J. Anim Sci. 2007. 85:267-275. doi:10.2527/jas.2005-591
© 2007 American Society of Animal Science
Serum hormone concentrations relative to carcass composition of a random allotment of commercial-fed beef cattle1,2
M. M. Brandt,
D. H. Keisler,
D. L. Meyer,
T. B. Schmidt and
E. P. Berg3
Division of Animal Science, University of Missouri, Columbia 65211
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Abstract
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Cattle (n = 995 steers and 757 heifers) were randomly selected from a commercial abattoir (Emporia, KS) to determine the relationships between USDA quality and yield grade characteristics and serum concentrations of leptin, IGF-I, and GH. Animals were randomly selected postexsanguination on the slaughter line on 4 occasions (March, May, August, and January). Blood was collected at exsanguination and transported to the University of Missouri for analysis. Sex and hide color were recorded. Carcass data included HCW, 12th-rib fat thickness, KPH, LM area, and marbling score, which were collected from each carcass approximately 24 h postmortem. Average serum leptin concentrations were greater (P = 0.008) for heifers (11.9 ng/mL) than steers (10.9 ng/mL). Heifers had lighter carcasses (331.9 vs. 352.2 kg, P < 0.001), greater 12th-rib fat measurements (1.3 vs. 1.1 cm, P < 0.001), greater KPH (2.5 vs. 2.4%, P < 0.001), and more marbling (Small40 vs. Small10, P < 0.001) than steers. Positive correlations (P < 0.01) existed between leptin concentration and marbling score (r = 0.28), 12th-rib fat depth (r = 0.37), KPH (r = 0.23), and USDA yield grade (r = 0.32). Negative correlations were found between leptin and IGF-I (r = –0.11; P < 0.001) and leptin and GH (r = –0.32; P < 0.001). Negative correlations (P < 0.01) were observed for IGF-I and KPH (r = –0.23) and marbling score (r = –0.20), whereas GH was most highly negatively correlated with KPH (r = –0.23; P < 0.001). Leptin concentration accounted for variation (P < 0.001) in a model separating least squares means across USDA quality grade, separating USDA standard (8.5 ng/mL), select (10.3 ng/mL), low choice (12.2 ng/mL), and upper 2/3 choice/prime (>12.9 ng/mL) carcasses. There was no difference (P = 0.31) observed in leptin concentrations between the upper 2/3 choice and prime carcasses (12.9 and 14.2 ng/mL, respectively). Relationships within endocrine profiles and between endocrine concentrations and carcass quality characteristics may prove to be a useful tool for the prediction of beef carcass composition.
Key Words: beef cattle • carcass • fat • insulin-like growth factor-I • leptin
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INTRODUCTION
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The hormone leptin was discovered by Zhang et al. (1994)
in the mouse and is primarily secreted from white adipocytes with the task of regulating food intake, energy expenditure, and energy balance in the body (Houseknecht et al., 1998). The relationships between leptin and carcass could prove useful for the meat-animal industry. Minton et al. (1998) and Geary et al. (2003) reported correlations between serum leptin and 12th rib back fat, USDA yield grade, marbling score, KPH, and USDA quality grade in beef cattle. McFadin et al. (2003) found positive correlations between serum leptin and 12th rib backfat, USDA yield grade, and marbling score in beef cattle. Results of these experiments suggest that serum leptin concentrations may be a means to predict beef carcass merit before slaughter.
Likewise, bovine GH and IGF-I have been documented as endocrine regulators for lean muscle growth in cattle. Trenkle and Topel (1978) reported correlations between GH, percentage carcass fat, and percentage carcass muscle. Anderson et al. (1988) reported negative correlations with percentage of carcass fat, GH, and IGF-I in beef bulls. To establish their true role, the relationship of these hormones to carcass merit deserves further study.
If quantification of circulating hormones is to be effectively used as a means of predicting economically important production factors such as subcutaneous or intramuscular fat, the technique must be applicable across the wide spectrum of US market cattle.
The objectives of this study were to evaluate (1) correlations between leptin, IGF-I, GH, and beef quality characteristics from a random allotment of commercial beef cattle; (2) hormone (leptin, IGF-I, GH) concentrations relative to one another and to the various estimates of beef carcass composition and quality; and (3) sex differences of leptin, IGF-I, and GH concentrations across variations in carcass compositional endpoints.
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MATERIALS AND METHODS
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The protocol for this experiment was reviewed by the University of Missouri Animal Care and Use Committee. The handling of the animals was found to be acceptable within the guidelines of the USDA Humane Slaughter Act for transport, lairage, stunning, and exsanguination.
Cattle were randomly selected immediately after exsanguination from a commercial abattoir (Emporia, KS) on 4 separate collection days (Table 1). Collection days were on March 30, 2004 (March-04), May 17, 2004 (May-04), August 17, 2004 (Aug-04), and January 3, 2005 (Jan-05). The initial collection day (March-04) yielded a random collection of 198 mixed-sex cattle, whereas collection in May-04, Aug-04, and Jan-05 each contained 526, 533, and 495 mixed sex cattle, respectively. The total number of steers and heifers used in the final analysis is outlined in Table 1. No preslaughter information was gathered for any of the cattle before initiation of the study. The random selection of cattle was assumed to represent the general population of market cattle sold through the commercial abattoir (based on McKenna et al., 2002), representing a variety of management strategies, transport distances, and genetic types.
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Table 1. Summary of collection days on which random sampling of commercial-fed beef cattle occurred at the commercial abattoir
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The length of lairage for all cattle received was estimated by plant personnel to be approximately 6 h. To ensure that cattle with lairage times greater than 6 h were not included in the study, the first 350 head of each day’s slaughter were not used in the study. It is a common practice for cattle to arrive throughout the night at commercial abattoirs to provide the appropriate number of cattle for the first hour or 2 of operation. The abattoir used in this study typically arranges for approximately 350 cattle to arrive before the beginning of the morning slaughter, which accounts for roughly 1.5 h of operation.
Cattle were randomly selected after stunning and while suspended from the bleed chain. Whereas collection was considered random, there was a conscious effort to avoid intact males (bulls and bullocks) and cattle with excessive age (through the use of dentition scores; Lawrence et al., 2001).
Typically, in the process of exsanguination, the carotid arteries and the jugular vein are severed at the base of the neck. After the initial blood surge, a moderate flow of blood is present for approximately 3 to 5 min. During this time, an individual from the university placed an open, 15-mL, sterile, polypropylene conical tube (S50712, Fisher Scientific, Morris Plains, NJ) directly into the flow of blood. Once it was full, the tube was capped and placed into a Styrofoam rack. Blood samples were allowed to clot for 30 min at room temperature and then chilled by placing them into a cooler containing ice. At the end of collection day, the chilled blood samples were transported for approximately 3.5 h to the University of Missouri, where the samples were refrigerated at 4°C for 24 h. Before centrifugation of the samples, caps were removed, and blood was separated from the sides of the tubes using wooden rods to aid in serum separation. Samples were centrifuged at 2,500 x g for 45 min. After centrifugation, serum was pippetted from the tubes, placed into 48-well plates (5 mL/well; ABgene Inc., Rochester, NY), and stored at –20°C until analyses.
Leptin concentrations were determined by a double-antibody RIA, as described by Delavaud et al. (2000). Leptin assay inter- and intraassay CV were 2 and 3% (n = 5 and 35, respectively). Insulin-like growth factorI and GH concentrations were assayed as described by Lalman et al. (2000). The IGF-I assay inter- and intraassay CV were 3 and 3% (n = 9 and 27, respectively). Serum samples were thawed, mixed thoroughly, and then 10 uL of serum was pipetted into deep 96-well plates. Immediately thereafter, 400 uL of 1 M glycine (pH 3.2) was used to acidify the samples. Then, 500 uL of PABET (0.01 M phosphate, 0.15 M NaCl, 0.1% gelatin, 0.02% sodium azide, 0.01 M EDTA, and 0.05% Tween 20, pH 7.2) was added, and the samples were capped and incubated in a constant-temperature oven at 37°C for 48 h. Subsequently, samples were neutralized by addition of 90 uL of 0.5 N NaOH to each sample before being submitted to the IGF-I assay. Bovine GH assay inter- and intraassay CV were 2 and 2% (n = 7 and 38, respectively).
Hide color was recorded at the time of blood collection according to a scheme reported by McKenna et al. (2002). Color scores were assigned as follows: 1 (black), 2 (black white face), 3 (black Holstein), 4 (red), 5 (red white face), 6 (red Holstein), 7 (white), 8 (gray), 9 (gray white face), 10 (brown), 11 (brown white face), 12 (yellow), 13 (yellow white face), and 14 (brindle).
Hot carcass weight and carcass identification number assigned by the packer were recorded before carcass entry into the cooler, where each carcass was chilled for approximately 24 h at 2°C. After chilling, carcasses were ribbed at the 12th-13th rib interface, allowed to bloom, and marbling score and skeletal maturity were recorded. Carcass measurements, including KPH, 12th-rib fat depth, and 12th-rib LM area, were collected while the carcasses remained on the bloom chain. Longissimus muscle area was determined using the reverse blot image technique described by Martin (1991). Subcutaneous 12th-rib fat depth was determined using a USDA preliminary yield grade ruler (USDA, 1997) and then adjusted to correct for atypical fat distribution, to hide removal defects, or both. Marbling scores were determined by a trained university evaluator. To minimize variation, the same evaluator determined marbling for the entire duration of the study. Though we did not physically determine carcass cutout yield (cut-ability), yield grade was calculated with standardized means using 12th-rib LM area, 12th-rib subcutaneous fat depth, estimated percentage of KPH, and HCW. These calculated yield grades are the common industry tool used as a means to segregate carcasses into 5 cut-ability categories (USDA, 1997) and thus served as an adequate means of categorical separation of carcass yield in the current study.
Statistical Analysis
The GLM procedure (SAS Inst. Inc., Cary, NC) was utilized to examine the fixed effects of kill day and sex on serum hormone profiles. All data were pooled to evaluate relationships between serum endocrine concentrations (leptin, IGF-I, and GH) and carcass traits, using Pearson correlation. Effects of USDA quality and yield grade categories on serum hormone profiles were evaluated by ANOVA for a completely random design. When significant effects were discovered, means were separated by the LSD method.
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RESULTS AND DISCUSSION
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Sample Population
Descriptive statistics for the beef carcass characteristics representing 4 d of processing at the commercial abattoir are reported in Table 2. The mean USDA yield grade was 2.9, similar to the average yield grade of 3.0 reported by McKenna et al. (2002) in the 2000 National Beef Quality Audit (data not reported in tabular form). Yield grade within the current study was distributed as follows: yield grade 1 (15.3%), 2 (40.1%), 3 (35.6%), 4 (8.0%), and 5 (0.9%). The average USDA quality grade for the current study was low choice (not shown in tabular form) with an average marbling score of small10 (Table 2).
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Table 2. Mean, SD, minimum, and maximum values for endocrine and carcass measurements of commercial-fed beef cattle
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Serum Leptin
Concentrations of leptin within this study (10.8 ± 0.2 ng/mL) are similar to those described by McFadin et al. (2003; 8.8 ng/mL) but lower than that reported by Geary et al. (2003; 22.8 ng/mL). Sampling and leptin quantification protocols used in the current study were similar to that utilized by McFadin et al. (2003) and Geary et al. (2003). McFadin et al. (2003) found concentrations of leptin in show steers from the Missouri State Fair to be lower than leptin concentrations reported by Geary et al. (2003). McFadin et al. (2003) concluded that the difference was due to stress factors associated with show-ring exhibition and extended lairage (
12 h). Geary et al. (2003) reported that steers and heifers that had been on feed for 130 d had greater serum leptin concentrations than those on feed for 90 d. The variation in mean serum leptin concentrations seen in this study and the aforementioned studies (Geary et al., 2003; McFadin et al., 2003) may be due to time on feed, length of time the cattle were held off feed, other aspects associated with their respective production environment, or a combination of these. Time of blood collection in relation to time since displacement from normal environment (feed, water, pen, etc.) may possibly be the most influential factor affecting variation in mean concentrations of leptin across studies.
For this population of cattle, serum leptin concentrations were positively associated with 12th-rib fat depth (P < 0.001), USDA yield grade (P < 0.001), marbling score (P < 0.001), and KPH (P < 0.001; Table 3). Correlations between leptin and carcass measurements found within this study agree with findings of Minton et al. (1998), Geary et al. (2003), and McFadin et al. (2003). Geary et al. (2003) reported a negative correlation (r = –0.45; P < 0.01) between leptin and LM area in a group of market steers, whereas the correlation between leptin and LM area in the current study was much smaller (r = –0.05; P = 0.03). The correlations between the concentrations of leptin and marbling score (r = 0.28, P < 0.01) in the current study were, however, lower than that reported by Geary et al. (2003; r = 0.50, P < 0.01) for a sample population of mixed-sex cattle and similar to the group of show steers evaluated by McFadin et al. (2003; r = 0.26, P < 0.05). The association between serum leptin concentrations and phenotypic color score revealed positive correlations (P = 0.001) between hide color and leptin. The correlation of r = 0.08 was ignored in further analysis due to weakness of the relationship.
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Table 3. Pearson correlation coefficients for leptin and carcass traits in the complete set of 1,682 randomly sampled, commercial-fed beef cattle
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Research up to this point (Minton et al., 1998; Geary et al., 2003; McFadin et al., 2003) comparing serum leptin concentrations to adipose-related carcass characteristics of fed beef cattle have been inconsistent in determining the anatomical adipose depot that is most highly correlated to leptin. Geary et al. (2003) reported the strongest correlation existed between leptin and KPH, although a meaningful correlation existed between leptin and subcutaneous fat measurements. McFadin et al. (2003) reported no correlation (P > 0.10) between leptin and KPH, though a correlation (r = 0.35; P < 0.01) with 12th rib backfat was observed. Correlations between leptin concentration and 12th rib backfat for our study (r = 0.37; P < 0.001) were similar to the correlation observed by Minton et al. (1998; r = 0.32; P < 0.001). Minton et al. (1998) and our study found positive correlations between leptin and KPH (r = 0.18 and r = 0.23; P < 0.001, respectively). The inconsistency across studies between leptin and the various fat depots could be attributed to a breakpoint in subcutaneous adiposity whereby concentrations of leptin begin to decline. Unpublished research from our lab has shown that as beef 12th-rib fat increased, serum leptin concentration increased and then began to decline as subcutaneous fat thickness reached its apex. Therefore, differences in subject adiposity might have been a factor contributing to differences within the body of literature.
Least squares means for serum endocrine concentrations and beef carcass composition characteristics within each random collection are shown in Tables 4 (steers) and 5 (heifers). Information related to heifers within March-04 should be read with caution due to the low number of heifers slaughtered that day (heifers n = 22). Heifers had lighter weight carcasses (331.9 vs. 352.2 kg, P < 0.001), greater 12th-rib fat depth (1.3 vs. 1.1 cm, P < 0.001), greater KPH (2.5 vs. 2.4%, P < 0.001), and more marbling (Small40 vs. Small10, P < 0.001) than steers. Statistical evaluation of the sex x yield grade interaction term in the leptin model revealed a trend (P = 0.08). Figure 1 depicts the relationship between steer and heifer leptin concentration across yield grade classification. Due to the low number of steers (n = 3) and heifers (n = 13) representing yield grade 5 carcasses, yield grade 4 and 5 classifications were combined to increase the power of test for the interaction term. As steer carcass cutability declined (increased numeric yield grade), leptin concentration continued to rise; each yield grade category differed (P < 0.05) from the previous one. A different pattern emerged for heifers. Serum leptin concentrations associated with yield grade 1 carcasses differed from yield grades 2 and 3; however, yield grades 3, 4, and 5 did not differ (P > 0.05). The observance of this leptin plateau indicates the possibility that in young heifers as the proportion of whole body fat increases, leptin concentration will elevate in a like manner until reaching a break point at or near the yield grade 3 classification. The plateau in leptin levels may be attributed to saturation of leptin receptors and excessive concentrations of bound leptin circulating in the blood stream, leptin resistance in young females at higher levels of adiposity, or both. Sex difference in leptin concentrations observed in the current study are inconsistent with the observations of Geary et al. (2003) who found no differences (P > 0.10) in serum leptin concentrations between steers (26.4 ng/ mL) and heifers (27.7 ng/mL) despite variation in live weight, HCW, marbling score, dressing percent, and quality grade. In other species, barrows (Berg et al., 2003) and geldings (Buff et al., 2002) have been reported to have greater leptin concentrations than their female counterparts. Barrows are commonly fatter than gilts (Berg et al. 2003), as are geldings compared with mares (Buff et al. 2002). For cattle, fat deposition is generally least for bulls, intermediate for steers, and greatest in heifers. With regard to sex differences in carcass composition and means for concentrations of leptin in steers, our research is consistent with other species as reported by Berg et al. (2003) and Buff et al. (2002).
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Figure 1. Interaction of sex and yield grade (YG; P < 0.08) on serum leptin concentrations at the time of exsanguination. a–eBars lacking a common superscript letter differ (P < 0.05). Because of the low numbers of steers (n = 3) and heifers (n = 13) representing YG5 carcasses, yield grade 4 and 5 were combined (YG4&5).
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Least square means for steer serum leptin concentration varied between the 4 different collection days, with Aug-04 steers having a greater (P < 0.05) leptin concentration compared with Jan-05 steers, which had the least serum leptin concentration; March-04 and May-04 steer populations were intermediate (Table 4
). During the same period of time, Aug-04 steers had the greatest (P < 0.05) amount of fat at the 12th rib, marbling, and final yield grade compared with the other 3 sampling periods (Table 4). Excluding March-04 (due to a small sample size), heifers sampled during Aug-04 and Jan-05 had greater (P < 0.05) serum leptin concentration, 12th-rib fat depth, marbling score, and final yield grade compared with May-04. Figure 2 displays means of serum leptin concentrations across quality grades from lowest to highest; standard, select, low choice, premium choice (upper 2/3 choice), and prime carcasses, respectively. Concentrations of leptin in serum from cattle whose carcasses graded prime and upper 2/3 choice were the greatest (P < 0.05) compared with all other quality grades, whereas the lowest concentration of leptin (P < 0.05) was observed in cattle with carcasses grading standard.
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Figure 2. Main effect of USDA quality grade [standard (Stand), select, low choice (Low Ch; small degree of marbling), premium choice (Prem Ch; modest and moderate degree of marbling), and prime] on serum leptin concentrations at the time of exsanguination. a–dBars lacking a common superscript letter differ (P < 0.05).
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Bovine Growth Hormone
Anderson et al. (1988) and Trenkle and Topel (1978) documented the associations between carcass composition and GH in cattle. In the current study, GH concentration were negatively correlated (P < 0.001) with KPH, 12th rib backfat depth, and yield grade (Table 3). Anderson et al. (1988) found that GH was negatively correlated with carcass fat percentage and fat thickness within growing beef bulls. This is to be expected as GH aids in rapid growth and development of premature animals and has been documented to decrease in market animals as the animals grow and age (Althen and Gerrits, 1976; Trenkle and Topel, 1978; Anderson et al., 1988). Trenkle and Topel (1978) described associations among GH and lean carcass measurements (total muscle weight) in crossbred beef steers. We found no correlations between GH and LM area (P = 0.98), or for GH and HCW (P = 0.10). A negative correlation existed between GH and USDA yield grade (r = –0.11; P < 0.001), though at a much lower association than that of GH and muscle mass (r = 0.42; P < 0.05) described by Trenkle and Topel (1978). The negative correlation between GH and yield grade in the current study is in contrast with that of Trenkle and Topel (1978) who described a positive association between GH and cut-ability. Concentrations of GH in serum collected from steers and heifers at time of exsanguination declined with the reduction of carcass cutability (categorized by a higher numeric yield grade; P = 0.02), whereby GH concentrations of animals with carcass yield grades 1 and 2 differed from yield grades 3, 4, and 5.
Steers had greater (P < 0.001) serum concentrations of GH than heifers, with mean values of 46.8 ± 2.6 and 30.6 ± 2.6 ng/mL, respectively. With steer carcasses having numerically lower yield grades and heavier HCW, GH would be an obvious factor contributing to sex-based differences. Evaluation of the sex x yield grade interaction term in the GH model approached a trend (P < 0.11). Figure 3 displays steer and heifer GH concentration across yield grade classification (pooling yield grades 4 and 5). Circulating concentrations of GH at time of exsanguination were similar among steers with carcass yield grades 1, 2, and 3, which were greater (P < 0.03) from the combined yield grade 4/5 classification. Young heifers of similar carcass maturity had lower concentrations of GH at each yield grade category vs. steers and had a more graded decline in GH concentration as numeric yield grade increased (Figure 3). Keller et al. (1979) documented lower average GH concentrations in heifer calves in comparison with their male counterparts at 5 mo of age. Means for endocrine profiles and carcass measurements for steers and heifers within each collection date are shown in Tables 4 and 5. Mean serum GH concentrations for steers within Jan-05 (74.5 ± 1.9 ng/mL) stand apart. As presented in Table 4, animals within Jan-05 recorded the heaviest HCW, whereas marbling and adipose measurements remained similar to steers represented in the other collection dates. In most cases, these factors would lead to lower numerical yield grade calculation, though the small mean 12th rib LM area of Jan-05 cattle resulted in similar yield grade classifications compared with the steers in the other collection dates. Reports of seasonal fluctuations of GH secretion in relation to length of photoperiod are inconsistent. Suttie et al. (1985) found seasonal differences in GH secretion within red deer, whereas several studies have shown no seasonal (photo-period) or temperature change effect in cattle (Tucker and Wettemann, 1976; Peters and Tucker, 1978; Zinn et al., 1989). Increased GH concentrations have been reported within vertebrates in response to a stressor to divert resources from continued growth to that of survival (Bruggeman et al., 1997; Carroll et al., 1998; McCusker, 1998). Govoni et al. (2003) reported average somatostatin concentrations in yearling Hereford steers and heifers to be 3.7 and 0.8 ng/mL, respectively, whereas Kendal et al. (2003) documented 29.4 and 33.2 ng/mL averages for Holstein steer calves (~100 d of age) subjected to different photoperiod lengths. Mean concentrations of GH (46.8 and 30.6 ng/mL) within this trial are greater than prior reports, possibly due to factors associated with unknown preslaughter stressors or with pulsatile secretions of GH.
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Figure 3. Interaction of sex and yield grade (YG; P < 0.11) on serum GH concentrations at the time of exsanguination. Because of the low number of steers (n = 3) and heifers (n = 13) representing YG 5 carcasses, yield grade 4 and 5 were combined (YG 4&5). a–dBars lacking a common superscript letter differ (P < 0.05).
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Insulin-Like Growth Factor-I
Insulin-like growth factor-I was negatively correlated (P < 0.001) with 12th rib back fat depth, KPH, marbling score, and yield grade (Table 3). These relationships correspond with those documented by Anderson et al. (1988) in relation to carcass fat deposition. Bishop et al. (1989) documented low but favorable (negative) correlations between KPH, yield grade, and 12th-rib fat depth, as well as varying positive correlations with LM area at the end of a 140-d feeding study. Connor et al. (2000) reported that bulls with a greater concentration of serum IGF-I at the beginning of a feeding experiment produced larger LM area as determined by ultrasound measurements taken at the conclusion of 140 d on feed. This outcome would be anticipated; IGF-I is known to aid in the growth and development of most body tissues (Muller et al., 1999) and is in direct association with GH. Muscle cells are thought to produce IGF-I to promote cell proliferation and differentiation during times of muscle hypertrophy (Oksbjerg et al., 2004), which would support positive correlations with lean tissue mass. At the same time, GH is known to decrease as animals mature, thus producing fewer stimuli for production of IGF-I during the latter period of time in the feedlot. Mean concentrations of IGF-I, as well as mean carcass measurements across slaughter dates, are shown for steers and heifers in Tables 4 and 5, respectively.
Thermal stress (Sarko et al., 1994, Matteri et al., 2000), stage of development (Anderson et al., 1988; Bishop et al., 1989; Connor et al., 2000), and feed intake (Beede and Collier, 1986) are factors that influence circulating concentrations of IGF-I. Within the current study, concentrations of IGF-I measured in the spring (March-04 and May-04) were elevated in comparison with summer (Aug-04, P < 0.001) and winter (Jan-05, P < 0.001). Lower concentrations of IGF-I in Aug-04 and Jan-05 may be attributed to factors associated with thermal stress.
In the current study, mean IGF-I concentration in steers (16.70 ± 0.4 ng/mL) were greater (P < 0.001) than heifers (12.0 ± 0.4 ng/mL). Yield grade (P < 0.001) was associated with IGF-I concentrations at the point of exsanguination, but the interaction term (sex x yield grade) was not (P = 0.24). Figure 4 displays the decrease in serum IGF-I concentration as carcass cutability classification declines. Yield grade 5 carcasses did not differ (P > 0.05) from any other yield grade classification, quite possibly due to the large variation in this the smallest sample category. The relationship between sex difference and change in IGF-I secretion has been well documented. Bishop et al. (1989) documented bulls peaked in serum IGF-I at a greater concentration than heifers, whereas Govoni et al. (2003) reported greater serum concentrations in Holstein steers vs. heifers. Elevated IGF-I concentrations can most likely be attributed to increased lean tissue mass and growth rate normally experienced within male beef animals when compared with heifers. Elevated IGF-I concentrations at weaning have been linked to increased growth rates, final live weight, and G:F in beef cattle (Lund-Larson et al., 1977; Bishop et al., 1989; Govoni et al., 2003), whereas Connor et al. (2000) found that Angus bull calves with elevated IGF-I concentrations at weaning exhibited slower ADG throughout a 140-d feeding trial. Even though we cannot draw firm conclusions based on serum IGF-I concentrations and growth rate within the current study because only a single blood collection was taken at slaughter, an association to measurements of carcass composition was recognized within this study and does warrant further investigation.
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Figure 4. Main effect of USDA yield grade (YG) on serum IGF-I concentrations at the time of exsanguination. a–cBars lacking a common superscript letter differ (P < 0.05).
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Relationships Between Endocrine Concentrations
Correlations among the endocrine hormones within the current study can be found in Table 3. Leptin was correlated to IGF-I (r = –0.11; P < 0.001) and GH (r = –0.31; P < 0.001). A correlation also existed between GH and IGF-I (r = 0.16; P < 0.001). During active growth, GH acts directly on the liver to increase synthesis and release of IGF-I, which, in turn, acts as an indirect feedback regulator of GH (Bertherat et al., 1995). With low concentrations of GH, lower levels of IGF-I circulating in the blood can be expected, though greater concentrations of GH induced by stressors do not necessarily yield greater IGF-I concentrations. Negative correlations between leptin and both IGF-I and GH can be explained primarily through their release patterns. The changes in which these release patterns occur are confounded by genetics, plane of nutrition, environment, and many other factors. Within the US beef industry, animals ready for commercial slaughter have (for the most part) reached a plateau in their growth curve and continue to be fed to reach a desired subcutaneous fat depth, which, in turn, should result in a improved quality grade.
If preslaughter endocrine profiling is to be utilized as a means of segregating carcasses based on carcass quality and ultimate value, discovery of the relationships present within a random assortment of cattle was essential. This study has revealed the possibility of segregating USDA quality grades by concentrations of leptin present at the time of slaughter. However, further research is warranted to determine if measurements of leptin, GH, IGF-I, or a combination of these prior to slaughter might provide the ability to segregate cattle into slaughter groups based on live-animal estimates of quality and yield grade. Future research must expose the environmental and genetic factors responsible for the variability in endocrine response before leptin (or other circulating hormones) can be utilized with existing phenotypic evaluation methods (such as ultrasound) to segregate feedlot cattle based upon expected meat quality and carcass cutability.
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Footnotes
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1 This research was in part supported by the Missouri Agriculture Experiment Station project number 0569 and the Missouri Beef Industry Council. 
2 The authors wish to thank Chad Carr, Gregg Rentfrow, Emmy Burger, and David Kemp for assistance with data collection and Greg Davis and Steve Morgan with Tyson Fresh Meats, Emporia, KS.
3 Corresponding author: bergep{at}missouri.edu
Received for publication October 12, 2005.
Accepted for publication August 24, 2006.
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LITERATURE CITED
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Abstract
INTRODUCTION
