C. Pomar and M. Marcoux

Transcription

1 The accuracy of measuring backfat and loin muscle thicknesses on pork carcasses by the Hennessy HGP2, Destron PG-100, CGM and ultrasound CVT grading probes C. Pomar and M. Marcoux Agriculture and Agri-Food Canada, Dairy and Swine Research and Development Centre, P.O. Box 90, Lennoxville, Quebec, Canada J1M 1Z3 ( Received 9 May 2005, accepted 23 June Pomar, C. and Marcoux, M The accuracy of measuring backfat and loin muscle thicknesses on pork carcass by the Hennessy HGP2, Destron PG-100, CGM and ultrasound CVT grading probes. Can. J. Anim. Sci. 85: Research was undertaken to evaluate the accuracy of different grading probes measuring backfat (F) and loin muscle thicknesses (M). Thus, 270 pig carcasses were selected according to a factorial arrangement. Gender (barrows and gilts), fat thickness at the Canadian grading site (< 15.75, to and > mm), and hot carcass weight (75.5 to 81.8, 81.9 to 86.2 and 86.3 to 92.7 kg) were the main factors. The Hennessy (HGP2), Destron (PG-100) and CGM optic probes and the CVT ultrasound probe with two transducers [PCA-5049, 172 mm (CVT-1) and PCB-5011, 125 mm (CVT-2)] were evaluated. Grading measures were compared to the equivalent measures taken in a digitized image. The F and M precision was evaluated in terms of random bias (ED). Hennessy F and CVT-1 M had the lower ED. For F measurements, CGM, Destron, CVT-2 and CVT-1 ED was respectively, 1.65, 1.72, 1.78 and 2.14 times greater than Hennessy ED. For M measurements, ED of CVT-2, CGM, DPG and Hennessy was 1.02, 1.84, 2.03 and 2.20 times greater than CVT-1 ED. Measures of the intercostal muscles were not reliable in any of the probes able to take that measure. Key words: Pork, carcass grading, grading probes, HGP2, PG-100, CGM, CVT Pomar, C. et Marcoux, M Exactitude des mesures d épaisseur de gras et de muscle de la longe des carcasses de porc prises avec les sondes Hennessy (HGP2), Destron (PG-100), CGM et CVT. Can. J. Anim. Sci. 85: L exactitude des mesures des épaisseurs de gras (G) et de muscle (M) dorsaux obtenues avec différentes sondes de classement a été évaluée sur 270 carcasses sélectionnées selon un dispositif en factoriel dont le sexe (mâles castrés et femelles), le poids de la carcasse chaude (75.5 to 81.8, 81.9 to 86.2 et 86.3 to 92.7 kg) et l épaisseur de gras (< 15.75, to et > mm) étaient les facteurs principaux. Les mesures obtenues avec les sondes optiques Hennessy (HGP2), Destron (PG-100) et CGM et à ultrasons CVT avec les transducteurs PCA-5049 (172 mm, CVT-1) et PCB-5011 (125 mm, CVT-2) ont été comparées à celles prises sur une image digitalisée de la côtelette. La fidélité des mesures G et M a été évaluée par rapport au biais aléatoire (BA). Les mesures G avec Hennessy et M avec CVT-1 avaient le BA le plus petit. Pour les mesures de G, le BA des sondes CGM, Destron, CVT-2 et CVT-1 était respectivement 1.65, 1.72, 1.78 et 2,14 fois plus grand que le BA de Hennessy. Les BA des mesures M obtenues avec CVT-2, CGM, DPG et Hennessy était 1.02, 1.84, 2.03 et 2.20 fois plus grand que celui de la CVT-1. Les muscles intercostaux n ont pas été bien mesurés par les instruments capables de pendre cette mesure. Mots clés: Porc, classification des carcasses, sondes de classement, HGP2, PG-100, CGM, CVT In Canada as in many other countries, pork grading systems use weight and lean yield to determine the commercial value of carcasses (Canada Gazette 1986). Carcass lean yield is predicted based in the strong relationship that exists between this parameter and backfat and muscle thicknesses measured at specific carcass locations (Fredeen and Bowman 1968; Engel et al. 2003). These two thicknesses are measured using instruments specifically developed for grading pork carcasses on the slaughter line. Optical probes operate on the principle of light reflection to measure fat and muscle thicknesses. Tissue thickness measurements vary by type of probe, since each probe operates differently depending on the type of light emitted, the sensitivity of the reflection-measuring device and software interpretation. Consequently, the precision of the probes may be different and the equations developed for predicting the various yields are specific to each instrument (Sather 481 et al. 1989). The HGP2 Hennessy probe (HGP) and the DPG-100 Destron probe (DPG) are common devices used for grading carcasses in Canada and other countries. These two instruments have been evaluated in the past in order to determine their accuracy (Fortin et al. 1984; Usborne et al. 1987; Hulsegge et al. 1994; Hulsegge and Merkus 1997). Other optical instruments such as the Capteur Gras Maigre Abbreviations: HGP, Hennessy Grading Probe; DPG, Destron Pork Grader; CGM, Capteur Gras Maigre; CVT, Carcass Value Technology; CANSITE, Canadian grading site; CVTSITE, CVT grading site, R 2, coefficient of determination; RSD, residual standard deviation; MSPE, mean square prediction error; ECT, error in central tendency; ER, error due to regression; ED, error due to disturbances; F, backfat thickness; M, loin muscle thickness

2 482 CANADIAN JOURNAL OF ANIMAL SCIENCE (CGM) or the ultrasound-based Carcass Value Technology (CVT) system are also available for grading pork carcasses. There is increasing demand in Canada for the production of carcasses having a minimal backfat depth and loins with large muscle areas, especially for m. longissimus (Pomar et al. 2001). As a result, the agencies responsible for the carcass payment system are considering the inclusion of backfat and loin depth in the grading system. Some Canadian slaughtering facilities now pay premiums for carcasses with above-average muscle depth, in the hope of promoting production of hog carcasses with larger loin-eye areas. Furthermore, some muscle measurements may be used for grading quality control. Although available grading probes have been evaluated for their accuracy when predicting lean yields, the accuracy of their measurements have not yet been evaluated. Therefore, this research was undertaken to evaluate the precision of the different grading probes for measuring backfat and muscle thicknesses. MATERIALS AND METHODS Carcass Sampling Two hundred and seventy (270) carcasses were selected in a commercial slaughterhouse in Quebec, Canada, according to a factorial arrangement design in which gender (barrows and gilts), backfat thickness (thin, medium and thick) and hot carcass weight (light, medium and heavy) were the main factors. The fat thickness for the thin, medium and thick classes were <15.75 mm, to mm and >19.75 mm, respectively. The weight intervals for the light, medium and heavy weight carcass classes were 75.5 to 81.8 kg, 81.9 to 86.2 kg, and 86.3 to 92.7 kg, respectively. These fat and weight intervals were chosen to increase the number of extreme carcasses. Each backfat thickness carcass weight class had 15 carcasses of each gender. The fat thickness used to assign the carcasses to each backfat thickness class was the one obtained with the HGP probe. One carcass from each weight, fat thickness and gender class was selected on Monday of each of the 15 collection weeks, for a total of 18 carcasses per week. Only carcasses properly split in the middle of the spinal column were used in the project. Carcasses requiring trimming or tissue removal of any kind were not retained. It is important to note that the gilts and barrows selected in this cutout project had the same average weight and fat thickness and that these values were similar to those of the population, gilts and barrows combined. Grading Equipment Fat and muscle thickness was measured with three optical probes: Hennessy (model HGP2, Hennessy Grading Systems Limited, New Zealand), Destron (model PG-100, Anitech Identification System Inc., ON) and Capteur Gras Maigre (CGM, Sydel, Lorient, France), and an ultrasound instrument (CVT, Animal Ultrasound Services Inc., NY; long transducer, PCA-5049: 3.5 MHz, 172 mm and short transducer, PCB-5011: 3.5 MHz, 125 mm). The backfat and muscle measurements are rounded by the instruments at 0.2, 0.5 and 1.0 mm by the HGP, DPG and CGM probes, respectively. The digitized image, CVT-1 and CVT-2 measurement were rounded at 0.01 mm. Data Collection Prior to data collection, the proper functioning of the devices and the reading of the standards were verified in accordance with manufacturer s recommendations. After splitting, carcasses were weighed and backfat thickness measured with an A-mode ultrasound device (Krautkramer SM2; Krautkramer, Cologne, Germany). A-mode ultrasound measurements were only used for the selection of carcasses. Carcasses having the right weight and fatness were diverted to the back rail where all carcass measurements were taken on the left sides. Measurements were taken by experienced operators on immobilized carcasses as opposed to moving carcasses on the rail. These measuring conditions were imposed to minimize the effect of multiple probing observed by Pomar and Rivest (1999) when carcasses are graded in the moving rail at normal slaughter speed. The ultrasound CVT probes were always used first since they are non-invasive and do not deform the subcutaneous fat or the muscle tissues. The readings with the ultrasound instrument were taken by placing in turn the long transducer (PCA-5049) (CVT-1) and the short transducer (PCB- 5011) (CVT-2) in a position parallel to, and 5 cm distant from, the split midline of the carcasses, considering the last rib as the reference point (CVTSITE). The operation of this system was described in Liu and Stouffer (1995) and Liu et al. (1993). Optical probe measurements were taken at the Canadian grading site between the 3rd and 4th last ribs, 7 cm off the mid-line (CANSITE). Probe measurements were taken using the alternation technique, which is believed to minimize the effect of multiple probing at the same grading site on fat and muscle thickness readings (Pomar et al. 2002) and has been extensively used in research (Usborne et al. 1987; Fortin et al. 1984; Diestre et al. 1989). Furthermore, this alternation technique was applied for the set-up of the current grading system in Canada (Canadian Pork Council 1994). Image Digitization The carcasses were delivered to the Agriculture and Agri- Food Canada Research Centre in Lennoxville every Tuesday after h of refrigeration at the slaughterhouse. After their arrival the carcasses were stored at 4 C in the meat laboratory s refrigerator until dissection. A clean-cut perpendicular to the middle line was made on the fresh loin between the 3rd- and 4th-last ribs of the left side of the carcasses using an electric meat saw. A digital image of the area of the chop stored and kept at 4 C was taken using a flatbed image scanner (Scanmaker 2; Microtek, U.S.A.) with a resolution of 100 pixels per inch (100 DPI) as previously described by Pomar et al. (2001). In brief, the contour of the m. longissimus thoracis was drawn manually on each image. The centre of the vertebral canal was taken as the origin of a cartesian plane with the Y-axis corresponding to the middle line and the centre of the apophyses, parallel to the split midline. Thickness of fat, loin and intercostal muscle was measured at the CANSITE to be compared with the

3 POMAR AND MARCOUX ACCURACY OF BACKFAT AND MUSCLE THICKNESS MEASUREMENTS 483 equivalent measurements taken by the optical probes and the thickness of fat and loin at the CVTSITE to be compared with the equivalent measurements taken by the CVT grading probes (Fig. 1). The intercostal mucle was included in the loin depth measurement of the CVTSITE as measured by the CVT probes. All these parameters were measured as previously described (Pomar et al. 2001). Statistical Analyses Probe carcass measurements and the corresponding measurements taken on the digitized image of the pork chop were studied following the procedure of Theil (1966). Thus, the mean square of the prediction error (MSPE) was calculated as the sum of the square of the difference between the probe and the corresponding image measurement divided by the number of experimental observations. The MSPE was decomposed into error in central tendency (ECT), error due to regression (ER), and error due to disturbances (ED) as suggested by Benchaar et al. (1998) and Pomar and Marcoux (2003). Error in central tendency indicates how the average of probe carcass measurements deviates from the average of corresponding image measurements. Error due to regression measures the deviation of the least square regression coefficient from one, the value it would have been if carcass and image measurements were in complete agreement. That is, the angle between measured and reference value. Error due to disturbances is the variation in image measurements that is not accounted for by a least squares regression of probe carcass measurements on image measurements. In fact, this error is the unexplained variance and represents the portion of MSPE that cannot be eliminated by linear correction of the predictions (Theil 1966). Finally, when expressed in percentage of MSPE, ECT, ER, and ED are called systematic bias, proportional bias (deviation of the regression slope from one), and random bias, respectively. Additionally, the relationship between the corresponding probe and digitized image measurements was evaluated by the adjusted coefficient of determination (R 2 ) and the total residual standard deviation (RSD). All calculations were performed using the appropriate statistical procedures of SAS (1999) or programmed within the same software. RESULTS AND DISCUSSION Descriptive Statistics The Destron probe measured deeper fat thickness than the other two optical probes, while the CGM probe has the lowest fat thickness values (Table 1). The standard deviations associated with each of these measurements are comparable among probes and are close to those measured on the digitized images. For muscle thickness, the DPG once again had the deepest and the CGM probe has the lowest measurements (Table 1). Also, muscle thickness measurements taken on the digitized images were higher than those measured by the three optical probes. Finally, the fat and muscle thickness measured by the ultrasound probe were comparable to those of the other probes (Table1) despite the difference in location and angle of measurement of the ultrasound probes compared with the others (Fig. 1). Evaluation of the Accuracy of the Various Measuring Instruments of Fat and Muscle Thickness Evaluating the accuracy of an instrument implies the evaluation of the closeness between its measurements and the accepted reference values in terms of trueness and precision. The trueness of a measurement indicates the degree of agreement between the expected and reference value while the precision indicates the degree of internal agreement between independent measurements made under specific conditions. A device is said to be accurate when it is true, i.e., when its measurements adjust to the true values, and precise when there is no spread around the true value (International Standardization Organization 1993). However, because in this experiment only one probe measurement is available for each reference measurement and because the accuracy of grading equipment may vary depending on the magnitude of the measurements, trueness is evaluated in this study in terms of the error of central tendency (ECT) and error of regression (ER), that is ECT + ER, while precision is evaluated in terms of the error due to disturbances (ED). Overall accuracy is evaluated by the magnitude of the mean square prediction error (MSPE). The limiting factor in this analysis is the fact that the real dimensions of backfat thickness, and loin or intercostal muscle thickness in hot carcasses are not available. However, as in a previous study (Pomar et al. 2001), measurements taken on the digitized image can be made very precisely and therefore, they are the closest available estimates of the real dimensions of the loin chops. For this reason, they are used in this study as the reference measurements. In this study however, the image of the loin chops were digitized at 4 C and not frozen as in the previous study (Pomar et al. 2001). Thus, the accuracy of the various measuring instruments is evaluated by comparing backfat and loin and intercostal muscle thicknesses measured by the grading probes to the equivalent measurement taken on the digitized images. The results obtained with the backfat measurement for all probes are presented in Figs. 2 to 13. The detailed results of all probes are presented in Tables 1 and 2. Comparing the trueness of backfat thickness measurements, the CVT-2 probe obtained the best scores (0.358) and the DPG the worst (2.163). Overall, the CVT-2 was 2.4, 2.5, 2.8 and 6 times truer than the HGP, CVT-1, CGM or DPG probes, respectively. In fact, for the truer probe (CVT- 2), the average difference between reference measurements and those made by the probe, which is an estimation of the ECT error, was 0.45 mm (Table 2). This same difference was 0.40, 0.65, 0.18 and 1.16 mm for the HGP, CVT-1, CGM and DPG probes, respectively. For the HGP probe, the observed difference between the carcass and digitized image is much lower that the 1.7 mm observed in a previous study (Pomar et al. 2001). On the other hand, the slope of the regression between reference and probe measurements, which is an estimation of the ER error, was 0.905, 0.805, 0.859, and for the CVT-2, HGP, CVT-1, CGM and DPG probes, respectively (Figs. 2 to 6). These slopes are all lower than 1, which means that all probes overestimate fat thickness measurements in lean carcasses and underestimate these measurements in fat carcasses (Figs. 2

4 484 CANADIAN JOURNAL OF ANIMAL SCIENCE Table 1. Descriptive statistics of backfat, loin and intercostal muscle thickness measurements obtained with the HGP2 Hennessy, PG-100 Destron and CGM optical probes, the CVT ultrasound probe and measurments made on the digitized image of the pork chops Probe measurements (mm) Grading site z n Mean Standard deviation Minimum Maximum CV y HGP (HGP2 Hennessy) Backfat CANSITE Muscle CANSITE Intercostal muscles CANSITE DPG (PG-100 Destron) Backfat CANSITE Muscle CANSITE CGM Backfat CANSITE Muscle CANSITE Intercostal muscles CANSITE CVT-1 x Backfat CVTSITE Muscle CVTSITE CVT-2 x Backfat CVTSITE Muscle CVTSITE Digitized image Backfat CANSITE Muscle CANSITE Intercostal muscles CANSITE Backfat CVTSITE Muscle CVTSITE Intercostal muscles CVTSITE z Canadian grading site (CANSITE): between the 3rd and 4th last ribs, 7 cm off the mid-line; CVT grading site (CVTSITE): parallel to, and 5 cm distant from the split midline considering the last rib as the reference point. y CV= 100 standard deviation/mean. x CVT-1: CVT with the long transducer (PCA-5049); CVT-2: CVT with short transducer (PCB-5011). Fig. 1. Digitized image. Measurements taken at the: (A) CVT grading site and (B) Canadian grading site.

5 POMAR AND MARCOUX ACCURACY OF BACKFAT AND MUSCLE THICKNESS MEASUREMENTS 485 Fig. 2. Hennessy (HGP) versus digitized image backfat thickness measurements at the Canadian grading site (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. Fig. 3. Destron (DPG) versus digitized image backfat thickness measurements at the Canadian grading site (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. to 6). Note that in Figs. 2 to 13, the solid line represents the perfect relationship between probe and reference measurements, while the dotted line represents the regression line between these same measurements. When the observations are below the solid line, the device s measurements are lower than the reference values. When the dotted line is below the solid line, the values predicted by the regression are systematically underestimated relative to the reference. The CVT-2 probe was characterised by its low ECT and ER errors and the DPG probe by its high ECT and ER errors. The HGP and CVT-1 probes had intermediate ECT and ER errors. However, the CGM probe had the lowest difference between reference and probe measurements, that is the lowest ECT error, but it also showed the highest ER error as indicated by the lowest slope between reference and probe measurements. In fact, backfat measurements taken by the CGM probe were systematically overestimated in lean carcasses and underestimated in fatter carcasses. These overand underestimation were similar in trend and magnitude to the ones observed using the Destron probe measurements. Nonetheless, ECT and ER errors are systematic and they can be corrected by regression. When comparing the precision of backfat thickness measurements by comparing the error due to disturbances (ED), the HGP probe obtained the best and the CVT-1 the worst scores of all evaluated probes. Overall, the CGM, DPG, CVT-

6 486 CANADIAN JOURNAL OF ANIMAL SCIENCE Fig. 4. Capteur Gras Maigre (CGM) versus digitized image backfat thickness measurements at the Canadian grading site (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. Fig. 5. Carcass Value Technology with long transducer (PCA-5049: 3.5 MHz, 172 mm) (CVT-1) versus digitized image backfat thickness measurements taken parallel to, and 5 cm distant from, the split midline of the carcasses (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. 2 and CVT-1 had 1.7, 1.7, 1.8, and 2.1 times greater EDs (or equivalently lower precision) than the HGP probe. This ED error is random in nature and can not be corrected by regression. Therefore, this fraction of the total error, which is measured in this study by the ED, would determine the ability of these grading probes to estimate the reference backfat thickness or to predict carcass lean yield. In fact the square root of ED is equivalent to the RSD obtained when regressing device measurements over reference data (see Table 2). In these regressions, the ECT and ER error components are corrected and only the ED component, which represents the random variation, remains unexplained. Nonetheless, comparing the overall accuracy of the grading probes in terms of the observed MSPE, the HGP probe was the most accurate and in relation to this probe, the CVT-2, CGM, CVT-1 and Destron were 1.3, 1.5, 1.7 and 2 times less accurate. When measuring loin muscle thickness, the lowest MSPE error was observed for the DPG probe and the highest for the CGM probe. In relation to the DPG probe, the CVT-1, CVT-2, HGP and CGM probes were 1.1, 1.5, 1.7 and 1.9 times less accurate (or equivalently higher MSPE). Decomposing this total error in its independent error components, the DPG probe was also the probe showing the lowest ECT but also the highest ER error. Thus, the CVT-1,

7 POMAR AND MARCOUX ACCURACY OF BACKFAT AND MUSCLE THICKNESS MEASUREMENTS 487 Fig. 6. Carcass Value Technology with short transducer (PCB-5011: 3.5 MHz, 125 mm) (CVT-2) versus digitized image backfat thickness measurements taken parallel to, and 5 cm distant from, the split midline of the carcasses (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. Fig. 7. Hennessy (HGP) versus digitized image loin muscle thickness measurements at the Canadian grading site (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. CVT-2, HGP and CGM probes had ECT errors of 1.8, 2.9, 2.9 and 3.5 times higher than the DPG probe. These ECT errors are in accordance with average differences between probe and reference measurements, which were 4.07, 5.38, 6.81, 6.83 and 7.49 mm for the DPG, CVT-1, CVT-2, HGP and CGM probes, respectively. For the HGP probe, this difference is close to the 9.8 mm observed in a previous study (Pomar et al. 2001). However, all probes tended to underestimated loin muscle thickness and this underestimation increased with the degree of leanness of the carcass as shown by the slopes of the regression line between probe and reference loin thickness measurements. These slopes were 0.674, 0.538, 0.557, and for the DPG, CVT-1, CVT-2, HGP and CGM probes, respectively (Figs. 7 to 11). The HGP and CGM probes had the highest slopes and, therefore, the lowest ER errors. In relation to these probes, the ER error component of the DPG, CVT-2 and CVT-1 probes was 1.05, 2.0 and 2.3 higher. Combining the ECT and ER components of MSPE, which are used in this study to evaluate the trueness of grading probe measurements, the DPG obtained the best scores and this probe was, respectively, 1.85, 2.62, 2.74 and 3.16 truer than the CVT-1, HGP, CVT-2 and CGM probes. In terms of the most important error component of MSPE for loin

8 488 CANADIAN JOURNAL OF ANIMAL SCIENCE Fig. 8. Destron (DPG) versus digitized image loin muscle thickness measurements at the Canadian grading site (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. Fig. 9. Capteur Gras Maigre (CGM) versus digitized image loin muscle thickness measurements at the Canadian grading site (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. thickness measurements that is the random error (ED), the CVT-1 showed the lowest value and the HGP the highest value. Thus, in relation to the CVT-1, the CVT-2, CGM, DPG and HGP probes increased ED by 1.0, 1.8, 2.0 and 2.2 times. These results indicate that ultrasound devices were more precise than the optical devices when measuring loin muscle thickness. Differences between measuring devices might result from the fact that backfat and muscle thickness measurements are specific to each optical probe, since the probes operate in their own distinctive way, based on wavelength of light emitted, the sensitivity of the reflection measurement device, and its software interpretation (Swatland et al. 1994; Swatland 2001). Also, ultrasound measurements were performed at 5 cm from the middle line, whereas grading probes were introduced at 7 cm from that line. Furthermore, differences between fat and lean measurements from ultrasound devices may be partially attributed to the velocity of sound, which is lower for fat than for muscle at normal body temperatures (Haney and O Brien 1986). For backfat and loin muscle thickness, differences between the readings of the grading devices on hanging hot carcasses or the equivalent measurements made on the digitized image of the refrigerated loin might be explained by differences in fat layer and muscles shape. In fact, hot carcasses rapidly undergo complex shifts in muscle and fat

9 POMAR AND MARCOUX ACCURACY OF BACKFAT AND MUSCLE THICKNESS MEASUREMENTS 489 Fig. 10. Carcass Value Technology with long transducer (PCA-5049: 3.5 MHz, 172 mm) (CVT-1) versus digitized image loin muscle thickness measurements taken parallel to, and 5 cm distant from, the split midline of the carcasses (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. Fig. 11. Carcass Value Technology with short transducer (PCB-5011: 3.5 MHz, 125 mm) (CVT-2) versus digitized image loin muscle thickness measurements taken parallel to, and 5 cm distant from, the split midline of the carcasses (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. areas as they hang in the vertical position. Most of the shift is toward the cephalic position although there is also considerable bowing of the vertebral column (Mersmann 1982). These tissues shifts probably disappeared when the digitized image was taken. Furthermore, the effect of cooling on backfat and muscle thickness may have also contributed to these differences. Besides the absolute differences observed between image and instrument measurements, the respective coefficient of variation between the fraction of the MSPE that cannot be corrected by regression is moderate for both, backfat thickness (between 7.0 and 10.5%) and loin muscle measurements (between 5.9 and 9.4%). However, the coefficients of determination (R 2 ) between probe and image measurements vary significantly between backfat and loin muscle measurements (Table 2). For the optical probes, the coefficient of variation for backfat thickness measurements is twice as much as the coefficient of variation observed for loin muscle thickness measurements (Table 1). These coefficients of variation are three times higher for backfat thickness than for loin muscle thickness in ultrasound and image measurements. These differences in variation between backfat and muscle thickness explain the higher R 2 values observed for backfat than for loin thickness predictive relationships.

10 490 CANADIAN JOURNAL OF ANIMAL SCIENCE Fig. 12. Hennessy (HGP) versus digitized image intercostal muscle thickness measurements at the Canadian grading site (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. Fig. 13. Capteur Gras Maigre (CGM) versus digitized image intercostal muscle thickness measurements at the Canadian grading site (n = 268). Regression line ( ); equality line ( ). Equality line has intercept = 0 and slope = 1. Additionally, the fact that these R 2 values observed for ultrasound loin muscle measurements are higher, and RSD and CVe lower, than for those made by the optical probes (see Table 2), despite higher coefficient of variation for optical than for ultrasound probes (see Table 1), corroborates the superior capability of ultrasound devices for measuring loin muscle thickness. In addition to backfat and loin muscle thickness measurements, the HGP and CGM probes can measure the thickness of the intercostal muscles, which are located in the loin between the end of the m. longissimus thoracis muscle and the point where the probe exits the carcass (Pomar et al. 2001). Results obtained from the HGP and CGM probes are shown in Figs. 12 and 13. The accuracy of these two optical devices in measuring these muscle layers were similar and in both cases very low (Table 2). For both probes, most of the observed variation remained unexplained as shown by the coefficients of determination that were lower that 1%. Therefore, the utilisation of these measurements for any quality control in grading seems inadvisable in the Canadian grading context. CONCLUSION The optical grading probes were generally more precise in measuring backfat thickness and the ultrasound grading probes were more precise in measuring the loin muscle

11 POMAR AND MARCOUX ACCURACY OF BACKFAT AND MUSCLE THICKNESS MEASUREMENTS 491 Table 2. Trueness, precision and accuracy evaluation of the HGP2 Hennessy, PG-100 Destron, and CGM optical probes and the CVT ultrasound probe measurements relative to measurements taken on the digitalized image z MSPE Probe measurements y (mm) n Mean R 2 RSD CVe MSPE ECT ER ED HGP (HGP2 Hennessy) Backfat Muscle Intercostal muscles DPG (PG-100 Destron) Backfat Muscle CGM Backfat Muscle Intercostal muscles CVT-1 x Backfat Muscle CVT-2 x Backfat Muscle z Mean square prediction error (MSPE), error of central tendency (ECT), error due to regression (ER), and error due to disturbances (ED) are calculated from the differences between probe and image measurements. Coefficient of determination (R 2 ), residual standard deviation (RSD) and coefficient of variation of the residuals (CVe) are calculated from the regression of probe measurements on image measurements. Trueness is evaluated from ECT and ER absolute values, precision from ED values and overall accuracy from MSPE values. y Optical probe measurements were taken at the Canadian grading site, which is located between the 3rd and 4th last rib, 7 cm off the mid-line; CVT measurements were taken at the CVT grading site, which is located parallel to the split line, 5 cm off from it, considering the last rib as the reference point. x CVT-1: CVT with the long transducer (PCA-5049); CVT-2: CVT with short transducer (PCB-5011). thickness in pork carcasses. Between the two ultrasound probes studied, the short transducer (CVT-2, PCB-5011 transducer) was more precise and accurate than the long transducer (CVT-1, PCA-5049 transducer) in measuring the backfat thickness but opposite for the loin muscle thickness measurement. Among the optical probes studied, the HGP probe was more precise when measuring backfat thickness but less precise when measuring muscle thickness. The Destron probe was the least precise probe when measuring backfat thickness, but performed at an intermediate level when measuring loin muscle thickness. Measures of the intercostal muscles were not reliable in either of the probes able to take that measure. Random biases are relatively large, as they range from 1.3 to 1.9 mm for backfat thickness and from 3.6 to 5.3 mm for loin thickness, as estimated by the RSD. These biases should be expected to be larger in commercial conditions where carcasses are graded on the moving rail at high slaughtering speed. Therefore, considering the inclusion of these measurements in the grading system to pay premiums for carcasses with minimal backfat thickness or above-average muscle depths should be approached with caution. In lean carcasses, optical probes overestimate backfat thickness, and all probes underestimate muscle depths. The utilisation of Destron probe measurements, in particular, to penalize carcasses for excessive leanness should also be questioned as this device showed a large random bias (RSD = 1.7 mm). Because each probe operates differently depending on the type of light and reflection-measuring device, different thresholds or grids are required for each probe to establish premiums or penalties. Nevertheless, the utilisation of backfat and muscle thickness measurements to penalize carcasses for its excessive leanness or to pay premiums to those having above-average muscle depth should be considered in the hope of promoting the production of hog carcasses with sufficient backfat thickness and large loin-eye areas. ACKNOWLEDGEMENTS This research study was funded by the Fédération des producteurs de porcs du Québec, the Matching Investment Initiative of Agriculture and Agri-Food Canada, the Canadian Meat Council and the Centre de développement du porc du Québec Inc. The authors thank Danielle Pettigrew, Bernard Joly, Gérard Daumas and Jacques Chesnais for their assistance in setting up this project, James Stouffer and Jean-Paul Daigle for their support in the use of the ultrasound systems, and all the staff involved in this project, especially Diane Brodeur and the employees of the Olymel slaughter plant in Princeville and the employees of Agriculture and Agri-Food Canada in Lennoxville, Quebec. Benchaar, C., Rivest, J., Pomar, C. and Chiquette, J Prediction of methane production from dairy cows using existing mechanistic models and regression equations. J. Anim. Sci. 76: Canada Gazette Hog carcass grading regulations. Canada Gazette, Part II. 120: Canadian Pork Council National Pork Carcass Cutout Project (1992): a joint initiative of Agriculture and Agri-Food Canada, the Canadian Meat Council and the Canadian Pork Council. CPC, Ottawa, ON.

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