Ractopamine (Paylean) Response in Heavy-Weight Finishing Pigs

Ractopamine hydrochloride (RAC) is an orally active β-adrenergic agonist that is incorporated into feed rations of finishing swine. Numerous studies have demonstrated the efficacy of RAC in improving ADG, feed efficiency, carcass weight, and dressing percentage, and its negligible effects on meat quality (Watkins et al., 1990; Stites et al., 1991; Uttaro et al., 1993; Crome et al., 1996; Carr et al., 2005b, 2009; Mimbs et al., 2005; Apple et al., 2007). Effective in April of 2006, the US Food and Drug Administration approved the feeding of 5 to 10 ppm RAC to finishing swine for the last 20.4 to 40.8 kg of BW gain. The previous label, valid from December of 1999 to April of 2006, approved the use of RAC in finishing swine weighing from 68 to 109 kg of BW. This new label opens the possibility of feeding RAC to swine at heavier final farm BW. Feeding swine to heavier final farm BW may offer economic benefits, such as increased kilograms of pork produced per sow per year as well as increased packer production efficiency (Crome et al., 1996). Few studies have addressed the efficacy of feeding RAC to swine with final farm BW of 130 to 140 kg. Crome et al. (1996) compared the growth and carcass response of finishing swine in 2 BW groups (107 and 125 kg final farm BW). They demonstrated that the heavier BW group still responded favorably to 10 and 20 ppm RAC in regard to growth performance and  carcass traits. In a component of a recent study by Carr et al. (2009), pigs were fed 5 and 20 ppm RAC with 16% CP to a final farm BW of approximately 133 kg. With pigs fed 20 ppm RAC, Carr et al. (2009)  demonstrated a 3.6 cm2 increase in loin eye area (LEA), a 0.35-unit increase in muscle score, and an increase of 0.46 kg on a ham trimmed wholesale cut [Institutional Meat Purchase Specification (IMPS)-401]. This study investigated the effects of 10 ppm RAC on finishing performance, carcass characteristics, and meat quality of pigs slaughtered at approximately 147 kg.

The protocol for this study was approved by the Institutional Animal Care and Use Committee of the University of Illinois. The study was carried out as a randomized complete block design with a 2 × 2 factorial arrangement of treatments: 1) sex (barrow or gilt) and 2) RAC inclusion (0 or 10 ppm; Paylean 9G, Elanco Animal Health, Greenfield, IN). Pigs were the progeny of PIC 337 × C22 matings. The experiment was carried out over 4 blocks. Each block had 2 replicates, composed of 4 pens per replicate (1 per sex × RAC subclass), resulting in 32 pens across 4 blocks. Each pen housed 4 pigs, resulting in 128 pigs across 4 blocks. 

Pigs were allotted to pens to create average pen BW of approximately 107 kg. Once pigs were allotted, they were allowed to acclimate to their new pens for 7 d. During acclimation, pigs had ad libitum access to a control diet (Table 1). The 28-d test phase commenced at the end of the 7-d acclimation phase. While on test, pigs were given ad libitum access to the assigned diet (Table 1). Pigs were weighed weekly during the 28-d trial period to determine ADG. Feeder weights and feed additions to each feeder were recorded to determine ADFI (as-fed basis) and G:F. 

On the final day of test (d 28), pigs were weighed off test in the morning and were housed in their test pens with access to food and water until they were loaded and transported to either the University of Illinois Meat Science Laboratory or the commercial slaughter facility. The 2 pigs closest to the pen mean BW at d 28 were selected for carcass characteristics and meat quality analysis, and these pigs were transported to the University of Illinois Meat Science Laboratory on the morning of slaughter. Meanwhile, the remaining 2 pigs per pen were sent to a commercial slaughter facility on the morning of slaughter, where HCW and last-rib fat were collected for subsequent calculation of percentage of lean (National Pork Producers Council, 2001).

A subset of pigs, totaling 64 pigs (2 pigs/pen), were used to collect detailed measurements on carcass characteristics and fresh pork quality traits. Briefly, pigs were transported the morning of slaughter, electrically stunned, exsanguinated, scalded, dehaired, decapitated, eviscerated, split, inspected, and placed immediately into a 4°C chill cooler. Approximate time from stun to cooler was 45 min. Of the 2 pigs selected from each pen, one was randomly selected for pH decline and temperature decline measurements. Pigs that were selected for pH decline were measured at 45 min, 1.5 h, 3 h, 4.5 h, and 6 h with an MPI pH meter (Meat Probes Inc., Topeka, KS) in the LM of the right side of the carcass. The first time point was collected at approximately the 13th rib, and subsequent time points proceeded anteriorly by 1-rib increments. The temperature decline from 45 min to approximately 20 h was recorded with a temperature recorder (Monitor Company, Modesto, CA) inserted into the right side LM posterior to the last rib. 

After chilling for approximately 20 h, the left side was ribbed at the 10th rib and allowed to bloom for approximately 15 min. Last-rib and 10th-rib fat depths, and 10th-rib LEA were measured. Meat quality traits measured at the 10th rib included subjective color and marbling scores (National Pork Producers Council, 1999), subjective firmness (National Pork Producers Council, 1991), Japanese color score, and objective color using a Minolta CR-300 with a D65 light source and a 0° observer (Minolta Camera Company, Osaka, Japan). A section of LM was dissected out posterior to the 10th rib, and chops were cut. Chop collection beginning from the 10th rib included a 1.0-cm-thick chop, which was discarded; a 1.3-cmthick chop for drip loss; a 2.54-cmthick chop for proximate composition analysis; and a 2.54-cm-thick chop aged for 14 d for Warner-Bratzler shear force determination. Drip loss was used to evaluate water-holding capacity. Chops selected for drip loss measurement were weighed, suspended from a fishhook in a Whirl-Pak bag (NASCO, Fort Atkinson, WI) for approximately 24 h at 4°C, and then reweighed. Results were reported on a percentage loss basis. Proximate composition on homogenized samples was determined by oven drying to determine moisture content and by extraction with an azeotropic chloroformand- methanol mixture to determine extractable lipid, as described by Novakofski et al. (1989).