An evaluation of the USDA standards for feeder cattle frame size and muscle thickness

Transcription

1 An evaluation of the USDA standards for feeder cattle frame size and muscle thickness A. D. Grona, J. D. Tatum 1, K. E. Belk, G. C. Smith, and F. L. Williams 2 Colorado State University, Department of Animal Sciences, Fort Collins ABSTRACT: This study was conducted to determine the live weights at which large-, medium-, and smallframed feeder steers and heifers attain a degree of finish associated with a carcass quality grade of low Choice and to examine the relationship of feeder cattle muscle thickness to carcass yield grade traits. Feeder steers (n = 401) and heifers (n = 463) representing three age classes (calf, yearling, long yearling) were selected randomly at a commercial feedlot to exhibit wide ranges in frame size and muscularity. Individual weights were recorded and a panel of five experienced evaluators scored each animal for frame size, muscle thickness, and flesh condition. The cattle were finished on a highconcentrate finishing diet and harvested at an estimated carcass fat thickness of 10 mm. Final weights and USDA carcass grade data were collected for all cattle. Frame size scores effectively predicted finished weight at a marbling end point of Small 00 for both heif- Key Words: Beef, Carcasses, Meat Grades, Muscle Weight 2002 American Society of Animal Science. All rights reserved. J. Anim. Sci : Introduction The U.S. Standards for Grades of Feeder Cattle, which were adopted for use in 1979, specify that grades for thrifty feeder cattle be determined by separate evaluations of frame size and thickness (USDA, 1979). According to the Standards, frame size evaluations reflect the live weight at which a steer or heifer would be expected to produce a carcass with a quality grade of U. S. Choice. Evaluations of thickness (indicative of differences in longissimus muscle area) reflect the yield grade of the carcass a feeder steer or heifer would produce when harvested at a given degree of finish (USDA, 1979). 1 Correspondence: phone: ; fax: ; dtatum@lamar.colostate.edu. 2 Livestock Div., Agric. Marketing Service, USDA (retired). Current address: P.O. Box 762, Barnsdall, OK Received July 9, Accepted October 23, ers (r 2 = 0.89, SE = 16 kg) and steers (r 2 = 0.94, SE = 13 kg). For heifers, the Small/Medium and Medium/ Large frame score intersects corresponded to live weights of 460 kg and 520 kg, respectively. For steers, the Small/Medium and Medium/Large frame score lines corresponded to live weights of 504 kg and 577 kg, respectively. These weights were greater than weights specified in the 1979 USDA grade standards. Evaluations of feeder cattle muscling, based on 1979 USDA Standards, were associated (P < 0.05) with differences in longissimus muscle area but were not related (P = 0.08) to differences in numerical carcass yield grades. An alternative muscle thickness classification scheme, involving the use of four thickness classes, was effective for stratifying feeder cattle according to eventual differences (P = 0.004) in carcass yield grade. Our findings suggest that USDA feeder cattle grade standards developed in 1979 are no longer adequate for describing today s population of feeder cattle. The USDA and others throughout the cattle industry have used the 1979 Standards as the basis for market reporting of feeder cattle prices. In addition, the Standards have been used for grouping feeder cattle in graded auction sales and for certifying the grades of cattle delivered on futures contracts. Though the Standards have served these purposes well, gradual genetic changes in the U. S. feeder cattle population, coupled with the adoption of new or different cattle management practices, have caused concerns regarding the continued validity of the grade specifications for frame size. Moreover, there has been concern that the 1979 thickness descriptions do not adequately categorize today s feeder cattle population according to differences in muscularity. Correspondingly, the USDA commissioned this study to evaluate and, if necessary, revise the 1979 feeder cattle grade standards. The objectives of this study were to determine the live weights at which large-, medium-, and small-framed feeder steers and heifers attain a degree of finish associated with a carcass quality grade of low Choice and to examine the relationship of feeder cattle muscle thickness to carcass yield grade traits. 560

2 Evaluation of feeder cattle grade standards 561 Materials and Methods Sampling, Measurement, and Scoring of Feeder Cattle. The experimental sample was collected over a period of 3 d and consisted of feeder steers (n = 401) and heifers (n = 463), approximately 6 to 18 mo of age. Seven large groups (lots) of cattle were randomly identified for use in the study from the inventory of recent arrivals at a commercial feedlot facility near Fort Morgan, CO. The groups originated from seven different ranches (sources) in Colorado, Kansas, Montana, Oregon, Texas, Utah, and Wyoming. Both steers and heifers (contemporaries from the same herd) were obtained from each source. All cattle had arrived at the feedlot 2 to 4 wk before the date of selection. Cattle comprising the experimental sample exhibited ranges in maturity, frame size, and muscularity that were considered to be representative of those typically encountered in application of the feeder grade standards. Because frame size and thickness are considered independently in application of the USDA feeder grades, the experiment was not conducted in a factorial arrangement with balanced numbers of cattle in each frame size muscle thickness subclass. When sampling was completed, each animal was identified with a uniquely numbered ear tag and individual initial weights were recorded. Hip height of each animal was measured using a metal ruler suspended from an overhead crossbar on the scale with the animal standing unrestrained, in a normal, resting position. Values for hip height were recorded as the difference between the distance from the crossbar to the floor of the scale and the distance from the crossbar to a point on the animal s dorsal midline between the tuber coxae. Also at this time, each animal was assigned an age classification (calf, C; yearling, Y; or long yearling, LY) based on knowledge of approximate birth dates. Breed was allowed to vary randomly in the experimental sample; however, breed type and hide color were recorded for each animal. When specific breed information was not available, breed type was estimated using phenotypic indicators of breed. Breed types represented in the experimental sample were British (55%), Continental European (41%), Dairy (2%), and Brahman-crossbred (2%). A panel of five experienced evaluators (professional livestock graders and market news reporters) visually evaluated and scored each animal for frame size, muscle thickness, and flesh condition. The evaluation panel consisted of individuals considered to be the nation s foremost authorities on the application of the USDA grade standards for feeder cattle, including one representative of USDA Agricultural Marketing Service (AMS) Livestock Standardization Branch, three representatives of USDA-AMS Market News, and one representative of the Virginia Department of Agriculture. Scores for frame size (Small, S; Medium, M; and Large, L) and muscle thickness (No. 1, No. 2, and No. 3; No. 1 = thickly muscled) were based on the 1979 USDA Standards (USDA, 1979). Evaluators scores for frame size and muscle thickness were recorded to the nearest 10% of a grade (e.g., S 00,S 10,S 20 S 100,M 00, M 10,M 20 M 100,L 00 L 100 for frame size, where S 00 is smallest and L 100 is largest and 1 00,1 10, , 2 00,2 10, , for muscle thickness, where 3 00 is least muscular and is most muscular). Flesh condition scores were assigned using a 9-point scoring system (1 = extremely thin, 2 = thin, 3 = moderately thin, 4 = slightly thin, 5 = average, 6 = slightly fleshy, 7 = moderately fleshy, 8 = fleshy, 9 = extremely fleshy; USDA, 1995). Single values representing each animal s frame size, muscle thickness, and flesh condition were calculated by averaging the five panelists scores for each trait. Mean values for frame size and muscle thickness were rounded to the nearest 10% of a grade. Initial data were collected within a 2-d period. Cattle Management. All processing and management practices were consistent with the conventional procedures used at the commercial feedlot. Cattle were penned by source (steers and heifers from each source were penned separately) for finishing. A series of five step-up diets were fed while acclimating the cattle to a high-concentrate finishing diet. The first two step-up diets were fed for 3 d each, whereas the remaining three step-up diets were fed for 5 d each. On d 21, cattle were started on the finishing diet, which consisted of 82% dry-rolled corn, 8% alfalfa hay, 6% liquid supplement, 2% fat, and 2% sunflower screenings. All cattle were given either one or two anabolic implants. Initial implants were administered at processing when the cattle entered the feedlot. Cattle receiving a second implant were reimplanted near the middle of the finishing period (at least 70 d before harvest). Cattle weighing over 363 kg when they entered the feedlot received a single implant: steers received a single Revalor-S implant (24 mg estradiol 17-β and 140 mg trenbolone acetate; Intervet, Millsboro, DE) and heifers received a single Finaplix-H implant (200 mg trenbolone acetate; Intervet). Two successive implants were administered to cattle weighing less than 363 kg when they entered the feedlot: steers received a Component E-S implant (20 mg estradiol benzoate and 200 mg progesterone; Ivy Animal Health, Overland Park, KS) followed by a Revalor-S implant, whereas heifers received two successive Finaplix-H implants. Heifers were fed melengestrol acetate at 0.05 mg/heifer daily during the entire finishing period to prevent estrus. Harvest and Carcass Data Collection. Feedlot personnel sorted each pen (based on visual appraisal), shipping cattle when they were estimated to have attained approximately 10 mm fat thickness over the longissimus muscle at the 12th rib. One to four truckloads of cattle were shipped on each shipping date (average size of shipping group = 82 animals). Overall means for adjusted carcass fat thickness and marbling score were 10 mm and Small 86, respectively; 63.4% of the carcasses graded low Choice or higher (USDA, 1997) and 99.7% had numerical yield grades of 3 or lower (USDA, 1997).

3 562 Grona et al. Individual final weights were recorded approximately 12 h before shipping. All cattle were transported to a commercial beef packing facility in Greeley, CO where they were harvested using conventional commercial procedures. Approximately 36 h postmortem, each carcass was measured or assigned scores for all factors used to determine U. S. quality grade and yield grade (USDA, 1997) by a USDA grader. Statistical Methods. Initial measurements and scores for feeder cattle, used to characterize the experimental sample, were analyzed using the Correlation and GLM Procedures of SAS (SAS Inst. Inc., Cary, NC). The least squares model used for analysis of variance included the fixed effects of age class, sex class, frame size classification, and muscle thickness classification. The F- tests for all effects were computed using the residual error mean square as the error term and comparisons among unadjusted main effect means were tested for significance using Tukey s Studentized Range test (SAS Inst. Inc.). In addition, Pearson correlation coefficients were computed (SAS Inst. Inc.) to examine the association among initial feeder cattle characteristics (weight, hip height, frame score, muscle score, and condition score). Analyses conducted to examine differences in weight, days on feed, and carcass characteristics among age, sex, and muscle thickness subclasses compared at a constant carcass fat thickness utilized statistical models (Mixed Procedure, SAS Inst. Inc.) that included the random effect of source; fixed effects of age class, sex class, frame size classification, and muscle thickness classification; and adjusted carcass fat thickness as a covariate. Preliminary analyses, using a full model, revealed no important interactions. Correspondingly, the data were reanalyzed using a reduced model that included only the main effects and the covariate. Comparisons among least squares means were tested for significance using protected t-tests. Two muscle thickness classification systems were tested: 1) the thickness classification system specified by USDA (1979) (No. 1 [1 00 to ], No. 2 [2 00 to ], No. 3 [3 00 to ], where No. 1 is thickly muscled) and 2) an alternative muscle thickness classification system (No. 1 [1 30 to ], No. 2[2 70 to 1 20 ], No. 3 [2 00 to 2 60 ], No. 4 [3 00 to ], where No. 1 is thickly muscled). A similar statistical approach was used to examine differences in final live weight and carcass characteristics among cattle classified according to flesh condition score and compared after a constant number of days on feed. The statistical model (Mixed Procedure, SAS Inst. Inc.) used for these analyses included the random effect of source; fixed effects of age class, sex class, frame size classification, muscle thickness classification, condition classification; and days on feed as a covariate. Comparisons among least squares means were tested for significance using protected t-tests. Analysis of covariance (GLM Procedure, SAS Inst. Inc.) was used to estimate the finished live weights at which heifers and steers of each frame size classification produced carcasses with a marbling score of Small 00. The statistical model included the effects of source, age class, muscle thickness classification, and frame size classification (expressed to the nearest 10% of a grade), along with marbling score as a covariate. Final live weight was the dependent variate and subclass means for final weight were adjusted to a constant marbling score of Small 00 using the overall regression of final live weight on marbling score. Adjusted final weights for each frame size classification were then regressed on frame size classification to estimate the weights corresponding to the grade intersects between the Small and Medium frame size classes (S 100 /M 00 = numerical frame score of 200) and between the Medium and Large frame classes (M 100 /L 00 = numerical frame score of 300). In the case of covariance and regression analyses, separate analyses were conducted for steers and heifers. Results and Discussion Feeder Cattle Characteristics. Data characterizing the sample of feeder cattle selected for use in the study are presented in Table 1. The experimental sample was composed of similar proportions of heifers (54%) and steers (46%). Calves constituted 31% of the sample, 47% of the cattle were yearlings, and 22% were long yearlings. Initial weights ranged from 167 kg to 474 kg (mean = 308 kg). As shown in Table 1, initial weights differed (P < 0.05) among age classes (LY > Y > C), frame size classes (L > M > S), and muscle thickness classes (No. 1 > No. 2 > No. 3). Hip height measurements ranged from cm to cm (mean = cm). Hip height measurements differed (P < 0.05) among age classes (LY > Y > C), frame size classes (L > M > S), muscle thickness classes (No. 3 > No. 1), and between sex classes (steers > heifers). Scores for frame size ranged from Small 40 to Large 100 (possible range = Small 00 to Large 100 ), and scores for muscle thickness ranged from 3 10 to (possible range = No to No ; No. 3 = thinly muscled, No. 1 = thickly muscled). With respect to frame size, evaluators classified 42% of the cattle as Large, 52% as Medium, and 6% as Small. With respect to muscle thickness, 56% of the cattle were classified as No. 1, 42% were classified as No. 2, and 2% were classified as No. 3. These distributions seemed to underscore the need to reevaluate current USDA specifications for frame size and muscle thickness. Differences (P < 0.05) in frame scores were observed among age classes (LY > Y > C) and among frame classes (Table 1). Muscle thickness scores differed (P < 0.05) only among muscle thickness classes (Table 1). Differences (P < 0.05) in flesh condition were observed (Table 1) among age classes (LY > Y > C), frame size classes (S > M > L), muscle thickness classes (No. 1 > No. 2 = No. 3), and between sex classes (heifers > steers).

4 Evaluation of feeder cattle grade standards 563 Table 1. Unadjusted means for feeder cattle characteristics measured or scored at the beginning of the experiment Initial Hip Flesh wt, height, Frame Muscle condition Effect n kg cm size a thickness b score c Age class Calf z z 286 z 311 x 5.4 z Yearling y y 278 y 285 x 5.6 y Long yearling x x 310 x 304 x 6.1 x Sex class Heifer x y 287 x 285 x 5.9 x Steer x x 288 x 311 x 5.4 y Frame size Large x x 334 x 304 x 5.3 z Medium y y 257 y 293 x 5.9 y Small z z 180 z 275 x 6.4 x Muscle thickness No x y 293 x 339 z 5.8 x No y xy 281 x 249 y 5.5 y No z x 261 x 141 x 5.0 y Residual SD d a Frame size (USDA, 1979): 100 to 199 = Small (S 00 to S 99 ); 200 to 299 = Medium (M 00 to M 99 ); 300 to 400 = Large (L 00 to L 100 ). b Muscle thickness (USDA, 1979): 100 to 199 = No. 3 (3 00 to 3 99 ); 200 to 299 = No. 2 (2 00 to 2 99 ); 300 to 400 = No.1(1 00 to ), where No. 1 is thick. c Flesh condition score(usda, 1995): 1 = extremely thin, 2 = thin, 3 = moderately thin, 4 = slightly thin, 5 = average, 6 = slightly fleshy, 7 = moderately fleshy, 8 = fleshy, 9 = extremely fleshy. d Standard errors of means may be calculated as 1/ n residual SD, where n = number of animals in that particular subclass. x,y,z Means in the same column, within an effect, without a common superscript letter differ (P < 0.05). Simple correlation coefficients (Table 2) provided insight into the interrelationships among initial measurements and subjective scores for various feeder cattle traits. Hip height, frame size, muscle thickness, and flesh condition all were positively correlated (P < 0.01) with initial weight. Scores for frame size were highly correlated (P < 0.01) with hip height measurements. However, frame size score also exhibited a low, positive correlation (P < 0.01) with muscle thickness score, reflecting a tendency for cattle perceived as muscular also to be scored higher for frame size than cattle perceived as light-muscled. A moderate, negative correlation (P < 0.01) between frame size score and condition score reflected a tendency for cattle perceived as thin to be scored higher for frame size than cattle perceived as fleshy. The correlation between hip height and condition score was not significant, but the correlation be- tween frame size score and condition score was highly significant (Table 2). This suggested that, in addition to skeletal size, evaluators considered body condition (perhaps as an indicator of compositional maturity) when assigning scores for frame size. Muscle thickness score was not correlated with hip height but exhibited a low, positive correlation (P < 0.01) with condition score. The correlation between scores for muscle thickness and condition reflected a tendency for cattle that were thin in condition to be perceived as less muscular than were fleshier cattle. Differences in Growth and Carcass Traits Among Sex and Age Classes. Least squares means comparing final live weight, days on feed, and carcass traits of cattle differing in sex class and age class are presented in Tables 3 and 4, respectively. These least squares means were computed using a statistical model that included Table 2. Simple correlation coefficients characterizing relationships among feeder cattle traits Trait Condition Muscle Frame Hip score score score height Initial wt 0.41** 0.14** 0.34** 0.72** Hip height ** Frame score 0.32** 0.13** Muscle score 0.14** **The correlation differed from zero (P < 0.01).

5 564 Grona et al. Table 3. Least squares means a for final live weight, days on feed, and carcass traits of heifers and steers compared at a constant carcass fat thickness (10 mm) Sex class Trait Heifer Steer SE P > F n Final wt, kg Days on feed Dressing percentage b Carcass wt, kg Longissimus muscle area, cm Longissimus muscle area adj. c KPH fat, % d Final yield grade Maturity e Marbling score f a Least squares means were computed using a least squares model that included the effects of source, age class, sex class, frame size classification, and muscle thickness classification, with adjusted carcass fat thickness as a covariate. b Dressing percentage was calculated using live weight (obtained 12 h before shipping) with a 4% pencil shrink (i.e., live weight 0.96). c Adjustment to the USDA yield grade equation (in yield grade units) for the difference between actual and required longissimus muscle area. d Estimated percentage of kidney, pelvic, and heart fat. e Maturity: 0 to 99 = A 00 to A 100. f Marbling score: 400 to 499 = Small; 500 to 599 = Modest. the effects of source, age class, sex class, frame size classification, and muscle thickness classification, along with adjusted carcass fat thickness as a covariate. Correspondingly, least squares means in Tables 3 and 4 represent comparisons of cattle differing in age and sex, respectively, but of the same frame size, muscle thickness and carcass fat thickness (10 mm), and originating from a common source. Heifers and steers differed (P < 0.05) in all traits except dressing percentage (Table 3) when compared at a constant carcass fat thickness (10 mm). Steers required more (P < 0.05) days on feed to attain the constant fat thickness end point. Moreover, at a constant end point of 10 mm carcass fat thickness, steers were heavier (P < 0.05) than heifers and produced heavier (P < 0.05) carcasses with larger (P < 0.05) longissi- Table 4. Least squares means a for final live weight, days on feed, and carcass traits of feeder cattle classified as calves, yearlings, or long yearlings compared at a constant carcass fat thickness (10 mm) Age class Trait Calf Yearling Long yearling SE n Final wt, kg 505 y 551 x 558 x 11.2 Days on feed 200 x 187 y 171 z 11.9 Dressing percentage b Carcass wt, kg 307 y 334 x 339 x 8.3 Longissimus muscle area, cm Longissimus muscle area adj. c 0.29 x 0.13 y 0.15 y 0.09 KPH fat, % d Final yield grade 2.39 y 2.55 x 2.52 xy ] 0.08 Maturity e 48 y 58 x 59 x 2.9 Marbling score f 478 y 516 x 508 xy 20.8 a Least squares means were computed using a least squares model that included the effects of source, age class, sex class, frame size classification, and muscle thickness classification, with adjusted carcass fat thickness as a covariate. b Dressing percentage was calculated using live weight (obtained 12 h before shipping) with a 4% pencil shrink (i.e., live weight 0.96). c Adjustment to the USDA yield grade equation (in yield grade units) for the difference between actual and required longissimus muscle area. d Estimated percentage of kidney, pelvic and heart fat. e Maturity: 0 to 99 = A 00 to A 100. f Marbling score: 400 to 499 = Small; 500 to 599 = Modest. x,y,z Means within a row without a common superscript letter differ (P < 0.05).

6 Evaluation of feeder cattle grade standards 565 mus muscle areas, lower (P < 0.05) percentages of kidney, pelvic, and heart (KPH) fat, and higher (P < 0.05) final numerical yield grades. The heifers yield grade advantage was due to their longissimus muscle area adjustment (Table 3); heifers had larger longissimus muscle areas relative to their carcass weights than did steers and, therefore, received larger (P < 0.05), negative longissimus muscle area adjustments when final yield grades were computed. Carcasses produced by heifers were evaluated as more mature (P < 0.05) but had higher (P < 0.05) marbling scores than carcasses produced by steers (Table 3). Cattle of different age classes (Table 4) differed (P < 0.05) in the number of days on feed required to attain the constant fat thickness end point (C > Y = LY) and, when compared at 10 mm fat thickness, calves had lighter (P < 0.05) live and carcass weights than yearlings and long yearlings. Longissimus muscle area did not differ among age classes, despite among-class differences in carcass weight (Table 4). Correspondingly, due to their lighter weights, carcasses produced by calves received larger (P < 0.05), negative longissimus muscle area adjustments than did yearlings and long yearlings when final yield grades were computed and, thus, had lower (P < 0.05) numerical yield grades than carcasses produced by yearlings (Table 4). Carcasses produced by calves also were evaluated as being less (P < 0.05) mature than carcasses of yearling and long yearling cattle and had lower (P < 0.05) marbling scores than carcasses produced by yearlings. Relationship of Feeder Cattle Frame Size to Live Weight at a Choice Grade End Point. Evaluations of frame size, as defined in the Official U. S. Standards for Grades of Feeder Cattle (USDA, 1979), are intended to reflect the live weights at which feeder steers and heifers are expected to produce U. S. Choice carcasses. According to the 1979 Standards, small-, medium-, and large-framed feeder steers are expected to produce Choice carcasses at live finished weights of less than 454 kg, 454 kg to 544 kg, and more than 544 kg, respectively. Small-, medium-, and large-framed feeder heifers are expected to produce Choice carcasses at weights of less than 386 kg, 386 kg to 454 kg, and more than 454 kg, respectively. Results of analyses conducted to test the validity of 1979 USDA feeder cattle frame size specifications are summarized in Figures 1 and 2. Coefficients of determination (r 2 ) for the regressions in Figures 1 and 2 were very high, suggesting that evaluators scores for frame size were effective for identifying eventual differences in live finished weight at a marbling end point of Small 00. Data in Figure 1 were used to determine the live weights corresponding to the S/M (y when x = 200 in Figure 1) and M/L (y when x = 300 in Figure 1) frame score intersects for heifers. Similar data for steers are provided in Figure 2. For heifers, the S/M and M/L frame score lines corresponded to live weights of 460 kg and 523 kg, respectively (Figure 1). For steers, the S/M and M/L frame score lines corresponded to live Figure 1. Relationship between frame size and final live weight (kg) adjusted to a marbling score of Small 00 for heifers. Frame size: 160 = Small 60, 180 = Small 80, 200 = Small 100 /Medium 00, 220 = Medium 20, 240 = Medium 40, 260 = Medium 60, 280 = Medium 80, 300 = Medium 100 / Large 00, 320 = Large 20, 340 = Large 40, 360 = Large 60, 380 = Large 80. weights of 504 kg and 577 kg, respectively (Figure 2). These weights were higher than weights specified in the 1979 USDA grade standards (USDA, 1979). Feeder Cattle Muscle Thickness and Carcass Yield Traits. According to the USDA (1979), differences in thickness among feeder cattle are reflected as differences in carcass longissimus muscle area. Therefore, feeder cattle muscle thickness evaluations are intended to relate to yield grade differences among the carcasses that feeder cattle eventually will produce. Comparisons of carcass yield traits for feeder cattle assigned to thickness classes of No. 1, No. 2, and No. 3, as specified in the 1979 USDA feeder cattle grade Figure 2. Relationship between frame size and final live weight (kg) adjusted to a marbling score of Small 00 for steers. Frame size: 160 = Small 60, 180 = Small 80, 200 = Small 100 /Medium 00, 220 = Medium 20, 240 = Medium 40, 260 = Medium 60, 280 = Medium 80, 300 = Medium 100 / Large 00, 320 = Large 20, 340 = Large 40, 360 = Large 60, 380 = Large 80.

7 566 Grona et al. Table 5. Least squares means a for carcass yield traits of cattle grouped according to 1979 USDA feeder cattle muscle thickness specifications or an alternative muscle thickness classification scheme compared at a constant carcass fat thickness (10 mm) 1979 muscle thickness Alternative muscle thickness classification classification b Trait No. 1 No. 2 No. 3 SE No. 1 No. 2 No. 3 No. 4 SE n Final wt, kg 541 x 530 y 542 x x 539 x 528 y 542 x 10.3 Carcass wt, kg 334 x 323 y 323 y x 330 x 322 y 323 x 7.7 Dressing percentage 64.2 x 63.3 y 62.0 z x 63.6 y 63.5 y 62.0 z 0.46 Longissimus muscle area, cm x 83.7 y 81.7 y x 84.6 y 83.9 y 81.7 y 2.14 Longissimus muscle area adj. c y 0.19 x 0.22 x 0.11 x 0.08 KPH fat, % d Final yield grade y 2.48 x 2.46 x 2.58 x 0.08 a Least squares means were computed using a model that included the effects of source, age class, sex class, frame size classification, and muscle thickness classification, with adjusted carcass fat thickness as a covariate. b Based on the following modified divisions of 1979 muscle thickness specifications: No. 1 = 1 30 to 1 100, No. 2 = 2 70 to 1 20, No. 3 = 2 00 to 2 60, No. 4 = 3 00 to c Adjustment to the USDA yield grade equation (in yield grade units) for the difference between actual and required longissimus muscle area. d Estimated percentage of kidney, pelvic, and heart fat. x,y,z Means in the same row, and within muscle thickness classification scheme, without a common superscript letter differ (P < 0.05). standards, are presented in Table 5. When comparisons were made at the same fat thickness (10 mm), feeder cattle classified according to differences in muscle thickness differed (P < 0.05) in dressing percentage, carcass weight, and longissimus muscle area (No. 1 > No. 2 = No. 3). However, among-class differences in carcass longissimus muscle area adjustment and final yield grade were not statistically different (Table 5). Tatum et al. (1982) reported similar differences in carcass yield grade traits among cattle differing in muscle thickness classification. Many of the graders who have utilized the 1979 USDA feeder cattle grades believe that the specifications for muscling do not adequately categorize and describe feeder cattle for marketing purposes. The two primary criticisms of the 1979 muscling standards are that 1) nearly all cattle of beef type qualify for the No. 1 grade, making the No. 1 thickness classification too inclusive, and 2) cattle with No. 3 muscling are almost nonexistent (F. L. Williams, Jr., personal communication). Correspondingly, a different classification scheme was tested in an effort to identify a more effective approach for stratification of feeder cattle into muscle thickness categories. Data presented in Table 5 show comparisons of carcass yield traits for cattle classified using the following modified divisions of the 1979 muscle thickness specifications: No. 1 (1 30 to ), No. 2 (2 70 to 1 20 ), No. 3 (2 00 to 2 60 ), and No. 4 (3 00 to ) (a score of 3 00 is least muscular, whereas a score of is most muscular). Results in Table 5 suggest that this alternative classification scheme tended to distribute cattle numbers more evenly among the No. 1, No. 2, and No. 3 classes than the 1979 muscle thickness classification system and would identify numerical carcass yield grade differences between the No. 1 and No. 2 muscle thickness classes. The No. 4 muscle thickness class (same as the No. 3 grade in the 1979 Standards) still would contain a relatively small number of cattle; however, the carcass traits of these cattle seemed different enough to warrant classifying them into a separate grade (Table 5). Phenotypic characteristics of feeder cattle in the No. 2 and No. 3 classes in the alternative classification scheme were quite different (beef-type vs dairy-type); however, cattle in the two groups produced carcasses with very similar grade traits and, on that basis, could be combined into one grade. Relationship of Feeder Cattle Flesh Condition to Final Live Weight and Carcass Grade Traits. Differences in flesh condition (fatness), though not used to determine USDA grades for feeder cattle, can affect feeder cattle prices (Lambert et al., 1983) and are sometimes noted in USDA market news reports (USDA, 1995) to more thoroughly describe feeder cattle market conditions. Differences in condition reflect differences in stage of compositional maturity (Berg and Butterfield, 1976) among feeder cattle of the same age class, sex class, and biological type. Therefore, feeder cattle that differ in condition score normally will produce carcasses that differ in fatness after a similar number of days on feed. Means presented in Table 6 may be interpreted as comparisons among cattle of the same sex class, age class, frame size classification, and muscle thickness classification originating from a common source and harvested after the same number of days on feed (175 d). Carcasses produced by cattle differing in condition score as feeders differed (P < 0.05) in all traits related to fatness. Preliminary yield grade, percentage of KPH fat, final yield grade, and marbling score all increased (P < 0.05) progressively as feeder cattle condition score increased (Table 6). Also, feeder cattle evaluated as being fleshier generally were heavier (P < 0.05) at har-

8 Evaluation of feeder cattle grade standards 567 Table 6. Least squares means a for final live weight and carcass traits of feeder cattle grouped according to flesh condition classification compared at a constant days on feed end point (175 d) Feeder cattle condition classification b Slightly Slightly Very Trait Thin thin Average fleshy Fleshy fleshy SE n Final wt, kg 528 x 525 x 530 x 535 x 548 w 561 w 13.4 Carcass wt, kg 315 x 318 x 321 wx 324 wx 333 w 336 w 9.0 Preliminary yield grade 2.79 z 2.86 z 2.91 z 3.03 y 3.12 x 3.31 w 0.07 Longissimus muscle area, cm Longissimus muscle area adj. c KPH fat, % d 1.94 x 1.95 x 2.00 x 2.05 w 2.07 w 2.13 w 0.04 Final yield grade 2.26 z 2.31 z 2.36 yz 2.47 y 2.66 x 3.02 w 0.16 Maturity e 50 wxy 49 xy 53 wx 56 w 54 wx 46 y 3.6 Marbling score f 463 x 471 x 480 x 509 w 535 w 562 w 21.1 a Least squares means were computed using a model that included the effects of source, age class, sex class, frame size classification, and muscle thickness classification, flesh condition classification with days on feed as a covariate. b Thin = condition score of 3 or lower, Slightly thin = condition score of 4, Average = condition score of 5, Slightly fleshy = condition score of 6, fleshy = condition score of 7, very fleshy = condition score of 8 or 9 (USDA, 1995). c Adjustment to the USDA yield grade equation (in yield grade units) for the difference between actual and required longissimus muscle area. d Estimated percentage of kidney, pelvic, and heart fat. e Maturity: 0 to 99 = A 00 to A 100. f Marbling score: 400 to 499 = Small; 500 to 599 = Modest. w,x,y,z Means within a row without a common superscript letter differ (P < 0.05). vest and produced heavier (P < 0.05) carcasses than cattle evaluated as thinner in condition as feeders. Variation in feeder cattle condition was not associated with differences in longissimus muscle area or longissimus muscle area adjustment. It should be noted that when carcass fat thickness was held constant (data not presented in tabular form) there were few differences in carcass traits among the condition classes. Implications Our findings suggested that feeder cattle frame size and muscle thickness specifications adopted for use by USDA in 1979 are no longer adequate for describing today s population of feeder cattle and emphasize the need to revise the Standards for grades of feeder cattle. Official U.S. Standards for Grades of Feeder Cattle, revised using results of this study, were adopted for use on October 1, Literature Cited Berg, R. T., and R. M. Butterfield New Concepts of Cattle Growth. Halsted Press, New York. Lambert, C., L. Corah, and O. Gruewald Factors affecting prices of calves and yearlings in Kansas. Kansas State Univ., Cooperative Extension Service, Manhattan, KS. Tatum, J. D., G. C. Smith, C. E. Murphey, Z. L. Carpenter, and L. M. Schake Feeder cattle frame size, muscle thickness, and subsequent beef carcass characteristics. Meat Sci. 6: USDA Official United States Standards for Grades of Feeder Cattle. Standardization Branch, Agric. Marketing Service, USDA, Washington, DC. USDA Preparation of reports. Feeder cattle reporting. LGMN Instruction No Agric. Marketing Service, Washington, DC. USDA Official United States Standards for Grades of Carcass Beef. Standardization Branch, Agric. Marketing Service, USDA, Washington, DC.