Advertisement

Effects of harvest season on carcass characteristics of lambs in the Intermountain West

      ABSTRACT

      Objective

      The objectives of this study were to survey characteristics including hot carcass weight (HCW), 12th rib fat thickness (RFT), body-wall thickness (BWT), longissimus muscle area (LMA), USDA yield grade (USDA YG), percentage closely trimmed retail cuts (RC), and calculated yield grade (Calc YG) of lamb carcasses in the Intermountain West to determine the effects of season of slaughter and interrelationships among carcass characteristics.

      Materials and Methods

      Lamb carcass characteristics were evaluated in 2 commercial Intermountain West processing plants over one year (n = 10,027). Carcasses were evaluated by season: spring (December–April, n = 2,322) and summer (May–August, n = 7,705).

      Results and Discussion

      Carcasses of lambs slaughtered in the spring had 3.4 kg heavier HCW (P = 0.04) than those slaughtered in the summer. Subcutaneous fat (RFT; P = 0.06) and Calc YG (P = 0.09) tended to be greater in the spring than summer. Correlation coefficients and models of fit with a linear covariate of HCW indicated negative relationship between HCW and RC and positive relationship with all other carcass traits (P < 0.001). Overall, graded lamb carcasses exceeded commercial processing plant preferred HCW (38.6 kg) by 5% (mean = 40.5 kg) and industry acceptable RFT (6 mm) by 25% (mean = 8.03 mm). Furthermore, 70% of lamb carcasses exceed 6 mm RFT.

      Implications and Applications

      Season of slaughter contributed to differences in HCW and USDA YG but no other carcass characteristics. Still, carcass data surveyed from the largest lamb-producing region of the United States suggests that the degree of fatness exceeds industry preferences. Although abattoirs mitigate adverse effects of excessive fat through trimming and diverse market outlets, industry-wide efforts that agree on acceptable standards of trimness are needed. Transparent dialog across industry segments should be prioritized in addition to consistent integration of value-based pricing to reduce the proportion of excessively finished lambs.

      Key words:

      INTRODUCTION

      Disappearance is the industry’s estimate for consumption and is measured as the amount of meat sold through restaurants and retail outlets, calculated as total supply minus exports and ending stocks (). Despite historical decreases in US lamb consumption since the 1970s, per capita disappearance of lamb in the United States has recently increased 56% from 2011 to 2020 (

      USDA ERS. 2021b. All supply and disappearance. ERS, USDA. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/livestock-meat-domestic-data/.

      ). To help meet this emerging demand, the United States imported 63% of its lamb supply in 2020 (), with 98% of total lamb imports from Australia and New Zealand (

      USDA ERS. 2021c. Annual and Cumulative Year-to-Date U.S. Livestock and Meat Trade by Country. ERS, USDA, Washington, D.C. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/livestock-and-meat-international-trade-data/.

      ). Therefore, improving the quality of and preference for domestically produced lamb is at the forefront of US industry efforts (

      American Lamb Industry. 2014. The American Lamb Industry Roadmap. Englewood, Colorado. Accessed Feb. 4, 2018. https://lambresourcecenter.com/wp-content/uploads/2015/09/Lamb-Industry-roadmap-3-Jan-2014.pdf.

      ). Historically, excessive fat on retail cuts has been shown to adversely affect consumer satisfaction and lamb purchasing behavior (
      • Carpenter Z.L.
      What is consumer-preferred lamb?.
      ;

      Hughes, D. R. 1976. Consumer Attitudes to Meat Cuts—A Further Study. University of Newcastle upon Tyne Dept. of Agricultural Marketing. No. 15.

      ;
      • Jeremiah L.E.
      • Gibson L.L.
      • Tong A.K.W.
      Retail acceptability of lamb as influence by gender and slaughter weight..
      ). This constraint is not attributable to a single sector of the US lamb industry but is an inherent challenge due to the seasonality of sheep production systems (
      • Beermann D.H.
      • Robinson T.F.
      • Hogue D.E.
      Impact of composition manipulation on lean lamb production in the United States..
      ). Approximately 85% of the US lamb crop is born between January and May (

      USDA APHIS. 2011. Part IV: Changes in Health and Production Practices in the U.S. Sheep Industry, 1996–2011. APHIS, USDA.

      ), which ultimately requires feeders to extend the time that lambs are fed, to fulfill year-round demand. Feeding lambs beyond optimal finish weight results in increased adipose deposition, and yet pricing structures of US slaughter lambs do not aggressively account for cutability differences (
      • Tatum J.D.
      • Savell J.W.
      • Cross H.R.
      • Butler J.G.
      A national survey of lamb carcass cutability traits..
      ;
      • Purcell W.D.
      Economic issues and potentials in lamb marketing: Keys to the future of the sheep industry..
      ;
      • Ward C.E.
      Slaughter lamb pricing issues, evidence and future needs..
      ). Although subcutaneous fat can be trimmed during fabrication, controlled studies have estimated that greater costs are required to trim carcasses that are high in external fat (
      • Hopkins D.L.
      • Wotton J.S.A.
      • Gamble D.J.
      • Atkinson W.R.
      • Slack-Smith T.S.
      • Hal D.G.
      Lamb carcass characteristics I. The influence of carcass weight, fatness, and sex on the weight of “trim” and traditional retail cuts..
      ), and intermuscular fat is more challenging to mitigate depending on fabrication methods. Despite these concerns, an in-depth lamb carcass survey has not been conducted in the US since the 1980s (
      • Tatum J.D.
      • Savell J.W.
      • Cross H.R.
      • Butler J.G.
      A national survey of lamb carcass cutability traits..
      ), let alone in the Intermountain West, where a large proportion of US lamb is produced. Therefore, the objectives of this study were to characterize seasonal differences and interrelationships of lamb carcass characteristics at 2 large commercial processing plants located in the US Intermountain West.

      MATERIALS AND METHODS

      Carcass Data Collection

      No live animals were used in this study, and, therefore, approval of an Institutional Animal Care and Use Committee was not required.
      Data from lamb slaughter and carcass characteristics was collected at 2 commercial processing plants located in the US Intermountain West region on 30 separate dates over the course of a calendar year (May 2018–May 2019). Data collection days were selected to target 1% and 5% of regional fabrication during the spring (December–April) and summer months (May–August), respectively. Lambs were slaughtered using standard procedures, and carcasses were assigned lot numbers based on slaughter day and order in which the lot was slaughtered. Hot carcass weight (HCW) and lot numbers were printed on carcass identification tags. After carcasses were chilled for approximately 24 to 48 h at 0 to 2°C, USDA AMS-certified graders applied a yield and quality grade (USDA YG and USDA QG, respectively) to each carcass based on official ovine grading standards (

      USDA AMS. 1992. Official United States Standards for Grade of Lamb, Yearling Mutton, and Mutton Carcasses. AMS, USDA.

      ). Carcasses were separated between the 12th and 13th rib by trained personnel, and research assistants captured images of the interface using a digital camera (SEREE, 24-megapixel). The camera was mounted on an aluminum support with a perpendicular crossbar for stabilization that ensured uniform placement and photograph distance across carcasses. Carcass identification tags, complete with corresponding HCW, USDA YG, and USDA QG, were included in the image frame to maintain carcass identity. A carcass probe was also held within the image frame and level with the cut surface of the longissimus dorsi for calibration reference during image analysis.
      ImageJ (v.1.52a, National Institutes of Health) was used to measure and calculate 12th rib fat thickness (RFT), body-wall thickness (BWT), and longissimus muscle area (LMA) from carcass images. Fat thickness was measured at the midpoint of the length of the longissimus dorsi muscle, and BWT was measured along the 12th rib at a point approximately 12.7 cm from the dorsal midline (

      USDA AMS. 1992. Official United States Standards for Grade of Lamb, Yearling Mutton, and Mutton Carcasses. AMS, USDA.

      ). Longissimus muscle area was measured by tracing the muscle and using the “area” function of ImageJ. Fat measurements, RFT and BWT, were measured (mm) on both sides of each carcass image and averaged. To verify the accuracy of image analysis, RFT, BWT, and LMA were collected on a subset of 20 lamb carcasses using traditional measurement methods (i.e., ribeye area grid and carcass grid) and compared with those estimated from digital images. Pearson correlation coefficients between these 2 methods were positive and strong (0.84–0.96; P < 0.001).
      Two prediction equations were used to assess carcass composition from measured traits. Calculated yield grade (Calc YG) was described by

      USDA AMS. 1992. Official United States Standards for Grade of Lamb, Yearling Mutton, and Mutton Carcasses. AMS, USDA.

      as follows: YGCalc = (0.394 × RFT), which was rounded down to the nearest whole number in the range of 1 to 5, to match the range of USDA YG values. Any Calc YG values less than 1 were rounded up to 1 (n = 89), and Calc YG values greater than 5 were rounded down to 5 (n = 660), respectively. Percentage of closely trimmed retail cuts (RC) was calculated according to
      • Berg E.P.
      • Neary M.K.
      • Forrest J.C.
      • Thomas D.L.
      • Kauffman R.G.
      Evaluation of electronic technology to assess lamb carcass composition..
      as follows: RC = 46.41 − (0.174 × HCW) − (0.12 × RF) − (0.154 × BWT) + (0.825 × LMA).

      Statistical Analysis

      Data were collected from a total of 10,027 carcasses. Descriptive statistics of traits were calculated across slaughter season for both graded and ungraded carcasses. Of the carcasses surveyed, 892 did not receive a grade by trained USDA graders, due to inferior quality indicators such as deficient fat cover, significant amounts of lean muscle removed from major primal cuts, or other carcass defects (

      USDA AMS. 1992. Official United States Standards for Grade of Lamb, Yearling Mutton, and Mutton Carcasses. AMS, USDA.

      ). Based on USDA grading standards (1992), yearling or mutton can receive grades, but this is not currently employed by trained graders, given that grading is a service paid for by the processing plant. Data from ungraded carcasses were not included in the remaining analyses, as these carcasses did not receive an official USDA grade due to inferior quality attributes. Hot carcass weight and RC were analyzed in the MIXED procedure of SAS (v9.4; SAS Institute Inc.) with the fixed effects of season (spring or summer), commercial processing plant (1 or 2), and their interaction. Slaughter lot (nested within season and commercial processing plant; 71 levels) was fit as a random effect, and those containing <5 carcasses were treated as missing values. All other carcass traits were analyzed in the same model but with an additional linear covariate of HCW to better estimate effects of season and commercial processing plant independent of carcass weight.
      Remaining analyses were pooled across slaughter season and commercial processing plant. Associations between HCW, RFT, BWT, and LMA were first assessed as Pearson correlation coefficients that were estimated using the CORR procedure. Carcasses were then classified as light (<29.5 kg), moderate (29.5–38.6 kg), or heavy (>38.6 kg) HCW based on communication with commercial processing plant personnel who indicated these levels as lightweight, ideal, and overweight, respectively, based on perceived efficiency of processing and potential market outlets. To evaluate the association of carcass traits with carcass weight and conformation, the MIXED procedure was used to analyze carcass traits with the fixed effect of HCW class (all traits except HCW) or USDA YG (all traits except USDA YG) and the random effect of slaughter lot (nested within season and commercial processing plant). Similarity between USDA YG and Calc YG was evaluated as their difference (i.e., USDA YG − Calc YG) and was analyzed in the previous model. Where possible, descriptive comparisons were made between present data and past studies conducted in the US, to infer trends in carcass characteristics over time. Additionally, results were contrasted to those from Australian and New Zealand sheep industries, as they are the largest competitors of the US lamb industry (

      USDA ERS. 2021c. Annual and Cumulative Year-to-Date U.S. Livestock and Meat Trade by Country. ERS, USDA, Washington, D.C. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/livestock-and-meat-international-trade-data/.

      ).

      RESULTS AND DISCUSSION

      Summary of Carcass Characteristics

      Compared with estimates from internal USDA documentation of historic fabrication frequency, the number of carcasses evaluated in this study represented approximately 1% and 4% (average number of carcasses sampled daily over a 30-d period, n = 334) of the regional fabrication occurring in the spring and summer months, respectively. Descriptive statistics of graded and ungraded carcasses across slaughter season are presented in Table 1. Frequency of ungraded carcasses was greater in summer than spring months (11% vs. 2%). Ungraded lamb carcasses had numerically greater HCW, RFT, BWT, and Calc YG and lower RC than graded carcasses. Although ungraded lamb carcasses are a part of the consumer supply chain, only graded carcasses were included in the remaining analyses.
      Table 1Descriptive statistics of graded and ungraded lamb carcasses across slaughter season (2018–2019) in the US Intermountain West
      Class, nTrait
      HCW = hot carcass weight, RFT = 12th rib fat thickness, BWT = body-wall thickness, LMA = longissimus muscle area, USDA YG = yield grade assigned by a USDA-certified lamb carcass grader, Calc YG = calculated yield grade rounded down to the nearest whole number, RC = percentage closely trimmed retail cuts.
      MeanSDMinimumMaximum
      Graded,

      n = 9,135
      HCW, kg40.58.761585.5
      RFT, mm8.033.64033.2
      BWT, mm31.08.877.2976.1
      LMA, cm217.03.017.1633.8
      USDA YG3.370.9715
      Calc YG2.971.2015
      RC, %47.62.6634.260.2
      Ungraded,

      n = 892
      HCW, kg43.111.810.790.1
      RFT, mm9.254.62030.5
      BWT, mm34.211.55.9271.1
      LMA, cm216.73.085.3232.1
      Calc YG3.311.2815
      RC, %46.43.1733.556.9
      1 HCW = hot carcass weight, RFT = 12th rib fat thickness, BWT = body-wall thickness, LMA = longissimus muscle area, USDA YG = yield grade assigned by a USDA-certified lamb carcass grader, Calc YG = calculated yield grade rounded down to the nearest whole number, RC = percentage closely trimmed retail cuts.
      Frequency distributions of traits measured on graded carcasses are displayed in Figure 1. In the current study, 55% of carcasses exceeded the upper HCW limit of packer preferences in the Intermountain West (29.5–38.6 kg).
      • Tatum J.D.
      • Savell J.W.
      • Cross H.R.
      • Butler J.G.
      A national survey of lamb carcass cutability traits..
      reported that 65% of US lamb carcasses slaughtered at commercial facilities in 1987 had an HCW range between 25 and 34 kg, and 11% of carcasses exceeded 34 kg. Lamb carcasses were heavier in this study, as 74% had HCW greater than 34 kg (Figure 1a). Historical USDA data support this continued increase in HCW over time, as lamb carcasses in 2020 were 9% heavier than in 1990 (

      USDA ERS. 2021d. Livestock and poultry live and dressed weights. ERS, USDA. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/livestock-meat-domestic-data/.

      ). Meanwhile, in 2019, Australian and New Zealand lamb carcasses weighed, on average, 23.3 kg and 19.1 kg, respectively (; ).
      Figure 1
      Figure 1Frequency distributions of hot carcass weight (HCW, A), 12th rib fat thickness (RFT, B), body-wall thickness (BWT, C), longissimus muscle area (LMA, D), percentage closely trimmed retail cuts (RC, E), and USDA yield grade (USDA YG) compared with calculated yield grade (Calc YG, F) for lambs slaughtered from the US Intermountain West, 2018–2019.
      Ideal RFT is 6.4 mm (

      USDA AMS. 1992. Official United States Standards for Grade of Lamb, Yearling Mutton, and Mutton Carcasses. AMS, USDA.

      ), and 64% of lamb carcasses in the present survey exceeded that level (Figure 1b). The average RFT of carcasses >6.44 mm was 10.02 mm. In the United States, RFT is used as an indicator of whole-carcass trimness. However, grading standards in Australia and New Zealand are based on grade rule (GR) measurement (

      New Zealand Meat Classification Authority (NZMCA). 2004. Guide to Lamb and Mutton Carcass Classification. Accessed Jun. 24, 2021. https://www.interest.co.nz/files/rural/lambgrade.pdf.

      ;

      AUS-MEAT. 2020. Handbook of Australian Sheepmeat Processing. Accessed Jul. 21, 2021. https://www.ausmeat.com.au/WebDocuments/Producer_HAP_Sheepmeat_Small.pdf.

      ), which is more similar to BWT than RFT. The GR measurement is recorded at 110 mm from the dorsal midline (compared with 127 mm in BWT) across the 12th rib interface (

      AUS-MEAT. 2020. Handbook of Australian Sheepmeat Processing. Accessed Jul. 21, 2021. https://www.ausmeat.com.au/WebDocuments/Producer_HAP_Sheepmeat_Small.pdf.

      ). In New Zealand, excessively fat carcasses are considered those with a GR measurement ≥15 mm (

      New Zealand Meat Classification Authority (NZMCA). 2004. Guide to Lamb and Mutton Carcass Classification. Accessed Jun. 24, 2021. https://www.interest.co.nz/files/rural/lambgrade.pdf.

      ). In the current study, nearly all carcasses (98%) had BWT ≥15 mm (Figure 1c). However, greater BWT should be expected, given a 10.9 kg (30.0 vs. 19.1 kg) greater average HCW in the United States than New Zealand in 2019 (

      USDA ERS. 2021c. Annual and Cumulative Year-to-Date U.S. Livestock and Meat Trade by Country. ERS, USDA, Washington, D.C. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/livestock-and-meat-international-trade-data/.

      ; ). Fat thickness differences between sheep industries may also be a function of differences in breeds and finishing methods (
      • Southam E.R.
      • Field R.A.
      Influence of carcass weight upon carcass composition and consumer preference for lamb..
      ).
      Across the industry, the area of the longissimus muscle is used to assess muscle composition of live animals either by palpation or by ultrasound. Although LMA is unlikely to be measured in commercial processing plants because lamb carcasses are typically not ribbed, it may be valuable in differentiating domestic from imported lamb and identifying consumer preference.
      • Hoffman T.W.
      • Belk K.E.
      • Woerner D.R.
      • Tatum J.D.
      • Delmore R.J.
      • Peel R.K.
      • LeValley S.B.
      • Pendell D.L.
      • Zerby H.N.
      • English L.F.
      • Moeller S.J.
      Preferences associated with American lamb quality in retail & foodservice markets..
      reported that the LMA of US loin chops (19.5 cm2) was greater than that of Australian or New Zealand-sourced loin chops (16.8 and 14.5 cm2, respectively) available in US retail stores. Although weight and size of lamb cuts may have less influence than other quality characteristics, such as eating satisfaction and composition (Hoffman et al., 2015), improving LMA can increase dressing percentage and carcass value (
      • Leeds T.D.
      • Mousel M.R.
      • Notter D.R.
      • Zerby H.N.
      • Moffet C.A.
      • Lewis G.S.
      B-mode, real-time ultrasound for estimating carcass measures in live sheep: Accuracy of ultrasound measures and their relationships with carcass yield and value..
      ).
      The RC equation used in the present study was developed by
      • Berg E.P.
      • Neary M.K.
      • Forrest J.C.
      • Thomas D.L.
      • Kauffman R.G.
      Evaluation of electronic technology to assess lamb carcass composition..
      using carcass data captured from commercial market lambs. It approximates the proportion of boneless lean that can be trimmed and fabricated into retail cuts from common carcass measurements by applying negative coefficients to HCW, RFT, and BWT and a positive coefficient to LMA. A similar RC equation was developed by

      Tschirhart, T. E., L. A. Rakowitz, D. R. McKenna, D. B. Griffin, and J. W. Savell. 2002. Development of a cutability equation for carcasses of show lambs. Pages 31–35 in Sheep and Goat/Wool and Mohair Consolidated Progress Report 2002. Texas A&M University Agricultural Research and Extension Center.

      for use in show lambs and uses the same component traits weighted by coefficients of the same direction but different magnitude from
      • Berg E.P.
      • Neary M.K.
      • Forrest J.C.
      • Thomas D.L.
      • Kauffman R.G.
      Evaluation of electronic technology to assess lamb carcass composition..
      . However, show lamb carcass composition is generally not reflective of the larger commercial lamb industry. Lamb carcasses in the present study had greater HCW (40.5 vs. 29.3 kg), RFT (8.03 vs. 5.3 mm), BWT (31.0 vs. 24.3 mm), and LMA (17.0 vs. 13.8 cm2) and, consequently, lower RC (47.6 vs. 48.3%) than those sampled in
      • Berg E.P.
      • Neary M.K.
      • Forrest J.C.
      • Thomas D.L.
      • Kauffman R.G.
      Evaluation of electronic technology to assess lamb carcass composition..
      , which may be due to differences in genetics and feeding strategies at that time compared with current practices.
      More common estimates of carcass lean yield are USDA YG and Calc YG, with USDA YG being assigned visually by a USDA grader (

      USDA AMS. 1992. Official United States Standards for Grade of Lamb, Yearling Mutton, and Mutton Carcasses. AMS, USDA.

      ) and Calc YG based on objective measurements of RFT. Greater USDA YG and Calc YG are indicative of increased carcass fat composition. In the present study, 38%, 30%, and 13% of carcasses received USDA YG of 3, 4, and 5, respectively. Similar percentages were observed among industry data from 2018, which summarized that 39%, 18%, and 11% of lamb carcasses were assigned USDA YG of 3, 4, and 5, respectively (

      USDA AMS. 2021. Beef, Lamb and Veal Percentage Charts. AMS, USDA. Accessed Nov. 11, 2021. https://www.ams.usda.gov/reports/meat-grading.

      ). Furthermore, the proportion of carcasses receiving USDA YG 4 and 5 was approximately 3% greater in 2018 than in the 2 years prior (

      USDA AMS. 2021. Beef, Lamb and Veal Percentage Charts. AMS, USDA. Accessed Nov. 11, 2021. https://www.ams.usda.gov/reports/meat-grading.

      ), which may further validate industry concerns over increased carcass fatness.
      Currently, the majority of graded lamb carcasses receive a USDA QG of Choice or Prime. For example, 95% and 4% of graded carcasses in the current study and 91% and 9% of industry-reported carcasses (

      USDA AMS. 2021. Beef, Lamb and Veal Percentage Charts. AMS, USDA. Accessed Nov. 11, 2021. https://www.ams.usda.gov/reports/meat-grading.

      ) in 2018 received Choice or Prime USDA QG, respectively. Quality grades in the US are assigned based on visual assessment of carcass conformation (i.e., flank streaking, thickness of muscling, and distribution of external finish;

      USDA AMS. 1992. Official United States Standards for Grade of Lamb, Yearling Mutton, and Mutton Carcasses. AMS, USDA.

      ). Grading standards in other countries also base standards upon muscle and fat composition, but fat indicators are based on GR and include the weight as part of the quality standard. New Zealand has the most differentiated grading system, with 7 fat classifications, 5 weight classifications, and a requirement for adequate muscle to qualify for export markets (

      New Zealand Meat Classification Authority (NZMCA). 2004. Guide to Lamb and Mutton Carcass Classification. Accessed Jun. 24, 2021. https://www.interest.co.nz/files/rural/lambgrade.pdf.

      ). A more refined quality grading system may better quantify the quality attributes of US lamb carcasses.
      Results from this survey indicate that lamb carcasses sampled in commercial processing plants in the Intermountain West during 2018 and 2019 were representative of recent carcass trends throughout the United States and were heavier and fatter than both domestic and international industry preferences.

      Effects of Slaughter Season and Commercial Processing Plant on Carcass Characteristics

      The slaughter season × commercial processing plant interaction was significant in the analyses of LMA and RC (P ≤ 0.04), and their least squares means are displayed in Table 2. For LMA, the interaction was due to a numerical re-ranking between commercial processing plants across slaughter season, but no subclass means were significantly different from one another (P ≥ 0.13). During the summer months, carcasses processed at commercial processing plant 2 had greater RC than those processed at commercial processing plant 1 (P = 0.02), but there were no commercial processing plant differences during the spring (P = 0.69). The slaughter season × commercial processing plant interaction did not influence any other trait evaluated (P ≥ 0.06), and least squares means for their main effects are displayed in Table 3.
      Table 2Least squares means (±SE) for the interactive effects of slaughter season and commercial processing plant on carcass traits of lambs slaughtered in the US Intermountain West, 2018–2019
      Lambs were slaughtered at 1 of 2 commercial abattoirs in the Intermountain West during the course of one calendar year; Spring = December–April, Summer = May–August.
      Trait
      LMA = longissimus muscle area, RC = percentage closely trimmed retail cuts.
      SpringSummer
      Commercial

      processing plant 1
      Commercial

      processing plant 2
      Commercial

      processing plant 1
      Commercial

      processing plant 2
      LMA, cm
      LMA = longissimus muscle area, RC = percentage closely trimmed retail cuts.
      17.6 ± 0.3117.0 ± 0.4016.7 ± 0.2317.4 ± 0.21
      RC, %48.1 ± 0.40a,b47.4 ± 0.50a,b47.4 ± 0.29b48.6 ± 0.26a
      a,bMeans within a trait with no common superscript are different (P ≤ 0.04).
      1 Lambs were slaughtered at 1 of 2 commercial abattoirs in the Intermountain West during the course of one calendar year; Spring = December–April, Summer = May–August.
      2 LMA = longissimus muscle area, RC = percentage closely trimmed retail cuts.
      Table 3Least squares means (± SE) for the main effects of slaughter season and commercial processing plant and solution for the linear covariate of hot carcass weight (by,HCW^) on carcass traits of lambs slaughtered in the US Intermountain West, 2018–2019
      Effect
      Lambs were slaughtered at 1 of 2 commercial processing plants in the Intermountain West during the course of one calendar year; Spring = December–April, Summer = May–August.
      LevelTrait
      HCW = hot carcass weight, RFT = 12th rib fat thickness, BWT = body-wall thickness, LMA = longissimus muscle area, USDA YG = yield grade assigned by a USDA-certified lamb carcass grader, Calc YG = calculated yield grade rounded down to the nearest whole number.
      HCW, kgRFT, mmBWT, mmLMA, cm
      HCW = hot carcass weight, RFT = 12th rib fat thickness, BWT = body-wall thickness, LMA = longissimus muscle area, USDA YG = yield grade assigned by a USDA-certified lamb carcass grader, Calc YG = calculated yield grade rounded down to the nearest whole number.
      USDA YGCalc YG
      Commercial processing plant140.5 ± 1.098.08 ± 0.2030.9 ± 0.4617.1 ± 0.193.53 ± 0.07a3.02 ± 0.06
      238.1 ± 1.218.11 ± 0.2431.1 ± 0.5417.2 ± 0.233.26 ± 0.08b2.87 ± 0.08
      SeasonSpring41.0 ± 1.38a8.38 ± 0.2630.9 ± 0.6117.3 ± 0.253.54 ± 0.09a3.03 ± 0.08
      Summer37.6 ± 0.86b7.80 ± 0.1631.2 ± 0.5417.0 ± 0.153.25 ± 0.06b2.86 ± 0.05
      Hot carcass weight, linear covariateby,HCW^0.21 ± 0.01
      ${\hat b_{y,HCW}}$ is different from zero (P < 0.001).
      0.70 ± 0.01
      ${\hat b_{y,HCW}}$ is different from zero (P < 0.001).
      0.24 ± 0.004
      ${\hat b_{y,HCW}}$ is different from zero (P < 0.001).
      0.06 ± 0.001
      ${\hat b_{y,HCW}}$ is different from zero (P < 0.001).
      0.06 ± 0.002
      ${\hat b_{y,HCW}}$ is different from zero (P < 0.001).
      a,bMeans within a trait and effect with no common superscript are different (P ≤ 0.04).
      1 Lambs were slaughtered at 1 of 2 commercial processing plants in the Intermountain West during the course of one calendar year; Spring = December–April, Summer = May–August.
      2 HCW = hot carcass weight, RFT = 12th rib fat thickness, BWT = body-wall thickness, LMA = longissimus muscle area, USDA YG = yield grade assigned by a USDA-certified lamb carcass grader, Calc YG = calculated yield grade rounded down to the nearest whole number.
      * ${\hat b_{y,HCW}}$ is different from zero (P < 0.001).
      There was an effect of season on HCW, as carcasses from lambs slaughtered in the spring (December–April) were 3.4 kg heavier than those slaughtered in the summer (P = 0.04). However, commercial processing plant location did not affect HCW (P = 0.15). From 2010 to 2020, American lamb carcasses weighed 31.3 kg on average, with heaviest dressed weights occurring in May (32.2 kg) and lightest occurring in September (30.1 kg;

      USDA ERS. 2021d. Livestock and poultry live and dressed weights. ERS, USDA. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/livestock-meat-domestic-data/.

      ). During the timeframe of this study, the average dressed weight of federally inspected lamb carcasses was 31.1 kg (

      USDA ERS. 2021d. Livestock and poultry live and dressed weights. ERS, USDA. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/livestock-meat-domestic-data/.

      ). Differences between the current results and USDA data may be attributable to the inclusion of smaller federally inspected plants and lamb carcass differences across the country. From USDA data, slaughter season also appears to exert a minor influence on carcass weights, with a 1.1-kg difference between the highest and lowest average monthly carcass weights (

      USDA ERS. 2021d. Livestock and poultry live and dressed weights. ERS, USDA. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/livestock-meat-domestic-data/.

      ).
      To investigate the effects of slaughter season on the remaining traits, independent of seasonal HCW differences, HCW was fit as a linear covariate in their analyses and was highly significant and positive (P < 0.001; Table 3) for all traits. The main effects of commercial processing plant and slaughter season influenced USDA YG and were greater (P = 0.01) for carcasses processed at commercial processing plant 1 than 2 and greater for lambs slaughtered in the spring than summer (P < 0.001). Although RFT (P = 0.06) and Calc YG (P = 0.09) tended to be greater in spring than in summer, no other traits were significantly influenced by slaughter season or commercial processing plant (P ≥ 0.06). The previous industry audit also found that RFT, USDA YG, and USDA QG increased with HCW (
      • Tatum J.D.
      • Savell J.W.
      • Cross H.R.
      • Butler J.G.
      A national survey of lamb carcass cutability traits..
      ).

      Interrelationships Among Carcass Characteristics

      Estimated correlation coefficients between HCW, RFT, BWT, and LMA are displayed in Table 4. All correlation coefficients were significantly different from zero and positive (P < 0.001) but varied in magnitude. The strongest correlations were between BWT and HCW (0.73) and BWT and RFT (0.66). Longissimus muscle area and RFT were correlated to a low degree (0.22), and all other correlation coefficients were moderate (0.48–0.58).
      • Snowder G.D.
      • Glimp H.A.
      • Field R.A.
      Carcass characteristics and optimal slaughter weights in four breeds of sheep..
      also found strong positive correlations between HCW and fat thickness measurements (RFT, 0.66; BWT, 0.84). The strongest correlation among carcass traits measured by
      • LeValley S.B.
      • Dewalt M.S.
      • Speer N.C.
      • Tatum J.D.
      • Glimp H.A.
      • Simms R.H.
      Carcass evaluation of western Colorado range lambs..
      was between HCW and RFT (0.42).
      Table 4Estimated Pearson correlation coefficients between carcass traits of lambs slaughtered in the US Intermountain West, 2018–2019
      HCW = hot carcass weight, RFT = 12th rib fat thickness, BWT = body-wall thickness, LMA = longissimus muscle area.
      TraitHCWRFTBWTLMA
      HCW0.57
      Correlation coefficient is different from zero (P < 0.001).
      0.73
      Correlation coefficient is different from zero (P < 0.001).
      0.58
      Correlation coefficient is different from zero (P < 0.001).
      RFT0.66
      Correlation coefficient is different from zero (P < 0.001).
      0.22
      Correlation coefficient is different from zero (P < 0.001).
      BWT0.48*
      LMA
      1 HCW = hot carcass weight, RFT = 12th rib fat thickness, BWT = body-wall thickness, LMA = longissimus muscle area.
      * Correlation coefficient is different from zero (P < 0.001).
      Least squares means for the effect of HCW class on other carcass characteristics are displayed in Table 5. As expected from previous models that fit a linear covariate of HCW (Table 3) as well as correlation analyses (Table 4), all traits were significantly affected by HCW class (P < 0.001), with a negative relationship for RC and positive relationship for all others. Differences between HCW classes for each trait were additive for LMA and USDA YG but numerically greater between heavy and moderate compared with moderate and light carcasses for other traits.
      Table 5Least squares means (±SE) for the main effect hot carcass weight (HCW) class on carcass characteristics of lambs slaughtered in the US Intermountain West, 2018–2019
      Trait
      RFT = 12th rib fat thickness, BWT = body-wall thickness, LMA = longissimus muscle area, USDA YG = yield grade assigned by a USDA-certified lamb carcass grader, Calc YG = calculated yield grade rounded down to the nearest whole number, RC = percentage closely trimmed retail cuts.
      HCW class
      Light: <29.5 kg; moderate: 29.5–38.6 kg; heavy: >38.6 kg.
      LightModerateHeavy
      RFT, mm5.46 ± 0.21c6.93 ± 0.19b8.81 ± 0.19a
      BWT, mm22.1 ± 0.52c27.5 ± 0.46b34.3 ± 0.46a
      LMA, cm
      Light: <29.5 kg; moderate: 29.5–38.6 kg; heavy: >38.6 kg.
      13.6 ± 0.16c16.0 ± 0.13b18.4 ± 0.13a
      USDA YG2.45 ± 0.06c3.07 ± 0.06b3.61 ± 0.06a
      Calc YG2.10 ± 0.07c2.58 ± 0.06b3.22 ± 0.06a
      RC, %48.7 ± 0.17a48.3 ± 0.15b47.4 ± 0.15c
      a–cMeans within a trait with no common superscript are different (P < 0.001).
      1 RFT = 12th rib fat thickness, BWT = body-wall thickness, LMA = longissimus muscle area, USDA YG = yield grade assigned by a USDA-certified lamb carcass grader, Calc YG = calculated yield grade rounded down to the nearest whole number, RC = percentage closely trimmed retail cuts.
      2 Light: <29.5 kg; moderate: 29.5–38.6 kg; heavy: >38.6 kg.
      Least squares means for the effects of USDA YG on other carcass characteristics are displayed in Table 6. All traits were significantly influenced by USDA YG (P < 0.001), with a negative relationship for RC and positive relationships for all others. Differences between USDA YGA within a trait were generally additive from 1 to 4 but greater between 4 and 5. This is likely due to the upper boundary of USDA YG being 5 and having a greater average and variation in carcass fatness than other classes.
      • Kosulwat S.
      • Greenfield H.
      • James J.
      Lipid composition of Australian retail lamb cuts with differing carcass classification characteristics..
      reported a 40% increase in overall fat content when GR measurement increased from 5 mm (fat score 1) to 15 to 20 mm (fat score 4). Fat deposition can be reduced by identifying accurate target end-point weights across genetically diverse breed types, selecting within breed for lean muscle growth (
      • Snowder G.D.
      • Glimp H.A.
      • Field R.A.
      Carcass characteristics and optimal slaughter weights in four breeds of sheep..
      ;
      • Karamichou E.
      • Merrell B.G.
      • Murray W.A.
      • Simm G.
      • Bishop S.C.
      Selection for carcass quality in hill sheep measured by X-ray computer tomography..
      ;
      • Kvame T.
      • Vangen O.
      Selection for lean weight based on ultrasound and CT in a meat line of sheep..
      ), strategic use of terminal sire breeds (
      • Mousel M.R.
      • Notter D.R.
      • Leeds T.D.
      • Zerby H.N.
      • Moeller S.J.
      • Lewis G.S.
      Evaluation of Columbia, USMARC-Composite, Suffolk, and Texel rams as terminal sires in an extensive rangeland production system: III. Prefabrication carcass traits and organ weights..
      ;
      • Shackelford S.D.
      • Leymaster K.A.
      • Wheeler T.L.
      • Koohmaraie M.
      Effects of breed of sire on carcass composition and sensory traits of lamb..
      ), and greater inclusion of NDF feedstuffs in the diet to reduce feed intake and rate of gain (
      • Blackburn H.D.
      • Snowder G.D.
      • Glimp H.
      Simulation of lean lamb production systems..
      ;
      • Beermann D.H.
      • Robinson T.F.
      • Hogue D.E.
      Impact of composition manipulation on lean lamb production in the United States..
      ). Lamb and feedstuff price volatility, restricted lamb abattoir capacity, and feedlot backlogs in heavy “old crop” lambs continue to create challenges in mitigating excessive fat deposition.
      Table 6Least squares means (±SE) for the main effect of USDA yield grade (USDA YG) on carcass characteristics of lambs slaughtered in the US Intermountain West, 2018–2019
      Trait
      HCW = hot carcass weight, RFT = 12th rib fat thickness, BWT = body-wall thickness, LMA = longissimus muscle area, RC = percentage closely trimmed retail cuts, Calc YG = calculated yield grade rounded down to the nearest whole number.
      USDA YG
      12345
      HCW, kg29.9 ± 0.64e32.7 ± 0.52d37.3 ± 0.51c42.1 ± 0.51b47.4 ± 0.54a
      RFT, mm3.70 ± 0.26e5.45 ± 0.15d6.98 ± 0.13c8.76 ± 0.14b11.7 ± 0.16a
      BWT, mm19.2 ± 0.63e23.5 ± 0.44d28.3 ± 0.41c33.3 ± 0.42b39.9 ± 0.46a
      LMA, cm214.6 ± 0.26e15.5 ± 0.17d16.4 ± 0.16c17.4 ± 0.16b18.7 ± 0.18a
      RC, %49.9 ± 0.21a49.2 ± 0.14b48.2 ± 0.13c47.2 ± 0.13d45.9 ± 0.15e
      YG USDA – YG Calc−0.53 ± 0.08
      Estimate for USDA YG − Calc YG is different from zero (P < 0.001).
      −0.05 ± 0.050.37 ± 0.04
      Estimate for USDA YG − Calc YG is different from zero (P < 0.001).
      0.74 ± 0.04
      Estimate for USDA YG − Calc YG is different from zero (P < 0.001).
      0.99 ± 0.05
      Estimate for USDA YG − Calc YG is different from zero (P < 0.001).
      a–eMeans within a trait with no common superscript are different (P < 0.001).
      1 HCW = hot carcass weight, RFT = 12th rib fat thickness, BWT = body-wall thickness, LMA = longissimus muscle area, RC = percentage closely trimmed retail cuts, Calc YG = calculated yield grade rounded down to the nearest whole number.
      * Estimate for USDA YG − Calc YG is different from zero (P < 0.001).
      Results suggest that increased HCW is, on average, associated with greater LMA as well as, unfortunately, greater subcutaneous fat content. Disincentivizing heavy carcasses may indirectly decrease subcutaneous fat levels and overall proportion of excessively fat lambs entering the supply chain. Price signals within the industry for slaughter lambs have taken precedence over biological efficiency, promoting feeding to greater carcass weights at the expense of optimal lean composition (
      • Blackburn H.D.
      • Snowder G.D.
      • Glimp H.
      Simulation of lean lamb production systems..
      ;
      • Field R.A.
      • Whipple G.
      The relation of slaughter and carcass weights to production and processing efficiency and market acceptability..
      ).
      Differences between subjective and objective yield grading methods (i.e., USDA YG − Calc YG) by USDA YG class are also shown in Table 6. Here, a positive (negative) value for USDA YG − Calc YG indicates that USDA graders estimated a greater (lesser) carcass fat content than suggested solely based on RFT. Carcasses assigned USDA YG 1 were, on average, 0.5 Calc YG fatter (P < 0.001). There was general agreement between yield grading methods at USDA YG 2, as USDA YG − Calc YG was not different from zero (P = 0.34). However, USDA YG − Calc YG became larger at USDA YG 3 and greater (P < 0.001), indicating that carcasses were leaner based on RFT alone. Overall, USDA YG and Calc YG methods were in agreement for 37% of lamb carcasses in the current study.
      • DeWalt M.S.
      • Tatum J.D.
      • Abraham H.C.
      • Dolezal H.G.
      • Garrett R.P.
      • Field R.A.
      • LeValley S.B.
      • Smith G.C.
      • Wise J.W.
      Comparison of methods for evaluating external fat thickness and body wall thickness in lamb carcasses..
      and
      • Heaton K.L.
      • Morgan J.B.
      • Tatum J.D.
      • Wise J.W.
      • Garret R.P.
      • Dolezal H.G.
      • Loveday H.D.
      • Smith G.C.
      Field studies to document the efficacy of visual assessments of lamb carcasses to appropriate USDA yield grades..
      verified the accuracy of objective, visual appraisal for assigning USDA YG. When compared with probe measurements, visual estimates of RFT were variable between 3 evaluators, and USDA YG was accurately assigned between 54% and 66% of the time. Correct assignment of USDA YG has a relatively small margin of error, with only 0.1 in fat thickness ranges, so even a small over- or underestimation of fat thickness can lead to incorrect classification of yield grades (
      • DeWalt M.S.
      • Tatum J.D.
      • Abraham H.C.
      • Dolezal H.G.
      • Garrett R.P.
      • Field R.A.
      • LeValley S.B.
      • Smith G.C.
      • Wise J.W.
      Comparison of methods for evaluating external fat thickness and body wall thickness in lamb carcasses..
      ). Vision-based grading may provide more precise indication of carcass composition and could be used to improve value-based pricing of lamb carcasses but lacks widespread adoption across the lamb industry (
      • Brady A.S.
      • Belk K.E.
      • LeValley S.B.
      • Dalsted N.L.
      • Scanga J.A.
      • Tatum J.D.
      • Smith G.C.
      An evaluation of the lamb vision system as a predictor of lamb carcass red meat yield percentage..
      )

      APPLICATIONS

      Based on current findings, season of slaughter influenced HCW and USDA YG but no other carcass characteristics of lambs slaughtered in the Intermountain West. In the current study, over 43% of the 10,027 carcasses surveyed were assigned USDA YG 4 or 5, indicating industry-wide room for improvement. Efforts to reduce the production of excessively fat lambs will require cross-sector strategies that involve all segments of the industry. Currently this challenge is being mitigated by the lamb processor by trimming fat and routing heavier carcasses to alternative market outlets. Additionally, industry-wide efforts that agree on acceptable standards regarding trimness are warranted.
      Prospective audits need to be conducted on an industry-defined routine basis and should include monthly sampling to provide a complete picture of carcass characteristics across the calendar year. Regular audits will promote accountability, with the end goal of improving the quality of American lamb and its differentiation from imported product. These audits need to be integrated into educational efforts, to ensure transparency and industry-wide communication for improvement across all segments (consumers, producers, feeders, and packers). Results from this study can provide lamb industry insights into areas of improvement as they relate to fat composition and current production trends.

      ACKNOWLEDGMENTS

      The authors thank the American Lamb Board (Denver, CO), the National Sheep Industry Improvement Center (Highlands Ranch, CO), and Hatch-Multistate “Increased Efficiency of Sheep Production” project accession no. 1025808 from the USDA National Institute of Food and Agriculture (Washington, DC) for financial support of this project. Additional gratitude is expressed to undergraduate research assistants M. Becker, K. Woodruff, and A. Brady, all from the University of Wyoming, Laramie, for their assistance collecting data.

      LITERATURE CITED

      1. American Lamb Industry. 2014. The American Lamb Industry Roadmap. Englewood, Colorado. Accessed Feb. 4, 2018. https://lambresourcecenter.com/wp-content/uploads/2015/09/Lamb-Industry-roadmap-3-Jan-2014.pdf.

      2. AUS-MEAT. 2020. Handbook of Australian Sheepmeat Processing. Accessed Jul. 21, 2021. https://www.ausmeat.com.au/WebDocuments/Producer_HAP_Sheepmeat_Small.pdf.

      3. Beef and Lamb New Zealand. 2021. New Season Outlook 2020–21. Accessed Nov. 17, 2021. https://beeflambnz.com/sites/default/files/data/files/B%2BLNZ%20New%20Season%20Outlook%202020-21.pdf.

        • Beermann D.H.
        • Robinson T.F.
        • Hogue D.E.
        Impact of composition manipulation on lean lamb production in the United States..
        J. Anim. Sci. 1995; 73: 2493-2502
        • Berg E.P.
        • Neary M.K.
        • Forrest J.C.
        • Thomas D.L.
        • Kauffman R.G.
        Evaluation of electronic technology to assess lamb carcass composition..
        J. Anim. Sci. 1997; 75: 2433-2444
        • Blackburn H.D.
        • Snowder G.D.
        • Glimp H.
        Simulation of lean lamb production systems..
        J. Anim. Sci. 1991; 69: 115-124
        • Brady A.S.
        • Belk K.E.
        • LeValley S.B.
        • Dalsted N.L.
        • Scanga J.A.
        • Tatum J.D.
        • Smith G.C.
        An evaluation of the lamb vision system as a predictor of lamb carcass red meat yield percentage..
        J. Anim. Sci. 2003; 81: 1488-1498
        • Carpenter Z.L.
        What is consumer-preferred lamb?.
        J. Anim. Sci. 1966; 25: 1232-1235
        • DeWalt M.S.
        • Tatum J.D.
        • Abraham H.C.
        • Dolezal H.G.
        • Garrett R.P.
        • Field R.A.
        • LeValley S.B.
        • Smith G.C.
        • Wise J.W.
        Comparison of methods for evaluating external fat thickness and body wall thickness in lamb carcasses..
        Sheep Res. J. 1992; 8: 38-42
        • Field R.A.
        • Whipple G.
        The relation of slaughter and carcass weights to production and processing efficiency and market acceptability..
        Sheep Goat Res. J. 1998; 14: 98-105
        • Heaton K.L.
        • Morgan J.B.
        • Tatum J.D.
        • Wise J.W.
        • Garret R.P.
        • Dolezal H.G.
        • Loveday H.D.
        • Smith G.C.
        Field studies to document the efficacy of visual assessments of lamb carcasses to appropriate USDA yield grades..
        Sheep Res. J. 1993; 9: 7-15
        • Hoffman T.W.
        • Belk K.E.
        • Woerner D.R.
        • Tatum J.D.
        • Delmore R.J.
        • Peel R.K.
        • LeValley S.B.
        • Pendell D.L.
        • Zerby H.N.
        • English L.F.
        • Moeller S.J.
        Preferences associated with American lamb quality in retail & foodservice markets..
        Meat Sci. 2016; 11: 138
        • Hopkins D.L.
        • Wotton J.S.A.
        • Gamble D.J.
        • Atkinson W.R.
        • Slack-Smith T.S.
        • Hal D.G.
        Lamb carcass characteristics I. The influence of carcass weight, fatness, and sex on the weight of “trim” and traditional retail cuts..
        Aust. J. Exp. Agric. 1995; 35: 33-40
      4. Hughes, D. R. 1976. Consumer Attitudes to Meat Cuts—A Further Study. University of Newcastle upon Tyne Dept. of Agricultural Marketing. No. 15.

        • Jeremiah L.E.
        • Gibson L.L.
        • Tong A.K.W.
        Retail acceptability of lamb as influence by gender and slaughter weight..
        Food Res. Int. 1993; 26: 115-118
        • Karamichou E.
        • Merrell B.G.
        • Murray W.A.
        • Simm G.
        • Bishop S.C.
        Selection for carcass quality in hill sheep measured by X-ray computer tomography..
        Animal. 2007; 1: 3-11
        • Kosulwat S.
        • Greenfield H.
        • James J.
        Lipid composition of Australian retail lamb cuts with differing carcass classification characteristics..
        Meat Sci. 2003; 65: 1413-1420
        • Kvame T.
        • Vangen O.
        Selection for lean weight based on ultrasound and CT in a meat line of sheep..
        Livest. Sci. 2007; 106: 232-242
        • Leeds T.D.
        • Mousel M.R.
        • Notter D.R.
        • Zerby H.N.
        • Moffet C.A.
        • Lewis G.S.
        B-mode, real-time ultrasound for estimating carcass measures in live sheep: Accuracy of ultrasound measures and their relationships with carcass yield and value..
        J. Anim. Sci. 2008; 86: 3203-3214
        • LeValley S.B.
        • Dewalt M.S.
        • Speer N.C.
        • Tatum J.D.
        • Glimp H.A.
        • Simms R.H.
        Carcass evaluation of western Colorado range lambs..
        Prof. Anim. Sci. 1991; 7: 7-11
      5. Meat and Livestock Australia (MLA). 2020. Global Snapshot: Sheepmeat. Accessed Jun. 26, 2020. https://www.mla.com.au/globalassets/mla-corporate/prices–markets/documents/os-markets/red-meat-market-snapshots/2020/global-sheepmeat-snapshot-jan2020.pdf.

        • Mousel M.R.
        • Notter D.R.
        • Leeds T.D.
        • Zerby H.N.
        • Moeller S.J.
        • Lewis G.S.
        Evaluation of Columbia, USMARC-Composite, Suffolk, and Texel rams as terminal sires in an extensive rangeland production system: III. Prefabrication carcass traits and organ weights..
        J. Anim. Sci. 2012; 90: 2953-2962
      6. New Zealand Meat Classification Authority (NZMCA). 2004. Guide to Lamb and Mutton Carcass Classification. Accessed Jun. 24, 2021. https://www.interest.co.nz/files/rural/lambgrade.pdf.

        • Purcell W.D.
        Economic issues and potentials in lamb marketing: Keys to the future of the sheep industry..
        Sheep Goat Res. J. 1995; 11: 92-105
        • Shackelford S.D.
        • Leymaster K.A.
        • Wheeler T.L.
        • Koohmaraie M.
        Effects of breed of sire on carcass composition and sensory traits of lamb..
        J. Anim. Sci. 2012; 90: 4131-4139
        • Snowder G.D.
        • Glimp H.A.
        • Field R.A.
        Carcass characteristics and optimal slaughter weights in four breeds of sheep..
        J. Anim. Sci. 1994; 72: 932-937
        • Southam E.R.
        • Field R.A.
        Influence of carcass weight upon carcass composition and consumer preference for lamb..
        J. Anim. Sci. 1969; 28: 584-588
        • Tatum J.D.
        • Savell J.W.
        • Cross H.R.
        • Butler J.G.
        A national survey of lamb carcass cutability traits..
        SID Res. J. 1989; 5: 23-31
      7. Tschirhart, T. E., L. A. Rakowitz, D. R. McKenna, D. B. Griffin, and J. W. Savell. 2002. Development of a cutability equation for carcasses of show lambs. Pages 31–35 in Sheep and Goat/Wool and Mohair Consolidated Progress Report 2002. Texas A&M University Agricultural Research and Extension Center.

      8. USDA AMS. 1992. Official United States Standards for Grade of Lamb, Yearling Mutton, and Mutton Carcasses. AMS, USDA.

      9. USDA AMS. 2021. Beef, Lamb and Veal Percentage Charts. AMS, USDA. Accessed Nov. 11, 2021. https://www.ams.usda.gov/reports/meat-grading.

      10. USDA APHIS. 2011. Part IV: Changes in Health and Production Practices in the U.S. Sheep Industry, 1996–2011. APHIS, USDA.

      11. USDA ERS. 2021a. Food Availability Documentation. ERS, USDA. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/food-availability-per-capita-data-system/food-availability-documentation/.

      12. USDA ERS. 2021b. All supply and disappearance. ERS, USDA. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/livestock-meat-domestic-data/.

      13. USDA ERS. 2021c. Annual and Cumulative Year-to-Date U.S. Livestock and Meat Trade by Country. ERS, USDA, Washington, D.C. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/livestock-and-meat-international-trade-data/.

      14. USDA ERS. 2021d. Livestock and poultry live and dressed weights. ERS, USDA. Accessed Jul. 27, 2021. https://www.ers.usda.gov/data-products/livestock-meat-domestic-data/.

        • Ward C.E.
        Slaughter lamb pricing issues, evidence and future needs..
        Sheep Goat Res. J. 1998; 14: 35-42