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J. Anim. Sci. 2003. 81:1-8
© 2003 American Society of Animal Science

Leptin as a predictor of carcass composition in beef cattle1

T. W. Geary*,2, E. L. McFadin{dagger}, M. D. MacNeil*, E. E. Grings*, R. E. Short*, R. N. Funston{ddagger} and D. H. Keisler{dagger}

* USDA-ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301; and {dagger} Animal Sciences Department, University of Missouri, Columbia 65211; and and {ddagger} Animal and Range Sciences Department, Montana State University, Bozeman 59717

2 Correspondence:
phone (406) 232-8215; fax (406) 232-8209; E-mail:
tom{at}larrl.ars.usda.gov.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 
Our objective was to determine if serum concentrations of leptin could be used to predict carcass composition and merit in feedlot finished cattle. Two different groups of crossbred Bos taurus steers and heifers were managed under feedlot conditions near Miles City, MT. The first group consisted of 88 1/2 Red Angus, 1/4 Charolais, and 1/4 Tarentaise composite gene combination steers (CGC) harvested at the ConAgra processing facility in Greeley, CO. The second group (Lean Beef Project; LB) consisted of 91 F2 steers and heifers born to Limousin, Hereford, or Piedmontese by CGC F1 cows crossed to F1 bulls of similar breed composition and harvested at a local processing facility in Miles City, MT. Blood samples were collected approximately 24 h before harvest (CGC) or approximately 3 d before and at harvest (LB). No differences in serum concentrations of leptin were detected (P > 0.10) between Hereford, Limousin, or Piedmontese F2 calves nor between LB steers and heifers. Positive correlations (P < 0.01) existed between serum leptin and marbling score (r = 0.35 and 0.50), fat depth measured between the 12th and 13th rib (r = 0.34 and 0.46), kidney, pelvic, and heart fat (KPH) (r = 0.42 and 0.46), and quality grade (r = 0.36 and 0.49) in CGC and LB cattle, respectively. Serum leptin was also positively correlated with calculated yield grade for CGC steers (r = 0.19; P = 0.10) and LB cattle (r = 0.52; P < 0.01). Longissimus area was not correlated with serum leptin in CGC steers (r = 0.12; P > 0.10). However, a negative correlation existed between longissimus area and serum leptin in the LB cattle (r = -0.45; P < 0.01). Serum concentrations of leptin were significantly associated with carcass composition (marbling, back fat depth, and KPH fat) and quality grade in both groups of cattle studied and may provide an additional indicator of fat content in feedlot cattle.

Key Words: Carcass Composition • Cattle • Fat • Leptin • Lipid


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 
The protein hormone leptin has been implicated in the control of food intake and body composition in mammals. The principal site of leptin production is adipocytes, and as adipocytes increase in mass, peripheral concentrations of leptin increase (Considine, 1997; Ahima and Flier, 2000). Once mammals reach their mature size, most subsequent growth occurs in the form of adipose tissue deposition; thus, it would seem reasonable that circulating concentrations of leptin would also increase.

Accrual of adipose tissue in the body first occurs as the result of hyperplastic adipocyte growth followed by hypertrophic changes (Owens et al., 1993). In the livestock industries, hypertrophy of adipose tissue is the major fat deposition involved in finishing animals to market weight (Hood, 1982); however, the rate of adipose tissue growth varies with location within the body. In growing cattle, sheep, and pigs, subcutaneous fat hypertrophy occurs faster than does intermuscular, intramuscular, or kidney, pelvic, and heart fat (KPH) hypertrophy (Kempster, 1980).

Adipocyte size may influence leptin synthesis and secretion because larger adipocytes contained more leptin mRNA (Auwerx and Staels, 1998). Adipocyte diameter varies according to tissue location, and in a study of 17-mo-old crossbred steers, diameter of adipocytes were classified in the following regions as containing the largest to smallest adipocytes: KPH, mesenteric, subcutaneous, intermuscular, intramuscular, and brisket fat, respectively (Cianzio et al., 1985). Because quality and yield grades are both influenced by fat deposition and are used to determine carcass value, our objective was to determine if peripheral concentrations of leptin in feedlot cattle could be used to predict carcass merit.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Implications
 Literature Cited
 
Animal and Data Collection

Two different groups of Bos taurus steers and heifers, managed under feedlot conditions in Miles City, MT, were used in this study. The first group consisted of 88 composite gene combination steers (CGC; 1/2 Red Angus, 1/4 Charolais, and 1/4 Tarentaise) castrated at approximately 1 yr of age and approximately 112 d before harvest. The CGC steers were fed a growing diet (Table 1Go) and allowed ad libitum access to water. Three weeks before harvest, steers were fed a finishing diet (Table 1Go). The second group of cattle (Lean Beef Project; LB) consisted of 91 F2 steers and heifers born to Limousin, Hereford, or Piedmontese by CGC F1 cows crossed to F1 bulls of similar genetic makeup. These steers were castrated at <60 d of age. The LB cattle were fed a growing diet from weaning until approximately 363 kg and were then fed a finishing diet (Table 1Go) for approximately 90 or 130 d. All diets were formulated on a DM basis, and neither group of cattle received ionophores or growth promotants. Heifers were not fed melengesterol acetate.


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  		Table 1. Content of diets (% DM basis) fed to CGCa steers and LBb steers and heifers during the growing and finishing phases
 
Blood samples were collected from CGC steers via coccygeal venipuncture approximately 24 h before harvest. Steers had access to fresh feed and water until the time of blood sampling. Blood samples were collected as steers were prepared for transport to the processing plant in Greeley, CO. Two blood samples were collected from LB steers and heifers. The first sample was obtained approximately 3 d before harvest, and the second sample was obtained at harvest. Samples collected 3 d before and at harvest did not differ in concentration of leptin; therefore, the mean of the two leptin values from each LB animal was used for analyses. All blood samples were allowed to clot for 6 to 18 h at 4°C, centrifuged at 2,500 x g for 30 min, and serum collected and stored at -20°C until assayed for leptin using the leptin RIA described by Delavaud et al. (2000). Intra- and interassay coefficients of variation for the leptin assay were less than 10%.

Animals in both groups were harvested according to humane cattle-processing procedures and carcass weight was measured immediately postharvest. Carcasses from both groups were chilled at 2°C for 24 h and ribbed between the 12th and 13th ribs to collect additional carcass data according to USDA (1989) guidelines. Traits measured for both groups of cattle were fat depth over the 12th rib, marbling score (scale of 1 = devoid to 28 = abundant +; with a small- [choice-] = 11; Short et al., 1999), ribeye area, and KPH. Yield and quality grades were calculated from these data (USDA, 1989). Numerical quality grades were assigned to carcasses based on the ConAgra grading system in which 1 [practically devoid] = standard, 2 [traces] = standard, 3 [slight] = select, 4 [small] = choice-, 5 [modest] = choice, 6 [moderate] = choice+, 7 [slightly abundant] = prime-, and 8 [moderately abundant] = prime.

The LB cattle were processed at a local packing plant near Miles City, MT. Additional data collected from these animals included live weight, shear force, KPH weight, and finishing average daily gain. Warner-Bratzler shear force was determined from 2.54-cm-thick loin steaks vacuum packaged for 14 d at 2°C and then stored at -20°C until shear force analysis was performed (Wheeler et al., 1996).

Statistical Analysis

Data from CGC steers were analyzed using the GLM procedure of SAS (SAS Inst. Inc., Cary, NC), using carcass traits as dependent variables and serum concentration of leptin as the independent variable. For CGC steers, relationships between serum concentrations of leptin and carcass traits were quantified by Pearson correlation coefficients and linear regression. Data from LB cattle were analyzed using the GLM procedure of SAS (SAS Inst. Inc., Cary, NC), using serum concentration of leptin and carcass traits as dependent variables and breed, sex, time on finishing ration, and all possible interactions as independent variables for the initial model. The final model was based on deleting one nonsignificant term at a time from the full model until all terms remaining in the final model approached significance for one or more traits. Only main effects remained in the final model. Separate analyses were conducted for each group of cattle because the linear models that are appropriate for each data set differed. Partial correlations of leptin with carcass traits were determined from the error sums of squares and cross products. For each carcass trait, a partial regression on leptin was calculated with the main effects fit simultaneously. These traits, fat depth, marbling score, and KPH weight (LB) or percentage (CGC) were each indicative of a part of the overall variability in carcass fatness. In order to develop a consistent test of the effect of serum leptin concentration on carcass fatness, a multivariate analysis of variance was performed considering these three dependent variables simultaneously. Models used for the multivariate analysis of variance were identical to those described above. Significance of the serum leptin concentration effect was established using Wilks’s criterion.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Implications
 Literature Cited
 
Mean serum concentrations of leptin and carcass measurements for CGC and LB cattle are presented in Table 2Go. Among LB steers and heifers, mean serum concentrations of leptin in Hereford (26.8 ng/mL), Limousin (26.3 ng/mL), and Piedmontese (28.8 ng/mL) F2 calves did not differ (P > 0.10). Adipose tissue measurements (fat depth, marbling score, and KPH) also did not differ (P > 0.10) between breeds (Table 3Go). Dressing percentage was different between breeds and was lower (P < 0.01) for Hereford than for Limousin or Piedmontese F2 calves. There was no difference (P > 0.10) in mean concentration of leptin between steers (26.4 ng/mL) and heifers (27.7 ng/mL; Table 3Go). Fat depth and percent KPH (P > 0.10) did not differ by gender, but steers had greater marbling score (P < 0.01) and KPH weight (P < 0.05) than heifers (Table 3Go). Serum concentrations of leptin were greater (P < 0.05) among steers and heifers that spent a greater amount of time on the finishing diet (Table 3Go). The only adipose tissue measurement that increased (P < 0.01) with increased time on finishing diet was KPH weight (Table 3Go).


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  		Table 2. Mean and standard deviation (SD) for leptin and carcass traits in CGCa steers and LBb steers and heifers
 

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  		Table 3. Probability values for sources of variation affecting serum concentrations of leptin and carcass traits in LB cattle, and the associated mean square errora
 
Serum concentrations of leptin were positively correlated (P < 0.01) with marbling score, fat depth, KPH, and quality grade in both CGC and LB cattle (Tables 4Go and 5Go). Serum leptin was also positively correlated (P < 0.01) with the ConAgra muscling grid value in CGC steers and with calculated yield grade in LB steers and heifers. Among CGC steers, there was only a tendency (P < 0.10) for serum concentrations of leptin and calculated yield grade to be correlated (Table 4Go). Dressing percentage and ribeye area were negatively correlated (P < 0.01) with serum leptin among LB steers and heifers (Table 5Go), while no relationship (P > 0.10) was detected between serum leptin and ribeye area among CGC steers, and only a tendency for a positive correlation (P < 0.10) existed between serum leptin and dressing percentage (Table 4Go).


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  		Table 4. Pearson correlation coefficients for leptin and carcass traits of CGC steersa
 

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  		Table 5. Partial correlation coefficients with effects of breed, sex, and time on feed removed for LB steers and heifersa
 
Coefficients of linear regression equations predicting carcass traits from serum concentrations of leptin in CGC and LB cattle are presented in Tables 6Go and 7Go, respectively. Serum leptin was a highly significant predictor of marbling score, fat depth, KPH fat, and quality grade in both groups of cattle. Among LB steers and heifers, serum leptin was also a highly significant predictor of ribeye area, calculated yield grade, and dressing percentage. Among CGC steers, serum leptin was a significant predictor of calculated yield grade and dressing percentage but was not indicative of ribeye area. The multivariate analysis of variance showed differences in serum concentrations of leptin predicted overall carcass fatness for both CGC and LB cattle (P < 0.01).


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  		Table 6. Summary of linear regressions to predict carcass traits from serum concentrations of leptin (ng/mL) in CGC steersa
 

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  		Table 7. Summary of partial linear regressions to predict carcass traits from serum concentrations of leptin (nanograms per milliliter) in LB steers and heifersa
 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Implications
 Literature Cited
 
Adipocytes store excess energy in the form of triglycerides when energy intake exceeds that which is needed for homeostasis and will subsequently release free fatty acids when dietary energy is inadequate (Kim and Moustaid-Moussa, 2000). Total adipose tissue mass increases via replication and differentiation of preadipocytes. Adipose tissue mass is influenced by volume and number of adipocytes (Prins and O’Rahilly, 1997). Adipocytes are also the principal site of leptin production in mice (Zhang et al., 1994), pigs (Mendiola et al., 1997), sheep (Dyer et al., 1997), cattle (Ji et al., 1998), and humans (Considine, 1997). In obese humans, the secretion of leptin per gram of adipose tissue is twice that of the leptin secreted per gram of adipose tissue in lean subjects (Fried et al., 2000). Among LB cattle, time on feed accounted for a significant amount of the variation in mean concentrations of leptin even though mean concentrations of leptin were similar between breeds. Diet and feeding times are variables known to influence serum concentrations of leptin in horses and sheep (Buff et al., 2001; Daniel et al., 2002). In humans, peripheral concentrations of leptin will peak at different times of the day, depending on the composition of the diet. A low-fat, high-carbohydrate diet will cause leptin levels to peak at night without affecting morning levels of leptin (Fried et al., 2000). In humans and sheep, fasting will decrease circulating levels of leptin, whereas overfeeding will increase circulating leptin without an appreciable change in fat mass (Considine, 1997; Daniel et al., 2002).

Mean serum concentrations of leptin were numerically lower in CGC steers (18.7 ng/mL) than LB steers (27.0 ng/mL). The fat depth and marbling scores were numerically lower in CGC carcasses (0.76 cm and 10.30, respectively) than LB carcasses (0.94 cm and 12.84, respectively) and likely contributed to the lower serum levels of leptin in CGC steers. The growing and finishing management, as well as variation in sample collection times, diets, and age at castration, may also have contributed to differences in leptin levels between the groups of cattle. Genetically, LB cattle were 1/2 CGC and either 1/2 Hereford, Limousin, or Piedmontese. The LB cattle used in this study were a subset of a population of animals used in a larger study to evaluate effects of sire breeds that differ in potential for lean tissue growth on traits measured from birth to slaughter (Short et al., 2002). The lack of difference in serum concentrations of leptin between breeds within the LB group was surprising because Short et al. (2002) reported large differences in fat depth and yield grade between these breeds in the larger population of LB cattle. While we observed differences in measures of fatness between breed of similar magnitude to those observed by Short et al. (2002), we were unable to detect differences in fat measurements between breeds. This lack of difference may be due to the smaller sample size in the current study. This lack of difference may also suggest that the leaner Piedmontese cattle express leptin from sites other than adipocytes or their expression per gram of adipose tissue is higher. The lower dressing percentage observed among the Hereford F2 calves agrees with that of Short et al. (2002) and supports our observation that the Limousin and Piedmontese F2 calves produced leaner carcasses without changes in serum concentrations of leptin.

While gender did not influence mean serum concentrations of leptin among LB cattle, the observation that steers had higher marbling scores and KPH weight without an increase in leptin suggests that leptin synthesis or degradation differs between steers and heifers. These results are consistent with those of Hellstrom et al. (2000) who reported that women typically have greater concentrations of circulating leptin than men. Our findings are inconsistent with results among horses and pigs in which serum leptin was greater in geldings or barrows than in mares or gilts (Berg et al., 2003; Buff et al., 2002). Others have also reported that peripheral concentrations of leptin may be influenced by testosterone and/or estrogen levels (Demerath et al., 1999; Horlick et al., 2000). Androgen effects on concentrations of leptin would have been minimal in the comparison of LB steers and heifers as the result of castration.

Significant correlations between subcutaneous fat thickness and circulating concentrations of leptin were observed among both groups of cattle in the present study, which agrees with previous research in cattle (Minton et al., 1998; Ehrhardt et al., 2000), sheep (Delavaud et al., 2000; Ehrhardt et al., 2000), pigs (Estienne et al., 2000), and horses (Buff et al., 2002). Percentage KPH and marbling scores were also significantly correlated with circulating concentrations of leptin in both groups of cattle. Minton et al. (1998) also reported positive correlations between serum concentrations of leptin and both KPH (r = 0.18) and marbling score (r = 0.28), but their correlations were considerably lower than the average correlations for these traits (r = 0.49 and r = 0.43 for KPH and marbling score, respectively) observed in the present study. A potential factor contributing to this difference in magnitude of correlations between these traits and leptin may be that serum was collected 30 d before slaughter in the study by Minton et al. (1998). Ehrhardt et al. (2000) reported strong linear relationships between peripheral concentrations of leptin and empty carcass fat content in Holstein calves but did not measure fat content from different sites. Others have reported no correlations between plasma concentrations of leptin and subcutaneous fat thickness, extracted longissimus muscle lipid content, or marbling score in Japanese Black steers (Kawakita et al., 2001). It was unclear from the study of Japanese Black steers when blood samples were collected relative to slaughter.

In humans, leptin mRNA and subsequent leptin production was greater in subcutaneous fat than in abdominal fat (Fried et al., 2000). Kidney and subcutaneous adipocytes are the largest and third largest adipose cells in steers (Cianzio et al., 1985), and adipocyte size influences leptin mRNA production and subsequent secretion of leptin in humans (Considine, 1997; Lonnqvist et al., 1997). In cattle, leptin mRNA expression was greatest from kidney fat followed by subcutaneous fat (Xie et al., 1999); thus, it is logical that such strong correlations exist between serum concentrations of leptin and these carcass traits.

Finished cattle can be sold by live weight, carcass weight, one of several muscling grids, or one of several marbling (quality) grids. The ability to predict an animal’s dressing percentage or carcass composition and quality before harvest would allow sorting and sale of pens of cattle similar in carcass value. Intramuscular fat content may influence palatability in beef as it has been reported to increase perceived tenderness by replacing protein with lipid and to increase perceived juiciness by stimulating the salivary glands (Savell and Cross, 1988). Adipocyte number, rather than size, was the main factor influencing intramuscular fat content in cattle (Hood and Allen, 1973; Cianzio et al., 1985) and circulating concentrations of leptin (Shillabeer et al., 1998) in rats. The CGC steers in the present study were sold using the ConAgra muscle grid formula pricing, in which quality grade was the major price incentive, and serum concentrations of leptin was strongly correlated with carcass value (r = 0.42; P < 0.01).

Estimation of longissimus muscle marbling on the live animal is currently possible using ultrasound equipment and trained personnel. Because quality grade is assigned based on marbling score and maturity, we hypothesized that serum concentrations of leptin may have potential as an accurate preslaughter indicator of quality grade in feedlot cattle. In two different groups of cattle, we obtained similar results strengthening the previously reported relationship between circulating leptin and fat deposition in the body (Minton et al., 1998). It appears that greater concentrations of leptin are indicative of greater adiposity in fed cattle. Both quality grade and yield grade are used for price differentiation among beef carcasses.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Implications
Literature Cited
 
The opportunity to modify body composition through selection or management can help cattle producers and feeders produce beef that more consistently meets expectations of consumers. Circulating concentrations of leptin may provide another indicator of fat content in live cattle and thus, facilitate more appropriate feeding and marketing management strategies. However, further research is needed to discern whether serum concentration of leptin coupled with other indicators may serve as a useful predictor of carcass value that can be applied before harvest.


   Footnotes
 
1 Mention of a proprietary product does not constitute a guarantee or warranty of the product by USDA, Montana Agric. Exp. Sta., or the authors and does not imply its approval to the exclusion of other products that may be also suitable. USDA, Agricultural Research Service, Northern Plains Area, is an equal opportunity/affirmative action employer, and all agency services are available without discrimination. Back

Received for publication April 18, 2002. Accepted for publication August 20, 2002.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 


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