Figure 4. Energy Partition in Pig Nutrition.
Energy is technically not a nutrient but is released by the metabolism of carbohydrates (starch) and fats (lipids) from the dietary components. In the United States, energy is expressed in kilocalories (kcal) or megacalories (Mcal). One megacalorie is equivalent to 1,000 kcal. Energy losses occur during the body's digestive and metabolic processes, with only part of the total energy from the feed available for body maintenance and productive functions (Net Energy).
The energy content of feedstuffs is normally expressed as either digestible or metabolizable energy. The digestible energy (DE) content of a feedstuff is determined by subtracting the amount of energy in the feed from the energy lost in the feces. Metabolizable energy (ME) is the digestible energy minus the energy lost in urine and gases (Figure 4). The energy lost as gas (in the gastrointestinal tract) is usually negligible (less than 1% of total energy intake). The National Research Council (NRC) is currently using the digestible energy (DE) value to formulate swine diets. Most feedstuffs can be converted to the ME value by multiplying DE by 0.96.
Figure 4. Energy Partition in Pig Nutrition.
Net energy is therefore used by the pig for maintenance (NEm) and for productive purposes (NEp) such as growth, development of products of conception, milk production, etc. As the pig approaches market weight, an increasing amount of energy is used for maintenance, which contributes to a poorer feed efficiency. In addition, there is an increasing amount of fat deposited in the carcass. The amount of energy needed for body fat gain is higher than that for muscle gain and is the major reason for the higher feed conversion of finisher pigs. For example, to deposit 1 lb. of muscle (0.20 lb. of protein plus 0.80 lb. of water) approximately 0.95 Mcal of energy is required. On the other hand, 1 lb. of body fat (0.83 lb. of lipid plus 0.17 lb. of water) requires approximately 4.84 Mcal of energy or approximately five times the energy cost of lean- tissue gain.
Energy must be provided in large amounts over that needed for maintenance to achieve optimum growth and reproduction responses. The major source of dietary energy for the growing pig is from the carbohydrate (i.e., starch) component of grains or their by-products. Feedstuffs such as corn, sorghum, and wheat contain high percentages of starch (>60%). Cereal grains such as barley also contain starch as their predominant energy source, but they also contain higher levels of the complex carbohydrates (e.g., fiber) that have a low digestibility.
Fats provide the most concentrated source of dietary energy to the body. Dietary fat provides 2.25 times more energy on a weight basis than does carbohydrate. The percent fat (oil) in most cereal grains is substantially less than the carbohydrate component and generally ranges from 1 to 4%. Dietary fats also contain the essential fatty acids that are used for the synthesis of various hormones. The presence of fat in the intestinal tract is essential for the absorption of fat-soluble vitamins.
Dietary fat and fiber can be used effectively to either reduce or increase body-heat production, respectively. When the environmental temperature is below the pig's comfort zone (LCT), the pig has a higher energy requirement, and if adequate energy is not provided in the feed, the pig must divert body fat toward the production of heat for body maintenance. In contrast, when environmental temperatures exceed the upper limit of the comfort zone (UCT), the pig's respiration rate increases, the pig consumes less feed, and a greater proportion of the consumed dietary energy is used for maintenance. Both of these conditions will increase the pig's feed conversion (Figure 3).
Dietary fat releases less heat when it is digested and metabolized, and thus will impose less heat stress on the animal when environmental temperatures are high. In contrast, dietary fiber releases more heat when digested and metabolized, thus providing a source of heat for the body when environmental temperatures are low. Because the lower heat increment from dietary fat produces less heat, the inclusion of fat in swine diets during the summer months can result in improved weight gains and feed conversion responses. Conversely, when environmental conditions are colder and additional body heat is needed, the use of fiber in the diets of swine may be beneficial. Consequently, the use of fats and fiber in swine diets during the appropriate season may be determined by production stage, economics, and environmental temperatures.
Other aspects of the diet can affect energy utilization and subsequent feed conversion (e.g., amino acids, fatty acids, and fiber). Diets that contain excess amino acids or improper balances of amino acids are utilized less efficiently than diets that contain amino acids in the correct balance. This is because the pig must use energy to get rid of excess dietary protein (amino acids), resulting in a lowered feed conversion. Dietary fiber is the complex carbohydrate found in grain, hulls, and plant forage material and is not efficiently digested by swine. Because of its lower digestibility, the dietary fiber component of the diet reduces the usable dietary energy content, which also results in a poorer feed conversion. Fiber is, however, often used in formulating diets for barrows where it reduces the amount of fat deposited. Fiber is also often incorporated into sow gestation diets where energy restriction is desired, or it is used under conditions to relieve animal constipation problems.