PorkNet Papers Collection
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Growth of the pig — without which, after all, pork production would not exist — is a process that depends on and is regulated by numerous factors. A primary biological goal in pork production is to support the growing pig so it can fulfill its potential for lean growth.
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The cost of feed is at least 50% of the total cost of pig meat production with the energy component representing the greatest proportion. Therefore, from a practical point of view, it is important to estimate precisely the energy value of feeds, either for least-cost formulation purposes or for adapting feed supply to energy requirements of animals. Evaluation of energy content of pig feeds is usually based on their digestible (DE) or metabolizable (ME) energy contents. However, the closest estimate of the " true " energy value of a feed is given by its net energy (NE) content which takes into account the metabolic utilization of ME. Therefore, NE systems have been proposed. In addition, NE is the only system in which energy requirements and diet energy values are expressed on a same basis which should be independent of the feed.
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Improved genetics, nutrition, housing and management systems have permitted significant increases in milk production of lactating sows. In 1972, Boomgaardt et al. (1972) studied sows that produced about 8.5 pounds of milk daily. In contrast, Johnston et al. (1993) reported daily litter growth rates of 4.86 pounds which equates to an estimated milk production of 19.5 pounds per day. In spite of these large improvements, the genetic potential for growth of nursing piglets is still not realized (Boyd et al., 1995) suggesting that improved nutritional programs that result in increased milk production will increase growth rate of nursing pigs. Heavier pigs at weaning may be of particular importance in modern production systems that implement relatively short lactations (14-18 days).
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The lipids in feedstuffs commonly consumed by swine consist mainly of neutral fats, specifically triglycerides. The concentration of fats in feedstuffs varies widely. Fat levels range from 1.5 to 4.5% among cereal grains, 4.5 to 10% among high oil corn lines, .5 to 9% in fibrous byproducts, .5 to 20% in common protein feedstuffs, and 90 to 99% in supplemental fat sources. Consequently, the level of fat in a particular diet is dependent on the type of basic feedstuffs included as well as the level of a supplemental fat source. For example, corn (standard yellow dent) and wheat contain about 3.9 and 1.7% fat, respectively. Inclusion of wheat for corn in a simple corn-soybean meal mix effectively lowers the fat content of the diet by one-half. Similarly, the replacement of half of the dehulled soybean meal (.8% fat) with full-fat beans (18% fat) as the supplemental protein source in a corn-soy mix raises the dietary fat content from 3 to above 5%. Supplemental fat sources commonly used in swine diets include animal fats (tallows, greases), vegetable oils (corn, soybeans), restaurant greases (spent oils and greases), and commercial blends (mixtures of vegetable oil, animal fats, restaurant grease).of fats in feedstuffs varies widely. Fat levels range from 1.5 to 4.5% among cereal grains, 4.5 to 10% among high oil corn lines, .5 to 9% in fibrous byproducts, .5 to 20% in common protein feedstuffs, and 90 to 99% in supplemental fat sources.
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The study of feed intake has been greatly facilitated by the recent development of computerized systems that monitor the feed intake of individual animals within a group. Such equipment was originally designed for use by breeding stock supply organizations in the performance testing of pigs in group situations. The majority of selection programs for pigs place considerable emphasis on feed efficiency because of its impact on production costs. Historically, genetic improvement in feed efficiency was achieved as a correlated genetic change resulting from selection for growth rate and carcass lean content for animals tested in groups, where individual feed intakes could not be measured. Alternatively, pigs were penned and fed individually to obtain feed intake data, a procedure that is expensive in terms of both labor and facility costs. In addition, animals that are genetically superior when penned as individuals may not perform the best in group-housed situations i.e. under commercial conditions. Studies that have compared the growth and carcass characteristics of boars that have been individually fed during the performance test with the performance of their progeny under commercial group-housed conditions. These have generally shown moderate to low correlations between performance in the two environments. This phenomenon is termed a genotype x environment interaction and suggests that different genes may be involved in controlling growth performance in group-versus individually housed animals. Thus, breeding organizations stimulated the development of feed intake recording equipment to allow them to test and select animals in a social environment similar to that experienced in commercial production.
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The cost of feed is at least 50% of the total cost of pig meat production with the energy component representing the greatest proportion. Therefore, from a practical point of view, it is important to estimate precisely the energy value of feeds, either for least-cost formulation purposes or for adapting feed supply to energy requirements of animals. Evaluation of energy content of pig feeds is usually based on their digestible (DE) or metabolizable (ME) energy contents. However, the closest estimate of the " true " energy value of a feed is given by its net energy (NE) content which takes into account the metabolic utilization of ME. Therefore, NE systems have been proposed. In addition, NE is the only system in which energy requirements and diet energy values are expressed on a same basis which should be independent of the feed.
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Energy balance is regulated by the central nervous system (CNS). The CNS coordinates the provision of energy and specific nutrients with the prevailing endocrine state and nutriture to support the metabolic needs of the animal. The appropriate internal environment for maintenance, physical activity, growth, and reproduction is maintained by constant adjustments in energy intake and energy expenditure.
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Pork producers continue to fine-tune their operations and practices so as to take advantage of available technology in their inexorable quest to maximize their economic opportunities. The cost of feed continues to be the major part of the cost of producing pork. The metabolizable energy in the feed is precious, as it is energy directly useful to a pig. Any metabolizable energy that is lost to the environment is an inefficiency.
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Dirty, less hygienic environments increase the level of immunological stress and depress growth and performance of pigs. In response to challenge by bacterial or viral pathogens, the pig's immune system responds by secreting cytokines. These molecules which promote inflammation were originally described for their ability to orchestrate the immune response against the infectious pathogen. It is now recognized that cytokines also act on other targets outside of the immune system. The cytokines released by activated macrophages have been found to alter the metabolism of carbohydrate, fat and protein substrates, regulate certain endocrine secretions, and reduce food intake. In short, many findings converge to suggest that a major component of the growth inhibition observed in immunologically challenged pigs is mediated by pro-inflammatory cytokines. The goal of this short paper is to provide an integrated view of how immunological stress through the secretion of cytokines depresses growth in pigs.
