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Fact Sheet: Carcass modifiers
Jay Y. Jacela, DVM; Joel M. DeRouchey, PhD; Mike D. Tokach, PhD; Robert D. Goodband, PhD; Jim L. Nelssen, PhD; David G. Renter, DVM, PhD; Steve S. Dritz, DVM, PhD
JYJ, DGR, SSD: Department of Diagnostic Medicine/Pathobiology, Kansas State University, Manhattan, Kansas. JMD, MDT, RDG, JLN: Department of Animal Science and Industry, Kansas State University, Manhattan, Kansas. Corresponding author: Dr Jay Y. Jacela, I-102 Mosier Hall, 1800 Denison Ave, Manhattan, KS 66506; Tel: 785-532-4845; E-mail:

RIS citationCite as: Jacela JY, DeRouchey JM, Tokach MD, et al. Feed additives for swine: Fact sheets – carcass modifiers, carbohydrate-degrading enzymes and proteases, and anthelmintics. J Swine Health Prod. 2009;17(6):325–332.
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Keywords: swine, carcass modifiers, carbohydrate-degrading, enzymes, proteases, anthelmintics
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Fast facts

Carcass modifiers, which are feed additives included in swine diets to improve carcass quality, include chromium, betaine, carnitine, conjugated linoleic acid, and ractopamine HCl.

Ractopamine HCl, which has shown the most consistent results among the carcass modifiers, acts as a repartitioning agent by redirecting nutrients away from adipose tissue and towards muscle growth.

Amino-acid levels need to be adjusted to meet the increased requirement for protein deposition with ractopamine supplementation.

Growth response to ractopamine HCl decreases over time.

More research is needed to validate the beneficial effects of the other carcass modifiers

There is increasing consumer demand for leaner and healthier pork products. Improvements in genetics, new technologies, and increased understanding of nutrition have become instrumental in helping producers meet this demand. Continued research also has led to the development of products that can be included in swine diets as carcass modifiers. A dietary carcass modifier is broadly defined as any component of the diet that alters the resulting carcass composition of pigs. Generally, the mechanism of action of carcass modifiers is aimed at increasing protein and muscle deposition while reducing fat deposition. These products vary in the mechanisms by which they modify carcass quality. In addition, not all carcass modifiers are approved for use in pig diets, for public-health reasons. Understanding the modes of action and differences between these products is important for safe and effective use.

What compounds are commonly used as carcass modifiers?

Carcass modifiers available for use in swine include chromium, betaine, carnitine, conjugated linoleic acid, and ractopamine.

Chromium. Chromium is an element essential for growth and development in animals. It plays an important function in metabolic processes involved in the regulation of glucose, proteins, lipids, and cholesterol. Chromium from organic complexes like chromium picolinate and chromium nicotinate is more readily absorbed than other inorganic forms, such as chromium chloride. A number of studies,1-3 mostly utilizing chromium picolinate, have shown that adding chromium to pig diets during the growing-finishing period can improve growth performance or lean meat yield. However, the responses have not been consistently observed in all studies.4-7 The exact physiological action of chromium that results in increased carcass leanness is not clear. One possible mechanism of action is improved insulin sensitivity of tissue, causing enhanced deposition of dietary protein and carbohydrate in the muscle cells.

Betaine. Betaine is a byproduct of molasses production from the sugar beet and plays a role in metabolic processes as a methyl donor. Interest in this product increased after studies8,9 indicated that it can increase carcass leanness and improve feed efficiency when added to finishing diets. However, results were not consistently repeated in other studies,10,11 indicating unreliability of the responses.

Carnitine. Carnitine is a vitamin-like compound essential for fatty-acid transport across the mitochondrial membrane. While results from earlier research12 were inconsistent, more recent studies13-15 have provided further evidence that the addition of carnitine in finishing diets results in a leaner carcass and thinner backfat. This has been attributed to the increased ability of the pig to more efficiently use fat for energy, divert carbon toward amino-acid synthesis, and spare branched-chain amino acids for protein synthesis.

Conjugated linoleic acid. Conjugated linoleic acid is a feed additive that has been shown16-17 to reduce whole-body fat accretion by repartitioning fat and lean tissue. The use of conjugated linoleic acid in pig diets also influences fat quality by lowering its iodine value. Lower iodine value is an indication of a more saturated (firm) fat. However, the high cost of conjugated linoleic acid limits its practical use in swine diets.

Ractopamine HCl. Among the substances categorized as carcass modifiers, ractopamine HCl has received the greatest amount of attention. Ractopamine HCl belongs to a group of compounds called β-agonists, that include zilpaterol, cimaterol, clenbuterol, and salbutamol. However, only ractopamine HCl is approved for use in pigs in the United States. It is also legal for use in swine diets in more than 20 countries, but not in some other parts of the world. It is recommended that this product be fed at concentrations of 5 to 10 ppm in the diet.

How does ractopamine improve carcass quality?

Ractopamine HCl, like the other β-agonists, acts as a repartitioning agent by redirecting nutrients away from adipose tissue and towards muscle growth. It modifies the metabolic signals within muscle and fat cells to direct more nutrients to lean growth. Pigs fed diets supplemented with ractopamine HCl also exhibit an increase in daily gain, accompanied, in many instances, by a slight decrease in feed intake. Efficiency of gain also is improved, because it takes less energy to deposit lean than fat. These improvements in growth performance have been consistently demonstrated in many experiments.18 However, it should be noted that the use of ractopamine in pig diets can also have potentially negative consequences. Ractopamine HCl affects behavior and stress-hormone profiles of finishing pigs, which makes them more difficult to handle.19 This potentially could lead to difficulty in handling and increasing susceptibility to transport stress at the time of marketing.

Do diet formulations need to be modified when ractopamine HCl is added?

Appropriate nutritional adjustments in finishing-diet formulations need to be made to capture the maximum benefits of ractopamine HCl. This is due to the increased requirement for nutrients to support the higher rate of muscle deposition that results with dietary ractopamine HCl use. According to the product label, diets should contain ≥ 16% crude protein when ractopamine HCl is added. However, because swine do not have a requirement for crude protein, but rather requirements for amino acids, it is important that the appropriate amino-acid levels be fed. The lysine requirement, in particular, is increased in pigs fed ractopamine HCl. It is recommended that diets supplemented with ractopamine HCl should have a standardized ileal digestible-lysine level that is 0.3% higher than that required by a pig of equal weight fed an unsupplemented diet.

At what stage of production should ractopamine be fed to pigs and for how long?

Ractopamine HCl is labeled for continuous feeding up to the last 90 lb before marketing. It is important to note that the response to the growth-promoting ability of ractopamine HCl is greatest during the first 2 weeks of feeding and progressively decreases over time. This is due to the down-regulation or desensitization of β-receptors that results from chronic administration of β-agonists. Therefore, feeding ractopamine HCl-supplemented diets longer than recommended will not translate to further improvement in performance. Also, pigs must be continuously fed ractopamine HCl-supplemented diets until market. Beneficial effects on performance will be lost once ractopamine HCl is removed from the diet. This beneficial effect can be lost with removal for as little as 7 days prior to market.

Is pork from a pig that was fed a diet containing ractopamine HCl safe for human consumption?

The use of ractopamine HCl as a feed additive in swine diets has been extensively studied for many years prior to its Food and Drug Administration approval in 1999. These studies20 have shown that pork from pigs fed diets containing ractopamine HCl is safe for human consumption. There is no withdrawal time required. A major limitation to the acceptance of β-agonists such as ractopamine HCl in animal production in other countries is the risk associated with drug residues in the meat products. This is especially true for clenbuterol, which has a rather long elimination time from the animal body (> 21 days),21 and thus may cause unsafe drug residues in meat and meat products.22 Consumption of pork containing clenbuterol residues can have adverse effects in humans.20 For this reason, clenbuterol and other related products have been banned for use as repartitioning agents in many parts of the world, including the United States.


Carcass modifiers are feed additives that can be used to increase lean-growth rates and improve efficiency. Among these, ractopamine HCl has shown the most consistent results. However, optimal results for ractopamine HCl use depend on the dose, duration of treatment, and nutrient levels in the diet.


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