. (1932). Body size and metabolism. Hilgardia. 6, 315-353.
Moe, P. W., & Tyrrell, H. F. (1972). The net energy value of feeds for lactation. J. Dairy Sci. 55, 945-958.
National Research Council. (1986). Nutrient requirements of cats. Washington, DC: National Academy Press.
National Research Council. (1989). Nutrient requirements of horses. Washington, DC: National Academy Press.
Pagan, J. D., & Hintz, H. F. (1986). Composition of milk from pony mares fed various levels of digestible energy. Cornell Vet. 76, 139.
Payne, P. R. (1965). Assessment of the protein values of diets in relation to the requirements of the growing dog. In O. Graham-Jones (Editor), Canine and Feline Nutrition Requirements. London: Pergamon Press.
Table 5-1 Energy requirement for selected animals Required, per Animal Animal per Day Fish, channel catfish, 100 g body weight 10 kcal of DE Fish, rainbow trout, 100 g body weight 5.04 kcal of DE Chicken, broiler, 5 weeks of age 439 kcal of Men (1) Chicken, white egg layer, 3 lb. body weight 296.90 kcal of Men (1) Pig, growing, 45 lb. body weight 4,529 kcal of DE 4,348 kcal of ME Dog, growing, 30 lb. body weight 1,294 kcal of Medog (2) Cat, growing kitten, 4.2 lb. body weight 337 kcal of ME Rabbit, growing, 5 weeks of age 377 kcal of DE Horse, light work, 1,100 lb. body weight 25,290 kcal of DE Goat, maintenance, 88 lb. body weight 1,970 kcal of DE 1,610 kcal of ME 910 kcal of NE (3) Ewe, maintenance, 110 lb. body weight 2,390 kcal of DE 2,000 kcal of ME 1,050 kcal of NEm Beef animal, growing, 800 lb. body weight Based on performance desired (4) Dairy cow, lactating, 1,400 lb. body weight Based on performance desired (4) Required Concentration, Animal Dry Matter Basis Fish, channel catfish, 100 g body weight 1,512 kcal/lb, DE Fish, rainbow trout, 100 g body weight 1,814 kcal/lb, DE Chicken, broiler, 5 weeks of age 1,619 kcal/lb, Men (1) Chicken, white egg layer, 3 lb. body weight 1,496 kcal/lb, Men (1) Pig, growing, 45 lb. body weight 1,714 kcal/lb, DE 1,645 kcal/lb, ME Dog, growing, 30 lb. body weight 1,665 kcal/lb, Medog (2) Cat, growing kitten, 4.2 lb. body weight 1826 kcal/lb, ME Rabbit, growing, 5 weeks of age 1,280 kcal/lb, DE Horse, light work, 1,100 lb. body weight 870 kcal /lb, DE Goat, maintenance, 88 lb. body weight 1,250 kcal /lb, DE 1,020 kcal /lb, ME 570 kcal /lb, NE (3) Ewe, maintenance, 110 lb. body weight 1,090 kcal /lb, DE 910 kcal /lb, ME 480 kcal /lb, NEm Beef animal, growing, 800 lb. body weight Based on performance desired (4) Dairy cow, lactating, 1,400 lb. body weight Based on performance desired (4) DE: Digestible energy; ME: metabolizable energy; NE: net energy; NEm: net energy for maintenance, dairy heifers. (1) Metabolizable energy, nitrogen-corrected. The nitrogen correction involves accounting for feed protein (nitrogen) that was retained in the body, rather than metabolized for energy. MEn is only used in poultry nutrition. (2) Metabolizable energy, specific to dogs. Few ME values have been determined for feedstuffs fed to dogs. In the absence of actual measured values, MEdog in feedstuffs is calculated using apparent digestibilities of 85% for nitrogen-free extract, 80% for crude protein, and 90% for ether extract (fat). Apparent digestibilities are multiplied by the gross energy values of 4.15 for nitrogen-free extract, 4.4 for crude protein, and 9.4 for fat. It is assumed that fiber digestion yields no ME for the dog. The resulting values of 3.53, 3.52 and 8.46 are kcal of ME available to dogs from nitrogen-free extract, crude protein, and fat. (3) The efficiencies of use of net energy for the different functions of maintenance, gain, pregnancy, lactation and mohair production have not been differentiated in goat nutrition. The value in this table is a sum of net energy requirements reported for maintenance, gain, pregnancy, lactation and fiber production. (4) In beef and dairy nutrition, rather than establish an energy requirement, the animal's performance is predicted based on the ration energy level. The energy requirement, therefore, is dependent upon the level of performance desired.
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|Author:||Tisch, David A.|
|Publication:||Animal Feeds, Feeding and Nutrition, and Ration Evaluation|
|Date:||Jan 1, 2006|
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