title: Effects of ractopamine on performance and composition of pigs phenotypically sorted into fat and lean groups.
abstract: Crossbred barrows (n = 144; 80 kg) from four farrowing groups were phenotypically selected into fat (FAT) and lean (LEAN) pens using ultrasound. The difference in 10th-rib fat depth between the LEAN and FAT groups was > or =0.5 cm. Within a farrowing group, pigs were assigned to pens (five pigs per pen and eight pens per phenotype) to equalize pen weight and fat depth. Pigs were fed a corn-soybean meal diet containing 19% CP, 1.0% added animal/vegetable fat, and 1.1% lysine (as-fed basis). Half the pens received 10 ppm (as-fed basis) of ractopamine (RAC) during the 28-d finishing phase. At 7-d intervals, live weight and feed disappearance were recorded to calculate ADG, ADFI, and G:F, and 10th-rib fat depth and LM area were ultrasonically measured to calculate fat-free lean and fat and muscle accretion rates. During the first 7 d on feed, LEAN pigs fed RAC gained less (P < 0.05) than FAT pigs fed RAC or LEAN and FAT pigs fed the control diet (RAC x phenotype; P = 0.02); however, RAC did not (P > 0.25) affect ADG after the second, third, and fourth weeks, or over the entire 28-d feeding period. Although wk-2 and -3 ADG were higher (P < or = 0.03) in LEAN than in FAT pigs, phenotype did not (P = 0.08) affect overall ADG. Dietary RAC decreased (P < or = 0.05) ADFI over the 28-d feeding trial, as well as in wk 2, 3, and 4, but intake was not (P > 0.20) affected by phenotype. Neither RAC nor phenotype affected (P > 0.10) G:F after 7 d on trial; however, RAC improved (P < or = 0.04) wk-3, wk-4, and overall G:F. Lean pigs were more efficient (P < or = 0.05) in wk 2 and 3 and over the duration of the trial than FAT pigs. Ultrasound LM accretion (ULA) was not (P > or = 0.10) affected by RAC; however, LEAN pigs had greater (P < or = 0.02) ULA in wk 2 and 4 than FAT pigs. Although fat depth was lower (P < 0.01) in RAC-fed pigs than pigs fed the control diet, ultrasound fat accretion rate indicated that RAC-pigs deposited less (P = 0.04) fat only during wk 4. In addition, calculated fat-free lean (using ultrasound body fat, ULA, and BW) was increased (P < 0.05) in RAC pigs after 3 and 4 wk of supplementation. In conclusion, RAC enhanced the performance of finishing swine through decreased ADFI and increased G:F, whereas carcass lean was enhanced through decreases in carcass fat and increases in carcass muscling.
authors.affiliation: Edgar L. Rhodes Center for Animal and Dairy Science, University of Georgia, Athens, 30602-2771, USA.
authors.forename: K J
authors.initials: KJ
authors.lastname: Mimbs
authors.forename: T D
authors.initials: TD
authors.lastname: Pringle
authors.forename: M J
authors.initials: MJ
authors.lastname: Azain
authors.forename: S A
authors.initials: SA
authors.lastname: Meers
authors.forename: T A
authors.initials: TA
authors.lastname: Armstrong
mesh_terms: D000273:Adipose Tissue / Q000000981:diagnostic imaging; D000821:Animal Feed / Q000032:analysis; D000818:Animals; D001823:Body Composition / Q000187:drug effects*; D004032:Diet / Q000662:veterinary*; D005838:Genotype; D006128:Growth / Q000187:drug effects; D006133:Growth Substances / Q000008:administration & dosage / Q000494:pharmacology*; D008297:Male; D010627:Phenethylamines / Q000008:administration & dosage / Q000494:pharmacology*; D010641:Phenotype; D013552:Swine / Q000254:growth & development* / Q000502:physiology; D013997:Time Factors; D014463:Ultrasonography
chemical_list: D006133:Growth Substances; D010627:Phenethylamines; C060897:ractopamine