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* Department of Animal Science, Iowa State University, Ames 50011
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Key Words: carcass • deposition • leanness • performance • selection • swine
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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). Utilization of incentive-based marketing systems became increasingly important to producers seeking to add value to the hogs they produced, corresponding to increased selection for lean percentage. As a result, the percentage of hogs sold on a carcass merit basis rose to 28% in 1988, to 78% in 1997 (Brørsen et al., 1998), and to 83% in 2002 (Grimes and Plain, 2005). In nearly 20 yr, pork producers have made tremendous strides toward providing a leaner product to the packer and ultimately to the consumer (Chen et al., 2002). The primary objective of this study was to compare growth performance and carcass composition of pigs sired by boars from the time period at which lean value-based marketing systems were introduced with the those representing the industry’s current position, where virtually all hogs are sold on a percentage lean basis. A second objective of the study was to assess changes in tissue deposition patterns of these traits that may have resulted from marketing scheme changes since the mid-1980s.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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). Matings to current time-period boars were made using fresh semen, whereas matings to old time period boars were made utilizing frozen semen.
Six matings per sire from the OTP and 5 matings per sire from the CTP were made to obtain a minimum of 3 litters per sire and to account for potential conception rate differences between frozen and fresh semen. Matings were performed across 2 breeding seasons. Females were mated to CTP boars in the first parity and then subsequently mated to OTP boars in the second parity, and vice versa, to reduce the effect of dam on pig performance across both lines. Pigs were housed in a mechanically ventilated, curtain-sided, finishing building with fully slotted floors and were allowed 0.77 m2 of floor space each in pens of 20 to 25 pigs from 34 kg of BW until they were marketed at an average BW of 109 kg. A 17.5% CP, 1.15% lysine, corn-soy diet was provided ad libitum from 34 to 68 kg of BW, followed by a 16.0% CP, 0.85% lysine corn-soy diet from 68 to 91 kg, and a 15.0% CP, 0.70% lysine corn-soy diet from 91 kg to market weight. The composition of each diet is presented on an as-fed basis and was formulated as recommended by NRC (1998).
Progeny Test
The total number of pigs evaluated within each line and sex for each trait category in 2 replications, representing 2 subsequent breeding seasons with similar management and housing, is presented in Table 1. In the first replication, boars, gilts, and barrows in each line were weighed and ultrasonically evaluated for 10th-rib LM area (LMA), off-midline backfat (BF10), and intramuscular fat percentage (IMF) every 2 wk beginning at a group mean BW of 63.5 kg. Off-test ultrasonic measurements were collected in both replications at a mean BW of 109 kg. Ultrasonic images were collected with an Aloka 500V SSD ultrasound machine fitted with a 3.5-MHz, 12.5-cm, linear-array transducer (Corometrics Medical Systems Inc., Wallingford, CT) by a National Swine Improvement Federation certified technician. Off-midline BF10 and LMA were measured from a cross-sectional image taken at the 10th-rib. A sound-transmitting guide conforming to the pig’s back was attached to the ultrasound probe, and vegetable oil was used as conducting material between the probe and skin. A minimum of 4 sagittal images was collected 7 cm off-midline across the 10th- to 13th-ribs. Final image parameters were generated using texture analysis software (Amin et al., 1997) and were included in a regression equation developed by Newcom et al. (2002) to estimate IMF.
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), were followed to obtain measurements of 10th-rib backfat, last-rib backfat, last lumbar backfat, and loin muscle area.
Kilograms of lean tissue at market weight and at trial entry were estimated using the following fat-free lean equations developed by the National Pork Producers Council (NPPC, 2000):
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Lean gain on test (LGOT) was calculated by subtracting the estimate of trial entry lean from market weight lean and then dividing by days on test.
Statistical Analysis
To evaluate the effect of sire time period on progeny growth performance, and carcass composition and tissue deposition rates, 2 types of analyses were utilized: phenotypic analysis of traits measured over the entire test period and phenotypic analysis of traits measured serially.
Off-Test Traits.
All traits evaluated at the end of the test period were analyzed with the following linear mixed model (SAS Inst. Inc., Cary, NC):
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where yijklmnp = the trait measured on the mth pig of the jth sex in the ith time period and in the kth contemporary group; TPi = the fixed effect of the ith time period; Sj = the fixed effect of the jth sex; CGk = the fixed effect of the kth contemporary group (based on on-test date); ODl = the fixed effect of the lth off-test date; OFFWTm = the linear effect of the off-test BW of the mth pig; SR(TPni = the effect of the nth sire nested within the ith time period, assumed random with SR(TP)ni 
N(0, 
SR2); DMp = the effect of the pth dam, assumed random with DMp N(0, DM2); and 
ijklmnp = residual with ijklmnp sim;N(0, 2).
The above model is the result of a stepwise process of fitting all 2-way interactions between fixed effects along with second and third order polynomial effects of the covariate OFFWT and subsequently removing nonsignificant (P > 0.05) individual effects sequentially.
Serially Measured Traits.
Traits measured serially were BW, BF10, LMA, and ultrasonically measured IMF. A random regression model was fitted to the serial data using SAS to model covariances between repeated records. Fixed and random curves were added to the previous model to evaluate deposition rates of serially measured traits. Interactions of second order polynomial terms with TPi were also fitted for the evaluations of BW, LMA, and IMF, whereas the interaction of a first order polynomial term with TPi was fitted for BF10. A first order polynomial was fitted for the random curves of BW, BF10, LMA, and IMF. An unstructured covariance structure was fitted for the random terms, and an auto-regressive covariance structure was fitted for the residuals. A significance level of P = 0.05 was established as a maximum level for an effect to remain in the model, and effects were dropped from the model sequentially by backward elimination.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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.
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) between time periods (particularly barrows) may explain the significant effect of sex x time period in the evaluation of ADG. The relative growth performance differences between sexes have been well documented (NPPC, 1995) and are similar to the findings of this study. Genetic and phenotypic trends reported by Chen et al. (2002) and Kaplon et al. (1991) indicate that a difference should be expected in ADG between time periods and thus contrast the findings of this study. Results of the current study may substantiate the industry’s strong selection pressure placed on lean percentage relative to that of growth performance since the mid- to late-1980s.
Lean Gain on Test.
Pigs sired by CTP boars had more (P < 0.05) LGOT than pigs sired by OTP boars. Differences in LGOT can be substantially affected by small differences in carcass composition, particularly LMA. Due to the fact that there were no differences found for ADG in this study, this difference in LGOT is consistent with time period differences of in-plant and ultrasonic measurements of carcass composition.
In-Plant Measures of Carcass Composition
The effect of sire time period was significant for all 3 measures of carcass backfat; pigs sired by CTP boars had less 10th-rib backfat, last-rib backfat, and last lumbar backfat when compared with pigs sired by OTP boars. Additionally, pigs sired by CTP boars had more (P < 0.05) LMA at the 10th-rib. Differences were detected (P < 0.05) between sexes (Table 2) for all measures of carcass composition, consistent with findings of NPPC (1995) and Stewart and Schinckel (1989). Findings of this study confirm the success of industry-wide selection for carcass leanness and coincide with genetic and phenotypic trends reported by Chen et al. (2002) and Kaplon et al. (1991).
Ultrasonically Measured Traits
Off-Test Traits.
Differences for traits measured ultrasonically were similar to those measured on the carcass. Ultrasonic evaluation revealed more (P < 0.01) LMA and less (P < 0.05) BF10 for pigs sired by CTP boars. Similar sex differences in ultrasonically measured compositional traits were found when compared with the in-plant carcass evaluation in this study (Table 2). Pigs sired by OTP boars, however, had more (P < 0.01) IMF when compared with pigs sired by CTP boars (4.43 vs. 4.00%). A similar result was found in pigs harvested (Schwab et al., 2006), where pigs sired by OTP boars had greater (P < 0.05) amounts of IMF than pigs sired by CTP boars.
Serially Measured Traits
Mean deposition rates for 10th-rib backfat, LMA, and intramuscular fat are listed by time period and sex in Table 3. Cumulative tissue deposition patterns, as well as daily accretion curves for each time period, are plotted in Figures 1 to 4 for BW, BF10, LMA, and IMF, respectively.
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) was detected for pigs sired by boars from both time periods. Similar daily BW accretion rates were reported by Casey (2003) and Moeller et al. (1998). As previously mentioned, results of this study for daily BW accretion are contrary to what would be expected from the genetic and phenotypic trends reported by Chen et al. (2002) and Kaplon et al. (1991) and may be explained by the unequal distribution of records among sexes in this study (Table 1).
Analysis of serial backfat measurements revealed a linear pattern of cumulative backfat deposition between 73 and 118 kg. Daily accretion of BF10, however, followed a curvilinear pattern (Figure 2) when evaluated per kilogram of BW gain. Pigs sired by OTP boars deposited more backfat (P < 0.05) at a faster rate (P < 0.05) per kilogram of BW gain than pigs sired by CTP boars throughout the entire test period (Table 3). The daily accretion rate difference for backfat between time periods explains the greater difference in BF10 between pigs sired by CTP and OTP boars at the conclusion of the test when compared with the onset of the test.
Barrows deposited BF10 at a faster rate (P < 0.05) than boars, whereas gilts had the lowest daily BF10 accretion rate (Table 3), a finding supported by Moeller et al. (1998), who reported BF10 deposition rates measured over a similar BW range of 0.309 ± 0.005 mm/kg and 0.222 ± 0.005 mm/kg for barrows and gilts, respectively. Smith et al. (1992) reported a curvilinear deposition pattern for BF10 over a BW range of 20 to 118 kg estimated from serial ultrasonic measurements of backfat. Gu et al. (1992), in a serial slaughter study involving 5 distinct genotypes, reported BF10 increased linearly with BW between 59 and 127 kg. These studies represent plausible ranges of daily backfat accretion and deposition patterns representative of the genetics of pigs sired by CTP boars evaluated in this study. It is also reported in previous studies (Hetzer et al., 1956; Noffsinger et al., 1959; Quijandria and Robison, 1971) that backfat has a linear relationship with BW during the finishing phase of production. These early reports confirm the deposition patterns found for OTP-sired pigs in the current study.
A quadratic relationship between serially measured LMA and BW, similar for each time period, was detected (Figure 3). Pigs sired by CTP boars deposited LMA at a faster (P < 0.05) and relatively constant rate through the course of the test when compared with pigs sired by OTP boars. Old time period-sired pigs, however, began the test period with a lower LMA deposition rate and had an accelerated rate of deposition (in a linear fashion) to a point where there was no difference in daily LMA accretion rates between lines at the conclusion of the test. A time period x sex interaction was found (P < 0.01) for LMA accretion rate. Within both time periods, gilts deposited LMA at the fastest rate (Table 3), a result supported by findings of Moeller et al. (1998). However, boars sired by CTP boars had the lowest rate of LMA deposition compared with gilts and barrows. Among pigs sired by OTP boars, barrows had the lowest rate of LMA deposition when compared with the other 2 sexes. Casey (2003) reported curvilinear daily LMA accretion curves in a study evaluating boars and gilts on commercial and electronic feeders. Regardless of feeder type, gilts deposited LMA at a faster rate than boars through the course of the test period. Gu et al. (1992) suggested that cumulative LMA accretion followed a relatively linear pattern of deposition from 55 to 135 kg of BW, similar to the findings for pigs sired by boars from both time periods in this study.
Analysis of serially measured IMF revealed a curvilinear cumulative deposition pattern for both time periods in which pigs sired by OTP boars deposited greater amounts of IMF from 73 to 118 kg of BW (Figure 4). Daily IMF accretion patterns illustrate that OTP-sired pigs had more IMF at 118 kg due to a faster rate of accretion early in the test period. In contrast, pigs sired by CTP boars increased the rate at which they deposited IMF in a linear fashion through the test period and concluded the test with a daily deposition rate not different (P > 0.05) from pigs sired by OTP boars. Effects of sex and time period x sex were not significant for IMF deposition rates within each time period.
Due to only recent developments in ultrasound technology to estimate IMF in the live animal, the evaluation of previous serial slaughter experiments is the only useful method to compare results found in this study. Such studies to evaluate IMF deposition patterns have only been conducted in beef cattle. In a serial slaughter experiment in beef cattle (Bruns et al., 2004), linear changes in marbling score relative to HCW were reported. In this study, steers had higher marbling scores as they increased in carcass weight, and the rate at which they deposited IMF decreased from 200 to 400 kg of HCW. They concluded that IMF is not necessarily a late-developing tissue but is a tissue that has the opportunity to develop early in growth if nutritional management permits. Others (May et al., 1992; Van Koevering et al., 1995) have reported increases in marbling when the feeding time was extended and indicated that marbling developed quadratically as time on feed increased before reaching a plateau at approximately 112 to 119 d. As days on feed increase, growth slows; however, our research in swine indicates that cumulative IMF deposition as a component of growth (not time) increased through the duration of the test period for both lines evaluated. Cumulative IMF deposition patterns for both lines presented in the current study are consistent with the findings of marbling deposition patterns as a component of growth in beef cattle.
Weis et al. (2004) explored the relationship of energy intake and BW on body composition in growing pigs. This study emphasized that energy intake may limit the expression of lean tissue growth for much of the growing-finishing period in modern pig genotypes with high lean tissue growth potential. This result, in agreement with Schinckel and de Lange (1996), may explain the difference in IMF accretion rates observed between lines in the current study. Pigs in this study, regardless of sire time period, were fed the same finishing diet formulated to meet current NRC (1998) requirements. Differences in IMF deposition rates observed between time periods may be partially due to a distinctly different genetic potential for lean tissue growth inherent between lines, in combination with corresponding energy requirements needed to reach maximum lean growth. Ultimate IMF content at slaughter appears to be a function of a pig’s genetic propensity to deposit IMF, in combination with nutritional management throughout the finishing period.
Results from this study illustrate that significant progress toward the enhancement of carcass composition, whether measured in-plant or via ultrasound, has been realized within the Duroc breed since the mid-1980s. Long-term selection response in carcass leanness has also yielded similar alterations in deposition patterns of correlated traits of LMA and ultrasonically measured intramuscular fat. Unfortunately, this increased carcass leanness over time has occurred without equivalent progress in growth rate and has been at the expense of meat quality traits, namely IMF.
Market hog procurement systems will likely continue to emphasize lean percentage in the future; however, it is important for producers to use a balanced approach to selection that includes meat quality traits. Results of this study provide a foundation for a better understanding of the development of IMF and its relationship to lean accretion, which may lead to research investigating the possibility of manipulating underlying biological mechanisms associated with this relationship. The findings also indicate that sex differences exist in deposition rates of IMF and should be considered in genetic evaluation programs for more accurate estimation of breeding values. Further research is warranted investigating the possible relationship energy intake has with lean tissue growth and accumulation of IMF.
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Footnotes |
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2 Corresponding author: tjbaas{at}iastate.edu
Received for publication May 26, 2006. Accepted for publication January 29, 2007.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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  A. P. Schinckel, M. E. Einstein, S. Jungst, C. Booher, and S. Newman Evaluation of the Growth of Backfat Depth, Loin Depth, and Carcass Weight for Different Sire and Dam Lines Professional Animal Scientist, June 1, 2009; 25(3): 325 - 344. [Abstract] [PDF]
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