Pork Quality and Exhibitions: Understanding Quality and Factors that Influence Quality in Youth Swine Shows

Dr. Steven J. Moeller, Associate Professor, Department of Animal Sciences, The Ohio State University

Introduction
Youth swine exhibitions have taken on an entirely different persona over the last five to ten years, much to the disdain of a swine industry that is focused on improving domestic and international demand for pork and pork products.  Swine exhibitions have and (or) are becoming a “niche” market, separate from the commercial segment, where the focus has shifted from youth education and producing a profitable, high-quality market hog to an industry driven largely by competition, monetary opportunities, and entertainment value.  In response, the show-pig industry has evolved into a complex, highly sophisticated industry of its’ own that is supported by a series of specialized entities that supply genetic resources, feed supplies, showing and fitting supplies, and general advice on how to produce a champion.

Following the trend set by a judge’s opinion of the “ideal” market hog, youth exhibition enthusiasts have gone to great lengths to produce the pig that will win the next show.  It is not uncommon for a single dose of semen from a prized sire at stud to cost over $250/dose when semen from high-quality, commercial sires generally is less than $10/dose, or to see barrows, without any chance of producing offspring, sell for thousands of dollars as feeder pigs.  In addition, specialized diets, containing very high levels of protein and appearance enhancing additives are now commonly used throughout the show-pig industry to enhance muscle, reduce fat, and change outward appearance.  To many show pig industry observers, the practices used to breed, feed, and fit show pigs have resulted in questions regarding the ethical and animal welfare aspects of show industry and the lessons that are being taught to youth through the 4-H and FFA educational programs.

 

Due to the evolution in show industry practices, traditional market outlets for show pigs are continuing to disappear due to the uncertainties revolving around show pig industry.  Many packers are no longer willing to purchase show pigs due to: 1) fear of legal and (or) illegal drug residues, 2) the high incidence of Pale, Soft, Exudative (PSE) pork, 3) the inability to remove hair from carcasses that are clipped too short and (or) of colored breeds, particularly Duroc, that have deep rooted, coarse hair, and 4) the increased incidence of dead on arrivals (DOA’s) and downer pigs upon arrival that the packing plant.  In response to the declining market base, show pig enthusiasts have self proclaimed that future options for disposal might include: 1) “tanking” or turning all carcasses into rendered product, or 2) euthanasia followed by burial or composting of the resulting carcasses.  I suspect this stance, at least partially, is due to the limited value of the market pig when compared against the breeding, purchase, feeding, and travel costs incurred to produce a show winner and also the entertainment value of showing pigs.  However, when an estimated 1,000,000 pigs (~ 1% of the U.S. annual harvest) annually, from birth to harvest, are the result of show pig production systems, the commercial U.S. swine industry has a tremendous amount to lose.  Thus, a swine industry must take a stand on the concepts and practices being performed in the youth exhibitions within the U.S.  This remainder of this paper will focus on defining quality of the product (pigs and pork) produced from youth exhibitions.

 

What is Quality?

‘Functionality’ is a term often used to describe quality of pork.  Functionality includes: 1) factors that affect consumer acceptance or demand for pork and pork products, and 2) factors that affect the processing and value-added opportunities for pork and pork products.  When evaluated in this context it is easier to work backwards, from the consumer and processor perspective to the farm level, where choice of inputs plays a role in the ultimate quality or functionality of the product. Today, consumers are more in-tune with their food than ever before.  Wholesomeness, food safety, appearance, taste, and cost drive many consumer buying decisions, but increasingly, issues regarding production practices on the farm including the feed stuffs utilized, use of (sub)therapeutic antibiotics, production facilities utilized, and use of growth promoting agents enter into consumer purchasing decisions.

 

Appearance is an important characteristic that defines both wholesomeness and eating quality of pork.  Color is often an indicator of freshness of pork.  A consumers’ first impression of pork color will determine their willingness to purchase pork.  Muscle color is related to the amount and form of myoglobin in the muscle.  Greater levels of myoglobin will yield darker color.  The form of myoglobin is also important.  In the presence of oxygen, myoglobin will be in the form oxymyoglobin resulting in pinkish, red colored pork.  In the absence of oxygen, myoglobin is in the form of deoxymyoglobin, and pork is purplish in color.  Pork exposed to oxygen for extended periods of time becomes brownish due to the conversion of myoglobin to metmyoglobin and oxidation of iron.  Brown, off-colored pork turns off consumers and reduces demand for pork.

 

Aside from freshness, fresh pork color (loin and ham color in particular) is also associated with taste or palatability of pork.  Fresh pork color is subjectively assessed by using a six-point, color scale where 1 = pale, pinkish-gray to white, 2 = grayish pink, 3 = reddish pink, 4 = dark reddish pink, 5 = purplish red, 6 = dark purplish red (NPPC, 2000).  Scores of 3, 4, or 5 are considered ideal from a palatability perspective.  Scores of 1 and 2 are associated with the PSE condition (Figure 1) of pork.  The pale color, coarse textured surface, and excessive loss of moisture result in dry, flavorless, tough pork when consumed.  A score of 6 is considered too dark and thus is often discriminated against by the consumer.

   

Intramuscular fat, or marbling, also plays a significant role in the appearance and perception of pork products.  Health conscious consumers tend to avoid greater levels of internal fat to reduce caloric intake, thus they choose pork with very little visible fat (marbling) within the muscle and trim all external fat.  However, fat is what give pork flavor.  Selecting pork with little internal fat often results in a dry, less juicy, less flavorful piece of pork that is not desired by the consumer. Fat levels are measured both chemically (percent fat) and by using a subjective marbling score that relates closely to percent fat.  Subjective scores of 1 = one percent intramuscular fat, 2 = two percent intramuscular fat, 3 = three percent intramuscular fat, 4 = four percent intramuscular fat, 5 = five percent intramuscular fat (NPPC, 2000).  Ideally, from the consumer perspective, marbling scores of 3, 4, and 5 (i.e. 97, 96, or 95% fat-free, respectively) provide juicy, flavorful fresh pork to the consumer (NPPC, 1995).  When marbling scores are less than 2 (98% fat-free or greater) fresh pork is much less flavorful and juicy, particularly when overcooked (> 160° F, final temperature).  The vast majority of show pigs harvested have very low (<2%) intramuscular fat within the loin and ham muscles, creating a lower quality pork product.

 

Figure 1.  Acceptable loin color and marbling

 

Water-holding capacity of pork has major effects on consumer perception and further processing opportunities for pork.  Water holding capacity is measure a muscle’s ability to bind the water present in the muscle and to absorb curing solutions in products that are further processed.   Water-holding capacity is closely, but not fully related to color and the PSE condition of pork (Figure 2).  Extremely poor water holding capacity is associated with pale pork color (score 1 or 2), a course, uneven surface texture, and excess purge or water loss in the package (Figure 3).  For consumers, the negative perception of purge in packages and the thought of paying money for water rather than meat results in decreased demand.  For processors, the inability of pork muscle to bind the curing solutions results in uneven curing, variability in color of processed products, inconsistent taste and appearance, and increases the cost of production.  Packers dislike carcasses with poor water holding capacity because carcasses shrink following water loss, resulting in lost yield, and payment to the producer based on weight prior to fluid loss.

 

Figure 2.  Pale, Soft, Exudative Pork.

Figure 2.  Excess purge from PSE meat.

 

 

Various measurements of water-holding capacity exist in the industry, but the most prevalent and most accurate measurement is muscle pH measured either very soon (45 minute pH) after harvest or at 24 to 48 hours post-harvest (ultimate pH).  Muscle pH measurements are an indicator of muscle protein degradation and water binding capacity.  Rapid decline in muscle pH within 45 minutes post-harvest results in the PSE condition.  Pork with a low ultimate pH (< 5.50) has poorer water holding capacity.  Both conditions result in moisture loss in the fresh product and a dryer product after cooking.  Muscle pH is also related to firmness of the muscle, whereby muscle protein (myosin) degradation occurs due to rapid pH decline following harvest and the muscle is unable to maintain a normal shape.  Thus, the term ‘soft’ in PSE is related to the rate of post-mortem muscle pH decline. 

 

Sensory attributes of pork are also quality important for consumer acceptance and satisfaction.  Juiciness and flavor are two important attributes of pork, and as described above, they are related to measures of appearance.   Juiciness and flavor characteristics of pork revolve around the amount of fat and moisture in the product.  Pork with too little fat and poor water holding characteristics will be dry and flavorless under almost all methods of preparation.  Pork with very little marbling but very good water holding capacity will be juicy, but may not have as much flavor as a product with a greater level of intramuscular fat.  The challenge in the swine industry, given selection and feeding for leaner pigs, has been to maintain appropriate levels of fat and moisture to ensure good eating quality.

 

Tenderness is a very important sensory attribute of pork, and one that is somewhat independent of fat and moisture.  Tenderness has a high economic value for consumers (NPPC 1997) and consumers are willing to pay more for more tender fresh pork.  Tenderness differences are related muscle fiber type and the amount or connective tissue present in the muscle.  Tenderness is assessed either through trained taste panels or the use of mechanical instruments that measure the force required to shear muscle fibers in the cooked product.  Less force equates to a more tender product.  Breed differences clearly exist for pork tenderness; however, additional factors including animal age, ageing of pork, and cooking methods will also influence tenderness measures. 

 

A more recent quality issue in the pork industry is bacon quality.  Bacon quality measures are a function of belly thickness and the degree of (un)saturation of the fat in the belly.  Because bellies are mechanically sliced, thin bellies and bellies with greater levels of unsaturated fat (soft bellies) are not able to be sliced efficiently, reducing the value of the belly.   Industry-wide selection for leanness and feeding of high-unsaturated feedstuffs has led to a problem with belly quality.  Given the high economic value of the pork belly, packers are becoming very concerned about belly quality.

 

Improving Pork Quality in Youth Exhibitions

In the broadest sense, the observed quality attributes (phenotype) are a function of genetics (genotype), management (environment), and the interaction of these two factors.  The muscle quality attributes described all tend to be moderate to highly heritable; meaning the genetic makeup of the animal (breeds used, genetic selection practiced, mating system utilized) will play a direct and significant role in the observed quality of the final pork product.  The environmental portion (feeding, handling, pig health, stressors, transportation, packaging, cooking, etc) of pork quality variation also plays a direct role in the observed quality attributes.  Show pig breeders control the base genetic materials supplied, while show pig purchasers have a choice in the genetics they purchase and in the environment supplied to the genetics (the pigs) after purchase.  Thus, both the breeder and purchaser share responsibility, from conception to the packing plant, for producing quality pork. 

 

Industry wide selection for reduced backfat and increased muscle mass following the implementation of carcass merit buying systems has led to an industry-wide reduction in pork muscle quality.  Pigs with less than 0.60 inches of backfat and greater than 8.0 square inches of loin muscle area are particularly susceptible to producing poor quality pork, increasing the percent of thin, soft bellies, and in some cases increasing the frequency of downers and DOA’s.  Not all pigs under 0.60 inches of backfat will produce poor quality pork, but the frequency tends to go up in this portion of the pig population.  Recently, many packer buying systems have been changed to address the substandard quality issues presented by extremely lean, heavy-muscled pigs.  The changes have reduced premiums for very lean pigs or increased premiums for a carcass with a little more fat to ensure better quality. These efforts have resulted in the delivery of a higher quality pork product to the packer. 

Unfortunately, the show pig industry has not addressed the issues of acceptable levels of backfat and muscle mass in the production of show pigs.  Too frequently, the breeding, feeding, and management strategies utilized in the show pig industry are geared toward generation of the leanest, heaviest muscled pig that meets the judge’s criteria and not what the industry desires.

 

The incorporation of the Porcine Stress Syndrome (PSS) or Stress Gene (HAL-1843; Fuji et al., 1991) into show pig breeding programs for the purpose of increasing muscle mass and reducing backfat has led to a reduction in muscle quality.  Previous research (Christian and Rothschild, 1981; Leach et al., 1996; NPPC, 1997) has clearly demonstrated the negative impact of the stress gene on pork quality.  Recent results from the National Barrow Show (National Hog Farmer, Special Edition, 2000), where purebred progeny were compared for over 30 measures of pork quality, verify the negative effects of animals carrying one copy (Nn genotype) the Stress Gene including: paler color, lower marbling score, poorer loin firmness, less intramuscular fat, greater cooking moisture loss, tougher and chewier meat, and reduced taste panel juiciness and tenderness scores.  Previous reports indicate that between 30 and 60% of pigs carrying a single copy (PSS Carriers; Nn genotype) of the mutant Stress Gene will produce PSE carcasses regardless of how they are fed or handled prior to harvest.  In addition, for those pigs carrying two copies (PSS Positive; nn genotype) of the Stress Gene, > 90% will produce PSE carcasses regardless of how they are fed or handled prior to harvest.   The National Pork Producers Council (NPPC 1995) estimated a reduction in value of $3.91/pig for carriers (Nn) of the Stress Gene due to the poorer muscle quality attributes when compared with normal, PSS negative pigs (NN genotype).  

 

In cases of stress or exertion (hauling, mixing, showing, weighing, etc) PSS Positive (nn) genotype pigs have been known to die quickly and unexpectedly, leading to a negative consumer image of the swine industry and the welfare practices being employed.   In addition, the increased level of death and downer pigs due to the Stress Gene has created economic losses beyond muscle quality alone.  While research studies evaluating PSS carriers (Nn) show very limited death loss due the mutant gene, because of the extra muscle mass and excitability in the Stress carrier (Nn), very high rates of PSE are often found in show pigs after harvest.  It is unfortunate that the show pig producers and buyers readily admit the presence of the stress gene and encourage its use, while the commercial industry called for the elimination of the Stress Gene from the US pig population in 1996 (1996, Pork Forum Resolution).

 

Show pig feeding programs are generally designed to provide very high levels of protein and lysine to maximize muscle growth and minimize fat deposition while controlling rate of body growth.  In addition, various feed suppliers are supplying customized diets fortified with muscle enhancing additives, some which are science based and FDA approved, and others of which are not tested, to capture the efficiency of lean tissue growth.  Although the intentions of these feeding programs may be noble, the effects of the combinations of ingredients on the pig and quality of the product produced are not generally favorable.  Excessive levels of lysine and (or) crude protein in the diet creates tremendous strain on the kidneys as the pig tries to remove the excess nitrogen from the system.  Coupled with the common practice of holding a pig at a constant weight to meet a fair weight target or, more critically, the practice of feed and (or) water deprivation to cut weight to meet a weight target, the excess nitrogen in the pigs systems creates a significant nutrient imbalance that can lead to downer and dead pigs.  In these scenarios, not only is pork quality compromised, but the welfare of the pigs is compromised and should not be tolerated anywhere in the system.  Use of sound nutrition principles must be addressed in the show pig industry.

 

The use of Paylean™ to enhance growth rate and increase muscle mass in pigs at FDA approved levels is well documented (See et al., 2002; Main et al., 2002; Anderson et al., 1987; Crome et al., 1996) and its use in show pigs is very common.  Results of muscle quality research suggest that feeding Paylean™ at 9 g/ton (10 ppm) will not effect subjective assessment of loin quality and (or) palatability (Hancock et al., 1987; Watkins et al., 1990; Crome et al., 1996).  Conflicting results appear it the literature for the influence of Paylean™ on pork tenderness.  Uttaro et al. (1993) reported ractopamine-fed pigs produced loins that were 0.49 kg tougher than loins from pigs fed a control diet, and Herr et al. (2001a) reported an increase in Warner-Bratzler shear force of 0.19, 0.23, and 0.39 kg for loins from pigs fed ractopamine at 5, 10, and 20 ppm, respectively, when compared with loins from untreated pigs.  In contrast to the findings of Uttaro et al. (1993) and Herr et al. (2001a), McKeith et al. (1988) and Merkel et al. (1990) reported no difference in shear force values for loins from control pigs and pigs fed up to 20 ppm Paylean™.  Ivers et al. (2000) reported no difference between muscle pH from loins of pigs fed Paylean compared to a control diet.  These research results suggest that when fed at approved levels and for the prescribed duration prior to harvest, Paylean™ has little or no effect on muscle quality.

 

However, concerns over the use of Paylean™ in swine are still present due to the illegal use in feeder pig weight swine and the potential for feeding at levels outside the FDA approved label specifications. No research has been reported that describes the effect of feeding Payleanä to feeder pigs or the impact of feeding early in life followed by feeding again prior to harvest.  The issue regarding the use of Paylean in show pigs is not a major problem because the product has been FDA tested and approved.  What appears to the be the problem in the show pig industry is understanding the interactions of Paylean™ (at any level) or other growth promoting agents, when fed to pigs provided excessive levels of lysine and (or) protein, while being fed at or below maintenance energy levels, and to pigs known to carry the Stress Gene.  The implications of this potential combination of factors are not known and should not be tested.  From the perspective of the US swine industry and the pigs’ welfare, these practices must be stopped.

 

Following industry standard animal housing and handling procedures will improve pork quality.  Buildings should be cleaned, maintained, and properly ventilated to reduce the incidence of disease and reduce possible stressors that can negatively impact the pigs’ production efficiency and subsequent pork quality.  Over crowding pens can reduce access to feed and water and negatively affect pork quality.  Poor health management can reduce performance and negatively influence pork quality as well.

 

Body clipping/shaving is the newest fad to reach the swine exhibition circuit.  Fueled by the desire to make pigs appear fresh and young (i.e., to cover up old pigs) and to ‘enhance’ the features of the pig, body clipping has become the norm in pig exhibitions.  The process of clipping normally involves significant restraint and can cause excessive stress on the pig.  From the perspective of the packer, short/shaved hair creates a production process problem due the inability of the dehairing machines to remove the hair following scalding.  The presence of hair roots and hair follicles does not allow the USDA or state inspector to approve the carcass and requires that the plant hand skin affected carcasses.  The cost of harvest goes up significantly as a result of slower line speeds and the increased labor cost per carcass.  Many packers no longer purchase swine from exhibitions due to difficulties in removing hair.

 

Human interaction with swine has been shown to reduce the amount of stress on the pig.  In preparation for the exhibition, pigs that are routinely worked with by calm, knowledgeable people will handle easier in front of a crowd.  In preparation for unfamiliar facilities and sounds at the fair, exhibitors may wish to acclimate their pigs prior to movement by playing loud music, establishing a routine for weighing that simulates what is expected at the fair, and practicing their show ring techniques with other swine in the ring.  Invariably, when gilts are shown in market hog classes transportation triggers a pubertal response and standing heat making handling more difficult.  Patience and timing of movement may  help alleviate this issue.

 

While good animal handling practices may help prepare the pigs for what happens at the show, virtually nothing can be done to eliminate the stress involved in mixing unfamiliar pigs at the time of transport to the market and in lairage prior to harvest.  Fighting is natural when unfamiliar pigs are housed together.  To help alleviate additional stresses during loading and transport, the National Pork Board has developed the Trucker Quality Assurance (TQA) program (NPB, 2002) that addresses the key steps necessary to improve welfare of the pig and improve quality of the pork produced.  Common mistakes people make when loading hogs are: 1) moving groups that are too large, 2) loading chutes that are too steep, 3) use of electric prods, 4) overloading and crowding pigs on the truck, 5) improper truck ventilation in hot and cold seasons, and 6) failure to unload promptly when reaching the plant.   It is advisable for all individuals handling market swine to attend and receive certification under the TQA program.

 

Influences that exist beyond the production phase also can positively or negatively impact pork quality.  Packing plant conditions, rate and effectiveness of carcass chilling, processing and retail handling characteristics, and improper cooking can all influence the quality of pork and the eating experience.  Thus they must not be ignored.  However, given that all pigs from a given exhibition are generally handled similarly following transport to the plant (the same as with producer groups of hogs), differences in pork quality are generally an indication of what has happened prior to the packing plant, whether it be feeding, genetics, handling, or the combination of these factors, and when packing plant errors due occur it occurs across the entire plant for a given time (day, season, etc).  In essence, producers still share the lions share of responsibility.

 

Summary

The immediate issue relating to quality of the swine from derived from youth exhibitions is market access.  Packers are unwilling to purchase pigs derived from exhibitions due to the risk of residues, poor muscle quality, hair length issues, and the number of DOA’s and (or) downer pigs.  Future market access will necessitate change.  The Quality Assurance programs utilized throughout the nation have helped educate youth on residues and providing wholesome, safe food, but have not at this point addressed the other issues.  Unfortunately, resistance to change exists in the show pig industry.  When show pig producers’ attitudes evolve to the use of euthanasia and rendering as solutions to the problems, the entire swine and livestock industries are open to attacks from the public. 

 

Accountable breeding, feeding, and handling practices need to be employed and demanded of youth participants and their parents.  Accountability address the welfare well-being of the pigs and provide an understanding of the implications that current practices have on societies views of the agrarian philosophy of animals as sources of food and sustenance.  An avenue to teach the science-based principles of animal science to our youth is needed.  These changes only come about through communication and dialogue between current and future generations of parents, judges, youth, industry, and educators at all levels. 

 

References

Anderson, D. B., E. L. Veenhuizen, W. P. Waitt, R. E. Paxton, and D. H. Mowrey.  1987.  Effect of ractopamine on nitrogen retention, growth performance and carcass composition of finisher pigs.  J. Anim. Sci. 65 (Suppl.1):130-131 (Abstr). 

Christian, L. L. and M. F. Rothschild.  1981.  Performance and carcass characteristics of normal, stress    carrier, and stress susceptible swine.  AS-528-F.  Iowa State University. Ames, IA.  pp 1-3.

Crome, P. K., F. K. McKeith, T. R. Carr, D. J. Jones, D. H. Mowrey, and J. E. Cannon.  1996.  Effect of ractopamine on growth performance, carcass composition and cutting yields of pigs slaughtered at 107 kg and 125 kg. J. Anim. Sci. 74:709-716.

Fujii, J., K. Otsu, F. Zorato, S. de Leon, V. K. Khanna, J. Weiler, P. J. O’Brien, and D. H. MacLennan.  1991.  Identification of a mutation in the porcine ryanodine receptor that is associated with malignant hypothermia.  Science 253:448-451.

Hancock, J. D., E. R. Peo, Jr., and A. J. Lewis.  1987.  Effects of dietary levels of ractopamine (a phenethanolamine) on performance and carcass merit of finishing pigs.  J. Anim. Sci. 65 (Suppl. 1):309  (Abstr.).

Herr, C. T., S. L. Hankins, A. P. Schinckel, and B. T. Richert.  2001a.  Evaluation of three genetic populations of pig for response to increasing levels of PayleanÒ.  Available at:  http://www.ansc.purdue.edu/swine/porkpage/.   Accessed July 1, 2002.

Herr, C. T., A. P. Schinckel, L. Watkins, B. Weldon, and B. T. Richert.  2001b.  Optimal PayleanÒ sequence (step-up/step-down) when fed to late-finishing swine.  2001 Swine Research Report, Purdue University.  pp 35-48.

Ivers, D. J., W. C. Weldon, J. D. Muegge, S. N. Carr, M. B. England, F. K., McKeith, and D. B. Anderson.  2000.  The effect of Payleanä on lean yields and pork quality characteristics.  Proc. 53rd Recip. Meat Conf., Columbus, Ohio.  pp 131-132 (Abstr.)

Main, R. G., S. S. Dritz, M. D. Tokach, R. D. Goodband, and J. L. Nelssen.  2002.  Effects of ractopamine dose and feeding duration on pig performance in a commercial finishing facility.  J. Anim. Sci. 80(Suppl. 2):76 (Abstr.).

McKeith, F. K., C. R. Stites, S. D. Singh, P. J. Bechtel, and D. J. Jones.  1988.  Palatability and visual characteristics of hams and loin chops from swine fed ractopamine hydrochloride.  J. Anim. Sci. 66(Suppl. 1):306 (Abstr.).

Merkel, R. A., A. S. Babiker, A. L. Schroeder, R. J. Burnett, and W. G. Bergen.  1990.  The effect of ractopamine on qualitative properties of porcine longissimus muscle. J. Anim. Sci. 68(Suppl. 1):336 (Abstr.).

NPPC.  1995.  Genetic Evaluation/Terminal Line Program Results. R. Goodwin and S. Burroughs, ed.

Natl. Pork Prod. Counc., Des Moines, IA.

NPPC.  1997.  Effects of the Stress Gene on Quality - Proceedings of Pork Quality Summit. E. Dotson, ed.  Natl. Pork Prod. Counc., Des Moines, IA.  pp 35 - 48.

NPPC.  2000.  Composition & Quality Assessment Procedures.  E. Berg, ed.  Natl. Pork Prod. Counc., Des Moines, IA.

See, M. T., T. A. Armstrong, and W. C. Weldon.  2002.  Effect of ractopamine (PayleanÒ) feeding program on growth performance and carcass value.  J. Anim. Sci. 80(Suppl. 2):77 (Abstr.).

Uttaro, B. E., R. O. Ball, P. Dick, W. Rae, G. Vessie, and L. E. Jeremiah.  1993.  Effect of ractopamine and sex on growth, carcass characteristics, processing yield, and meat quality characteristics of crossbred swine.  J. Anim. Sci. 71:2439-2449.

Watkins, L. E., D. J. Jones, D. H. Mowrey, D. B. Anderson, and E. L. Veenhuizen.  1990.  The effect of various levels of ractopamine hydrochloride on the performance and carcass characteristics of finishing swine.  J. Anim. Sci. 68:3588-3595.