Response of barrows to space allocation and ractopamine 1,2,3

Transcription

1 Response of barrows to space allocation and ractopamine 1,2,3 M. C. Brumm* 4, P. S. Miller*, and R. C. Thaler *Haskell Agricultural Laboratory, University of Nebraska, Concord 68728; and Department of Animal and Range Sciences, South Dakota State University, Brookings ABSTRACT: An experiment using 264 crossbred barrows was conducted to examine the interaction between space allocation and dietary ractopamine addition on pig performance and carcass characteristics using a 2 2 factorial arrangement of treatments. Treatments were 0.55 (19 pigs per pen) or 0.74 (14 pigs per pen) m 2 /pig from start (29.7 ± 0.1 kg BW) to slaughter (108 kg BW) in a fully slatted facility and 0 or 10 ppm (asfed basis) ractopamine for 28 d before slaughter. There were few treatment interactions. Pigs given 0.55 m 2 / pig had a lower ADG (P = 0.010), ADFI (P = 0.088), 10th-rib backfat depth on d 86 (P = 0.010), and carcass loin muscle depth (P = 0.011) than pigs given 0.74 m 2 / pig. There was no difference in feed conversion (P = 0.210) as a result of space allocation. Pigs fed diets containing 10 ppm ractopamine had decreased (P = 0.004) ADFI and improved (P = 0.001) feed conversion efficiencies for the 28-d feeding period, along with greater loin depth (P = 0.005) and carcass lean percent (P = 0.001). The improvements in 28-d carcass lean growth associated with feeding 10 ppm ractopamine resulted in an improvement in overall daily fat-free lean gain (P = 0.046). Under these experimental conditions, the response to dietary ractopamine was similar for crowded and uncrowded pigs. Key Words: Pigs, Ractopamine, Space Allocation 2004 American Society of Animal Science. All rights reserved. J. Anim. Sci : Introduction The response to ractopamine by finishing pigs is dose dependent. At a low inclusion rate (5 ppm), ractopamine enhances gain, feed efficiency, and carcass leanness. As the amount of ractopamine in the diet is increased (5 to 20 ppm), there are generally further improvements in carcass leanness and feed efficiency (Watkins, et al., 1990; Crome et al., 1996). Management factors can alter the feed intake of finishing pigs, which would alter the daily intake of ractopamine and the potential response. When pigs are given less space, feed intake almost always decreases, with a concomitant decrease in ADG (Brumm and Miller, 1996; Brumm et al., 2001). However, feed conversion efficiency is generally minimally affected by a decrease in space allocation. The following 1 A contribution of the Univ. of Nebraska Agric. Res. Div., Lincoln, NE Journal Series No The authors acknowledge D. Forsberg for animal care and W. Kreikemeier for data collection and assistance with statistical analysis. 3 Supported in part by a grant from Elanco Animal Health, Indianapolis, IN. 4 Correspondence: Rd. (phone: ; fax: ; mbrumm1@unl.edu). Received December 18, Accepted August 2, experiment was conducted to investigate the potential interaction between dietary ractopamine and space allocation on pig performance and carcass characteristics. Materials and Methods The experiment was conducted at The University of Nebraska s Haskell Agricultural Laboratory near Concord, NE, under the approval of the University of Nebraska Lincoln Institutional Animal Care and Use Committee. Pigs were housed in a fully slatted, curtainsided facility with fresh water, under-slat flushing for daily manure removal (Brumm et al., 2002). Each pen measured m and contained a two-hole feeder (Farmweld Inc., Teutopolis, IL) and one cup drinker (Drik-O-Mat, Farmweld Inc.). Crossbred barrows (Line 671 [Y L], Danbred NA, Seward, NE) were ear-tagged and individually weighed upon arrival (BW = 29.7 ± 0.1 kg) following a 2-h transport. Pigs were ranked by BW and then randomly assigned to experimental treatments within BW rank. All pigs that died during the experiment were examined by a consulting veterinarian for cause of death. Pen size was not adjusted in the event of pig removal or death. Feed disappearance was adjusted for dead and removed pigs before data analyses. A 2 2 factorial arrangement of the experimental treatments of space allocation and dietary ractopamine 3373

2 3374 Brumm et al. Table 1. Experimental diets (as-fed basis) BW range or feeding period 59 kg 4 wk 4 wk 29 to 36 to to 4 wk preslaughter preslaughter Item 36 kg 59 kg preslaughter CON a RAC a Ingredient, % Corn Soybean meal (46.5% CP) Fat b Dicalcium phosphate Limestone Salt Vitamin premix c Trace mineral premix d L-Lysine HCl Tylosin premix e Ractopamine premix f 0.05 Calculated composition ME, kcal/kg 3,399 3,417 3,425 3,425 3,425 CP, % Lysine, % Avail P, % Total P, % Ca, % Laboratory analyses g CP, % Lysine, % NA h Ca, % P, % Particle size, m NA h a CON = no ractopamine final 28 d; RAC = 10 ppm dietary ractopamine final 28 d. b CW-3800, Feed Energy Co., Des Moines, IA c Provided the following per kilogram of premix: vitamin A, 811,525 IU; vitamin D, 103,285 IU; vitamin E, 3,995 IU; vitamin K, 398 mg; choline, 14 mg; niacin, 7,160 mg; pantothenic acid, 2,500 mg; riboflavin, 602 mg; vitamin B 12, 2.5 mg. d Provided the following per kilogram of premix: Zn, 27.3 g as zinc oxide; FE, 52.2 g as ferrous sulfate; Mn, 6.3 g as mangenous oxide; Cu, 2.95 g as copper sulfate; I, 113 mg as EDDI (ethylenediamine dihydroiodide); Se, 54.5 mg as sodium selenite. e Contained 88 g/kg of tylosin (Tylan-40, Elanco Animal Health, Indianapolis, IN). f Contained 19.8 g/kg of ractopamine (Paylean-9, Elanco Animal Health). g Ward Laboratories, Kearney, NE. h NA = not available. was utilized. The space allocation treatments were 14 (uncrowded) or 19 (crowded) pigs per pen (0.74 or 0.55 m 2 /pig). The ractopamine treatments initiated 4 wk before slaughter were 0 (CON) or 10(RAC) ppm ractopamine (Paylean, Elanco Animal Health, Indianapolis, IN) additions to the diet. There were four pens per treatment combination for a total of 16 pens. The experimental diets (Table 1) were fed in meal form and were formulated to meet or exceed published nutrient requirements of barrows (NRC, 1998). Diet changes were made on the week individual pens achieved target weights. All diets contained 110 ppm (as-fed basis) tylosin (Elanco Animal Health) from arrival to 36 kg BW, 44 ppm from 36 kg to 4 wk before slaughter, and 0 ppm for the last 4 wk before slaughter. Beginning 4 wk before slaughter, pigs on both the CON and RAC treatments were offered diets formulated to contain (as-fed basis) 16.1% CP and 0.92% lysine. Pigs were weighed and feed disappearance determined every 3 wk for the first 9 wk, 4 wk before slaughter, and the day before slaughter. Because target BW the day before slaughter was 109 kg, the uncrowded pigs were switched to the CON and RAC treatments on d 58 following arrival, whereas the crowded pigs were switched on d 65. On d 2, 23, 44, 65, and 86, all pigs were scanned by real-time ultrasound for backfat depth and LM area at the 10th rib by a National Swine Improvement Federation (Raleigh, NC) certified technician (Moeller, 2002). Either 10 (crowded treatment) or seven (uncrowded treatment) pigs per pen were bled via venipuncture on the same day as weighing and real time ultrasound scanning. Pigs identified for bleeding were randomly selected at the first bleeding and the same pigs were bled at subsequent samplings. Plasma was harvested and stored frozen ( 18 C) for analysis for the urea concentrations by the automated procedure of Marsh et al. (1965). Individually identified pigs were slaughtered in Madison, NE, by IBP, Inc. (Dakota Dunes, SD) for determi-

3 Space and ractopamine effects on barrows 3375 nation of carcass composition and merit. Individual carcasses were evaluated by IBP personnel using the Animal Ultrasound System (Animal Ultrasound Services, Ithaca, NY). Average fat and loin thickness, estimated carcass percent lean containing 5% fat, and carcass value were reported on individual pigs. Fat-free lean (FFL) and daily FFL gain were estimated on individual pigs using the fat and muscle depths reported by IBP, Inc., and NPPC (2000) equations. Statistical Analyses The pen of pigs was the experimental unit for statistical analyses. Analyses of variance as a randomized complete block design were conducted using the PROC MIXED procedures of SAS (SAS Inst., Inc., Cary, NC). The model before the final 28-d preslaughter period included only space allocation. The model for the final 28-d period and overall included space, ractopamine, and their interaction as fixed effects, and replication as a random effect. Backfat and loin muscle depth from the real time ultrasound scans and plasma urea concentrations were examined using the repeated measures option of the statistical package. The percentage of pigs that died or that were removed was analyzed by χ 2 analyses. Results There were no interactions between space allocation and ractopamine treatment for final weight, ADG, ADFI, or feed conversion efficiency (Table 2). There was no difference in the CV for within-pen weight between the two RAC treatments (9.0 vs. 9.0%) in the uncrowded treatment, but it increased in the crowded pigs fed RAC compared with those fed CON (10.4 vs. 7.5%), which resulted in a trend for the space allocation by ractopamine interaction (P = 0.092). Pigs given 0.55 m 2 /pig grew slower than pigs given 0.74 m 2 /pig for the period from d0to23(p = 0.001), d 23 to 44 (P = 0.058), and from d 0 to the beginning of the RAC treatments (P = 0.015). However, there was no effect (P = 0.117) of space allocation on ADG during the 4-wk period that the RAC treatments were fed. From arrival to slaughter, the crowded pigs gained 50 g/d less than the uncrowded pigs (P = 0.01). There was a decrease in ADFI for the crowded vs. uncrowded pigs for the period from d 0 to 23 (P = 0.049) andd23to44(p = 0.015). There was no difference in ADFI from d 44 to 58 (P = 0.369), resulting in no effect of space allocation on feed intake before the initiation of the RAC treatments (P = 0.446). Crowded pigs had a decrease in feed intake during the 4-wk RAC treatment period (P = 0.019), which resulted in a tendency (P = 0.088) for a decrease in feed intake due to a restriction in space allocation from arrival to slaughter. Crowded pigs had a poorer feed conversion compared with the uncrowded pigs from d 0 to 23 (P = 0.036), resulting in an overall poorer feed conversion from arrival to the initiation of the RAC treatments (P = 0.034). However, there was no effect of space allocation on feed conversion during the RAC treatment period (P = 0.762) or overall (P = 0.210). There was no effect of 10 ppm RAC for 4 wk before slaughter on final weight compared with 0 ppm RAC. There was no effect of RAC on ADG, either during the 4-wk period it was in the experimental diets (P = 0.541) or overall (P = 0.472). The addition of 10 ppm RAC to the diet resulted in a decrease (P = 0.004) in daily feed intake during the 4-wk inclusion period. The addition of 10 ppm RAC to the diet resulted in an improvement in G:F for the 4-wk treatment period (P = 0.001). This improvement was large enough to result in an overall trend toward improvement in G:F (P = 0.079) compared with the 0 ppm RAC treatment. There were minimal effects of space allocation on 10th-rib backfat depth for the first four sampling periods (Table 3). On d 86, crowded pigs had a decrease in backfat compared with uncrowded pigs (P = 0.010). On d 65, uncrowded pigs fed 10 ppm RAC had a larger LM area than pigs fed 0 ppm (P = 0.055), with the crowded pigs tending toward a smaller LM area (P = 0.074). On d 86, the LM area was 2.3 cm 2 larger (P < 0.001) for the 10 ppm RAC pigs on the uncrowded treatment vs. 2.1 cm 2 larger (P < 0.001) on the crowded treatment compared with 0 ppm RAC-fed pigs. Similar to the live pig data, there were minimal interactions between space allocation and RAC treatments for any of the carcass traits reported. Although not different (P = 0.134), pigs fed RAC had a 2 kg heavier carcass than CON pigs, which when combined with a 0.9 kg nonsignificant (P = 0.523) heavier final weight, resulted in a 0.7% increase in carcass yield (P = 0.097). Similar to the results from the real-time ultrasound on d 86, crowded pigs had a slight decrease in carcass backfat depth (P = 0.080) compared with uncrowded pigs. Loin muscle depth decreased (P = 0.011) for the crowded vs. the uncrowded pigs, but there was no effect of space allocation on carcass merit or FFL percent. The trend (P = 0.060) toward an interaction of space allocation and RAC treatments was due to a difference in the magnitude of response to RAC. For the crowded pigs, the carcass lean percent was 55.4 and 56.1% for the CON and RAC treatments, whereas the carcass percent lean was 55.7 and 55.9%, respectively, for the uncrowded pigs. Because of the slower daily live weight gain, there was a decrease in daily FFL gain for the crowded vs. uncrowded pigs (P = 0.003). Pigs fed 10 ppm RAC for 4 wk before slaughter had an increase in loin muscle depth (P = 0.005) and carcass lean percent (P = 0.001) compared with pigs fed 0 ppm RAC. There was no effect of RAC treatment on FFL percent, but there was an increase in daily FFL gain for the 10 ppm RAC treatment. There was no effect of experimental treatments on death loss or the number of pigs removed for tail biting or poor performance (Table 4). On d 44, pigs given 0.55 m 2 floor space had a trend toward lower plasma urea

4 3376 Brumm et al. Table 2. Effect of experimental treatments on pig performance Space, m 2 /pig Preslaughter diet P-values Item CON a RAC a SE Space RAC Space RAC No. of pens BW, kg d d d d RAC initiation b d Final c CV for within pen weight, % d d d d RAC initiation b d Final ADG, g d 0 to d 23 to d 44 to d 0 to RAC initiation b RAC to final c d 0 to final c ADFI, g (as-fed basis) d 0 to d 23 to d 44 to d 0 to RAC initiation b RAC to final c d 0 to final c G:F d 0 to d 23 to d 44 to d 0 to RAC initiation b RAC to final c d 0 to final c a CON = no ractopamine final 28 d; RAC = 10 ppm dietary ractopamine final 28 d. b Initiated on d 65 for the 0.55 m 2 /pig treatment and on d 58 for the 0.74 m 2 /pig treatment. c d 93 for 0.55 m 2 /pig treatment and d 86 for 0.74 m 2 /pig treatment. concentration compared with pigs given 0.74 m 2 /pig (P = 0.063; Table 5). There was an interaction for space allocation and RAC (P < 0.001) on d 65. On d 86, pigs fed RAC tended to have a decrease in plasma urea concentration (P = 0.105). Discussion In this experiment, crowding was achieved by increasing the number of pigs per pen, which is what occurs in production systems with fixed pen sizes; however, this introduces the possibility of a group size space interaction. In a review of the literature, Turner et al. (2003) concluded that during the grower stage (31 to 68 kg BW), ADG decreases 0.48 g/d for each additional pig in the pen (3 to 100 pigs/pen), with no effect of group size on performance during the finisher stage. Kornegay and Notter (1984) suggested a decrease in ADG of 1.9 g/d during the grower stage, and 1.2 g/d during the finisher stage for each additional pig. This suggests that ADG was decreased 2 to 10 g/d during the grower stage (d 0 to d 44) and 0 to 6 g/d during the finisher stage (d 44 to final) due to the five extra pigs per pen used to establish the crowded treatment. Given the magnitude of the response to crowding treatment before ractopamine, the effect of group size can be considered minimal. Petherick (1983) suggested that the requirement for space can be expressed by the following equation: k = A/BW 0.667, where A is area (m 2 ) and BW is in kilograms. Gonyou et al. (2004), in a summary of seven peer-reviewed articles, concluded that the appropriate k value for fully slatted floors was when using ADG as the response criteria. This suggests pigs on the 0.55

5 Table 3. Effect of experimental treatments on carcass measurements Space and ractopamine effects on barrows 3377 Space, m 2 /pig a Preslaughter diet a P-values Item CON b RAC b SE Space RAC Space RAC 10th-rib backfat depth, mm d d d d d th-rib LM area, cm 2 d d d d d < <0.001 Carcass traits c Carcass wt., kg Carcass yield, % Backfat, mm Loin depth, mm Lean, % NPPC standardized fat-free lean (FFL) d % FFL FFL daily gain, kg a Eight pens per treatment. b CON = no ractopamine final 28 d; RAC = 10 ppm dietary ractopamine final 28 d. c Recorded by in-plant personnel of IBP, Inc., Madison, NE. d National Pork Producers Council (NPPC, 2000). m 2 /pig treatment would not have a decrease in ADG due to space allocation until a BW of 68 kg. On the other hand, pigs given 0.74 m 2 /pig would not be expected to have a decrease in ADG until 120 kg BW, well above the final BW for all treatments in this experiment. Based on this equation, the 3 kg lighter BW for the crowded vs. the uncrowded pigs on d 44 was most likely not due to effects of space allocation. In addition, the approximately 65 g/d difference in ADG for this period is considerably higher than that predicted for the difference in group size. On this basis, neither group size nor space allocation can adequately explain the decrease in ADG noted for the crowded pigs before d 44. Table 4. Number of pigs per pen at initiation and termination of experiment No. of pigs Space, Preslaughter m 2 /pig diet Start b End b 0.55 CON c 19 19, 19, 18, 16 RAC c 19 19, 19, 18, CON 14 14, 13, 12, 11 RAC 14 14, 14, 13, 13 a No. of pigs per pen for a given treatment combination on d 0. b No. of pigs per pen for each of four pens for a given treatment combination at slaughter. c CON = no ractopamine final 28 d; RAC = 10 ppm dietary ractopamine final 28 d. Similar to previous results (Kornegay and Notter, 1984; Brumm et al., 2001), pigs mixed at the beginning of the growing-finishing phase of production and given less space grew more slowly from time of mixing to slaughter. There was no effect of space allocation on feed conversion efficiency, which also agrees with previous results. The increased number of pigs per pen for the crowded treatment resulted in a 40% increase in total weight gain for the same space. This increase in total weight gain for the 0.55 vs m 2 /pig treatments is misleading because pigs on the 0.55 m 2 /pig space allocation required 7 d more to attain slaughter weight. If the total gain per pen is divided by the number of days pigs were in the pen, total daily gain per pen was 15.1 vs kg for the 0.55 and 0.74 m 2 /pig treatments, respectively, a 29% increase. The interaction between space allocation and RAC treatment on 10th-rib backfat depth and LM area on d 65 and 86 was likely due to how the RAC treatments were initiated. The uncrowded pigs began the RAC treatments on d 58, whereas the crowded pigs began the RAC treatments on d 65. Thus, on d 65, the uncrowded pigs had been on the RAC treatments for 7 d vs. 0 d for the crowded pigs. On d 86, the uncrowded pigs had been on the RAC treatments for 28 d vs. 21 d for the crowded pigs. The interaction (P = 0.001) between space and RAC treatments for plasma urea on d 65 (Table 5) was due to the day RAC treatments began. Pigs on the 0.74 m 2 /pig treatment had been on

6 3378 Brumm et al. Table 5. Effect of experimental treatments on plasma urea (mg/100 ml) Space, m 2 /pig a Pre-slaughter diet a P-values Day CON b RAC b SE Space RAC Space RAC <0.001 < a Eight pens per treatment. b CON = no ractopamine final 28 d; RAC = 10 ppm dietary ractopamine final 28 d. RAC was initiated on d 65 for the 0.55 m 2 /pig treatment and on d 58 for the 0.74 m 2 /pig treatment. the 0.97% lysine diet associated with the RAC treatments for 7 d, whereas pigs on the 0.55 m 2 /pig treatment were switched to the higher lysine diet following sampling on d 65. The lack of difference in plasma urea concentrations for the crowded and uncrowded treatments supports the conclusions of Brumm and Miller (1996) and Edmonds et al. (1998). These authors concluded that the decrease in ADG when space is restricted and ADFI is decreased is not related to a decreased intake of lysine or other essential amino acids. Unlike previous trials (Watkins et al., 1990; Crome et al., 1996), ractopamine had no effect (P = 0.541) on ADG for the 4-wk feeding period, although there was a slight numerical increase for pigs fed 10 ppm. The improvements in feed conversion efficiency and carcass traits were similar to those reported by Stites et al. (1991). The increase (P = 0.105) in plasma urea for the CON vs. RAC pigs on d 86 is consistent with previous reports that indicate plasma urea increases when dietary AA are available in excess of the growing pig s metabolic needs (Chen et al., 1995; Coma et al., 1995). On d 86, CON pigs were consuming a diet that contained higher levels of lysine and other essential AA than the amount required for lean growth (NRC, 1998). However, RAC pig grew 18 g/d faster and consumed 183 g/d less feed during the 4-wk period when the ractopamine treatments were applied; thus, the lower plasma urea on d 86 for the RAC vs. CON pigs reflected the better use of the higher AA diet for lean and BW growth. The numeric but nonsignificant decrease in backfat depth and significant increase in LM area on d 86 for RAC vs. CON pigs also agrees with this conclusion. Carcass results were similar to those of Stites et al. (1991) and Uttaro et al. (1993), who reported a decrease in fat depth and an increase in LM area and carcass lean percent for pigs fed 10 vs. 0 ppm ractopamine. The decrease in carcass backfat for the crowded pigs agrees with the conclusion of Brumm and Gonyou (2001) that the decrease in ADFI associated with crowding results in a decrease in carcass backfat; however, the decrease in loin depth for crowded vs. uncrowded pigs has not been reported previously. In this experiment, there were few treatment interactions; therefore, the responses to ractopamine and space allocation over a range of 0.55 to 0.74 m 2 in finishing pigs seem to be independent. Although the inclusion of 10 ppm ractopamine in the diet for 28 d before slaughter had no effect on ADG, feed conversion and carcass traits were improved. Pigs given 0.55 m 2 /pig or space grew slower with no difference in feed conversion compared with pigs given 0.74 m 2 /pig. Literature Cited Brumm, M. C., A. K. Baysinger, R. W. Wills, and R. C. Thaler Effect of wean-to-finish management on pig performance. J. Anim. Sci. 80: Brumm, M. C., and H. W. Gonyou Effects of facility design on behavior and feed and water intake. Pages in Swine Nutrition. A. J. Lewis and L. L. Southern, ed. CRC Press, New York. Brumm, M. C., M. Ellis, L. J. Johnston, D. W. Rozeboom, D. R. Zimmerman, and the NCR-89 Committee on Swine Management Interaction of swine nursery and grow-finish space allocations on performance. J. Anim. Sci. 79: Brumm, M. C., and P. S. Miller Response of pigs to space allocation and diets varying in nutrient density. J. Anim. Sci. 74: Chen, H. Y., P. S. Miller, A. J. Lewis, C. K. Wolverton, and W. W. Stroup Changes in plasma urea can be used to determine protein requirements of two populations of pigs with different protein accretion rates. J. Anim. Sci. 73: Coma, J., D. R. Zimmerman, and D. Carrion Relationship of rate of lean tissue growth and other factors to concentration of urea in plasma of pigs. J. Anim. Sci. 73: Crome, P. K., F. K. McKeith, T. R. Carr, D. J. Jones, D. H. Mowrey, and J. E. Cannon Effect of ractopamine on growth performance, carcass composition, and cutting yields of pigs slaughtered at 107 and 125 kilograms. J. Anim. Sci. 74: Edmonds, M. S., B. E. Arentson, and G. A. Mente Effect of protein levels and space allocations on performance of growingfinishing pigs. J. Anim. Sci. 76: Gonyou, H. W., J. Deen, J. J. McGlone, P. L. Sundberg, M. Brumm, H. Spoolder, J. Kliebenstein, B. Buhr, and A. K. Johnson Developing a model to determine floor space requirements for pigs. J. Anim. Sci. 82(Suppl. 1). (Abstr.) Kornegay, E. T., and D. R. Notter Effects of floor space and number of pigs per pen on performance. Pig News Info. 5: Marsh, W. H., B. Fingerhut, and H. Miller Automated and manual direct methods for the determination of blood urea. Clin. Chem. 11: Moeller, S. J Evolution and use of ultrasonic technology in the swine industry. J. Anim. Sci. 80(E. Suppl. 2):E19 E27. NPPC Composition and Quality Assessment Procedures. Natl. Pork Prod. Council, Des Moines, IA.

7 Space and ractopamine effects on barrows 3379 NRC Nutrient Requirements of Swine. 10th ed. Natl. Acad. Press, Washington, DC. Petherick, J. C A biological basis for the design of space in livestock housing. Pages in Farm Animal Housing and Welfare. S. H. Baxter, M. R. Baxter, and J. A. D. MacCormack, ed. Nijhoff, The Hague, The Netherlands. Stites, C. R., F. K. McKeith, S. D. Singh, P. J. Bechtel, D. H. Mowrey, and D. J. Jones The effect of ractopamine hydrochloride on the carcass cutting yields of finishing swine. J. Anim. Sci. 69: Turner, S. P., D. J. Allcroft, and S. A. Edwards Housing pigs in large social groups: A review of implications for performance and other economic traits. Livest. Prod. Sci. 82: Uttaro, B. E., R. O. Ball, P. Dick, W. Rae, G. Vessie, and L. E. Jeremiah Effect of ractopamine and sex on growth, carcass characteristics, processing yield, and meat quality characteristics of crossbred swine. J. Anim. Sci. 71: Watkins, L. E., D. J. Jones, D. H. Mowrey, D. B. Anderson, and E. L. Veenhuizen The effect of various levels of ractopamine hydrochloride on the performance and carcass characteristics of finishing swine. J. Anim. Sci. 68: