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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
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* Igenity Livestock Production Business Unit, Merial Ltd.;
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Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, T6G 2P5, Canada;
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Agriculture and Agri-Food Canada Research Centre, Lethbridge, Alberta, T1J 4B1 Canada; and
Alberta Agriculture, Food and Rural Development, Lacombe Research Centre, 6000 C&E Trail, Lacombe, Alberta, Canada, T4L 1W1
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: beef cattle • carcass merit • feed efficiency • performance • serum leptin
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). It functions, through interactions with the leptin receptor in the hypothalamus, as a regulator of BW, feed intake, energy expenditure (Houseknecht et al., 1998; Woods et al., 1998), reproduction (Garcia et al., 2002), and the immune system (Lord et al., 1998). Differences in circulating leptin or tissue mRNA levels have been related to BW, food intake, and body fatness in humans and in animals (Larsson et al., 1998; Delavaud et al., 2002; Berg et al., 2003).
In cattle, circulating leptin concentrations are correlated with the regional distribution of body fat (Yamada et al., 2003) and carcass merit (Minton et al., 1998; Geary et al., 2003). Mutations in the leptin gene or its promoter are associated with differences in serum leptin concentrations and other economically relevant traits in beef and dairy cattle (Liefers et al., 2003; Nkrumah et al., 2005). Despite the number of studies on the physiological roles of leptin, little is known about the genetic and phenotypic relationships of endocrine leptin with the performance or efficiency of gain of farm animals (Berg et al., 2003; Richardson et al., 2004).
In addition, though studies have been carried out to determine the relationship of circulating leptin with a number of traits in cattle (Ehrhardt et al., 2000; Delavaud et al., 2002; Garcia et al., 2002), most previous studies involved relatively fewer animals or animals under fasting or underfeeding treatments, or after leptin administration. No genetic parameter estimates have been reported on serum leptin or its relationships with economically relevant traits in farm animals. This study evaluated the genetic and phenotypic relationships of serum leptin concentration with performance, efficiency of gain, and measures of carcass merit in beef cattle.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The animals used in the study were cared for according to the guidelines of the Canadian Council on Animal Care (CCAC, 1993).
Growth, feed intake, ultrasound, and carcass data were collected over 3 yr on beef steers sired by Angus, Charolais, or University of Alberta Hybrid bulls between 2002 and 2005. Dams were produced from crosses among 3 composite cattle lines, namely Beef Synthetic 1, Beef Synthetic 2, and Dairy x Beef Synthetic (Goonewardene et al., 2003). Cows and heifers for the study were bred in multiple-sire breeding groups on pasture, and the sire of each calf was later determined in a parentage test using a panel of bovine microsatellite markers. The steers were managed and tested under feedlot conditions using the GrowSafe automated feeding system (GrowSafe Systems Ltd., Airdrie, Alberta, Canada; Basarab et al., 2003) at the University of Alberta’s Kinsella Research Station. Details of the procedures for the feedlot tests were given by Nkrumah et al. (2004). The steers weighed 353.0 (SD = 61.3) kg and were 252 (SD = 42) d of age at the beginning of the tests. Two tests made up of approximately 80 steers per test were conducted each year for 3 yr.
In yr 1, steers were fed free choice a backgrounding diet of mainly alfalfa-brome hay with oats and supplemented with corn grain and feedlot mineral supplement to promote a growth rate of just under 1.0 kg/d for approximately 30 d. This period was followed by a 30-d pretest adjustment period, in which the amount of corn in the backgrounding diet was gradually increased to introduce the steers to the test diet and the feeding system. The test diet in yr 1 (as-fed basis) was composed of 80.0% dry-rolled corn, 13.5% alfalfa hay pellet, 5% feedlot supplement (32% CP beef mineral supplement), and 1.5% canola oil, supplying approximately 2.90 Mcal/kg of ME and 12.5% CP, on a DM basis. In yr 2 and 3, the same test procedures were used, but the test diet (as-fed basis) contained 64.5% barley grain, 20% oat grain, 9.0% alfalfa hay pellet, 5.0% beef feedlot supplement, and 1.5% canola oil, supplying 14.0% CP and 2.91 Mcal/kg of ME, on a DM basis. Corn was used in yr 1, instead of barley and oat, because of a feed barley shortage in that particular year.
Blood Collection and Leptin Assay
At the end of the efficiency of gain tests (1 wk before slaughter), blood samples were collected from each animal by jugular venipuncture into evacuated tubes. Steers were bled in the morning, and all steers were allowed unrestricted access to feed and water before bleeding. Blood samples were allowed to clot for approximately 18 h at 4°C. Samples were centrifuged at 2,500 x g for 30 min, and serum was collected and stored at –20°C until assayed for leptin using the commercially available leptin RIA kit (Multispecies Leptin Assay, Linco Research, St. Louis, MO). Intra- and interassay CV for the leptin assays were less than 5%. Steers in the study were grouped into high, medium, and low serum leptin concentration groups based on SD above and below the mean leptin concentration. Thus, steers with high serum leptin were >0.5 SD above the mean, medium serum leptin were within ± 0.5 SD of the mean, and low serum leptin were <0.5 SD below the mean.
Traits Analyzed and Their Derivations
Procedures for obtaining the measures of feedlot performance and efficiency of gain have been described previously (Nkrumah et al., 2004). Linear regression using PROC REG (SAS Inst. Inc., Cary, NC) of weekly or fortnightly BW measurements against time, in days, was used to derive ADG, final BW, and midtest metabolic BW (MWT, BW0.75) for each animal. The total feed intake of each animal over a 70-d test period was used to compute the daily DMI. Feed conversion ratio (FCR) was computed as the ratio of daily DMI to ADG on test. The partial efficiency of growth (PEG; i.e., energetic efficiency for ADG above maintenance) of each animal was computed as the ratio of ADG to the difference between average daily DMI and expected DMI for maintenance (Arthur et al., 2001), where expected DMI for maintenance was computed according to the NRC (1996).
Residual feed intake (RFI) was calculated from the phenotypic regression of ADG and MWT on DMI according to Arthur et al. (2001). In each case, individual RFI was computed as actual daily DMI minus the expected daily DMI predicted from the appropriate model. Ultrasound backfat thickness, ultrasound LM area (ULMA), and ultrasound marbling score (UMAR) were predicted from linear regression against time of measurements obtained every 28 d with an Aloka 500V real-time ultrasound with a 17-cm, 3.5-MHz linear array transducer (Overseas Monitor Corporation Ltd., Richmond, British Columbia, Canada).
At the end of the efficiency of gain tests, steers were weighed and shipped to a commercial packing plant 200 km away, where they were slaughtered the following day and standard industry carcass data collected after a 24-h chill at –4°C. Carcass traits were evaluated according to the Canadian beef carcass grading system (Agriculture Canada, 1992). Carcass weight of each animal was determined as the combined weights of the left and right halves of the carcass. The carcass 12th-rib fat (RF) was measured at the 12 to 13th rib of each carcass. Carcass marbling score is a measure of intramuscular fat and can be classified as: 1 to <2 units = trace marbling (Canada A quality grade); 2 to <3 units = slight marbling (Canada AA quality grade); 3 to <4 units = small to moderate marbling (Canada AAA quality grade), and 
4 units = slightly abundant or more marbling (Canada Prime). Lean meat yield (LMY) is an estimate of saleable meat and was calculated according to the equation: LMY, % = 63.65 + (1.05 x muscle score) – (0.76 x carcass 12th-rib fat). Yield grade (YG) is the proportion of lean meat and is classified as Y1 59 %, Y2 = 54 to 58%, and Y3 < 54%.
Statistical Analyses
Performance, efficiency of gain, and ultrasound data from 464 steers and carcass data from 381 steers were analyzed. Genetic (co)variances were obtained with the statistical software ASREML using an animal model (Gilmour et al., 2000). The PROC MIXED of SAS was used to test for fixed effects and to estimate least squares means. Effects of breed or serum leptin group on leptin concentration, growth, feed intake, efficiency of gain, and carcass merit were analyzed by least squares procedures using a statistical model in SAS that included fixed effects due to leptin group (high, medium, and low), breed (Angus, Charolais, and Hybrid), year of test (3 levels), test group nested within year (2 levels per year), all possible interactions, and linear and quadratic effects of age when P < 0.05. All interaction terms that did not account for a significant portion of the observed variance (P > 0.10) were subsequently excluded from the final model. The random effects in the models were sire and dam and the residual effects of animal. The error term for estimating breed of sire effects was sire within breed. The PROC CORR of SAS was used to obtain Pearson, partial phenotypic correlations adjusted for the effects of age and the fixed effect of test group.
A preliminary univariate analysis for each trait was carried out to obtain beginning genetic and phenotypic co(variance) parameters in ASREML that were then fitted in subsequent REML, 2-trait analyses. Pairwise, 2-trait analyses were performed between serum leptin concentration and each test trait. The 2-trait, individual animal models used to estimate (co)variance components included fixed contemporary group effects, random additive genetic and residual effects, and age as a covariate. Genetic variances and heritability estimates for any particular trait were calculated as the average value of the estimates from all pairwise, 2-trait analyses performed against all traits, and their SE were the medians of the SE estimates.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. The heritability estimate (±SE) for serum leptin concentration in the study was moderate (0.34 ± 0.13). Serum leptin concentration averaged 13.91 (SD = 5.74) ng/mL and ranged from 2.19 to 39.70 ng/mL. This gave a desirable range for relating serum leptin concentration to the different performance, efficiency of gain, and carcass traits studied. In Table 2, differences in serum leptin concentration and measures of growth, BW, efficiency of gain, and carcass merit among steers of different sire breeds are presented. Serum leptin concentration was 20 and 19% less (P < 0.001) for steers sired by Charolais- compared with Angus- and Hybrid-sired steers, respectively. Phenotypic RFI was 0.31 and 0.34 kg less (P = 0.03) for Charolais- compared with Angus- and Hybrid-sired steers (P = 0.03).
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The differences among steers with low, medium, and high serum leptin concentration in feed intake and measures of efficiency of gain are presented in Table 3. Daily DMI (P < 0.01) and RFI (P < 0.05) were greater for steers with high serum leptin concentration than for those with low serum leptin concentration. Consistently, PEG was less (P = 0.01) in high serum leptin than in low serum leptin steers. No differences in FCR were observed among the steers differing in serum lep-tin concentration. Daily DMI had a positive phenotypic correlation (r = 0.15; P < 0.01) with serum leptin concentration, but the corresponding genetic correlation was negative. The phenotypic correlations of serum leptin concentration with FCR, RFI, and PEG were not different from zero (P > 0.10; data not shown). Genetically, serum leptin concentration was negatively correlated with FCR (r = –0.44 ± 0.24), and RFI (r = –0.24 ± 0.38), but positively correlated with PEG (r = 0.34 ± 0.24). However, the large SE associated with these genetic correlation estimates make it difficult to determine whether these point estimates were indeed different from zero.
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; the phenotypic and genetic correlations among these traits are presented in Table 5. Steers with high serum leptin concentration had greater (P < 0.05) MWT, final BW, and slaughter BW compared with those with low leptin concentrations. Metabolic BW (P < 0.05) and slaughter BW (P < 0.10) had positive phenotypic correlations but negative genetic correlations with serum leptin concentration. Serum leptin concentration was not correlated genetically and phenotypically with ADG, and there were no differences (P > 0.10) in ADG among steers with different leptin concentrations. Ultrasound backfat had a moderate positive (P < 0.001) phenotypic correlation and strong positive genetic correlation with serum leptin concentration. Similarly, UMAR had positive phenotypic (P < 0.01) as well as genetic correlations with serum leptin concentration.
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The relationships of serum leptin concentration with carcass traits are presented in Tables 6 and 7. Steers with high serum leptin concentration had greater RF (P < 0.001), marbling (P < 0.01), YG (P < 0.001), and quality grade (P < 0.001), compared with steers with low serum leptin concentration. The RF, marbling score, and YG had moderate phenotypic correlations but moderate to strong genetic correlations with serum leptin concentration. However, serum leptin concentration was negatively correlated phenotypically (P < 0.001) and genetically with carcass LMY. Steers with high and medium serum leptin concentrations had lower LMY compared with those with low serum leptin concentration. In addition, carcass LM area had a weak but significant (P < 0.05) negative phenotypic correlation with serum leptin concentration; the corresponding genetic correlation was very strong and negative as well.
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) by Geary et al. (2003) for composite steers and heifers and also by Richardson et al. (2004) for Angus steers. The current study demonstrated considerable breed differences in serum leptin concentration associated with differences in ultrasound and RF, marbling score, YG, and LMY. Berg et al. (2003) observed that breed differences in serum leptin concentration in pigs were related to growth and carcass characteristics. Ren et al. (2002) reported that plasma leptin tended to be greater, and body fat content, omental and perirenal fat mass, as well as leptin mRNA expression in subcutaneous and perirenal fat depots, were significantly greater in German Holsteins compared with Charolais. Their results also showed that BW and LM area were greater in Charolais compared with Holsteins. Thomas et al. (2002) reported that serum leptin concentrations were greater in Angus bulls compared with their Brahman counterparts of similar ages. Similarly, Yonekura et al. (2002) showed that mean serum leptin concentration was greater in Japanese Black cattle compared with their Holstein counterparts. Breed differences in circulating leptin concentrations seem to correlate well with the differential abilities of the breeds to accrete body fat or lean.
Wegner et al. (2001) reported differences in plasma leptin concentration among crossbred cattle varying in percentage Wagyu breed composition. However, a recent study by Geary et al. (2003) showed no significant differences in serum leptin concentration among different composite breeds of cattle, despite considerable differences observed among the breeds in measures of body fatness. The breed differences in serum leptin concentration observed in the current study, as well as in most of the studies cited above, were considerably lower in magnitude compared with the differences in ultrasound and carcass merit observed between the same steers. It appears that only part of the breed differences in body composition are related to serum leptin concentration, and other factors may be involved as well. It has been suggested that genetically leaner cattle may be expressing leptin from sites other than adipocytes or their level of expression per gram of adipose tissue may be greater (Geary et al., 2003).
The current study is the first of its kind reporting genetic parameter estimates on the relationships of serum leptin with performance, efficiency of gain, and carcass quality. The phenotypic relationships between serum leptin concentration and ultrasound and carcass measures of body fatness obtained in the current study were generally in agreement with Minton et al. (1998) and Geary et al. (2003). Serum leptin concentration also showed significant positive phenotypic correlations with measures of body fatness in humans (Larsson et al., 1998). These authors reported percent body fat content to be 35.6 ± 3.6 vs. 27.4 ± 2.9 in humans with high vs. low serum leptin, respectively. It is therefore surprising that a number of studies (Kawakita et al., 2001, Yonekura et al., 2002) involving Japanese Black cattle failed to demonstrate a relationship between body fatness and plasma leptin concentration. Kawakita et al. (2001) attributed the differences in findings using other cattle compared with Japanese Black cattle to the greater ability of the latter breed to deposit intramuscular fat (Zembayashi et al., 1995).
Previous Wegner et al. (2001) reported that correlations of plasma leptin concentration with LM lipid content for cattle with 0, 50, and 75% Wagyu breed genetics were r = 0.62, r = 0.11, and r = –0.60, respectively. It appears that the superior ability of Japanese Black cattle to deposit intramuscular fat somehow alters the relationship of circulating leptin with body fatness, possibly because the relationship of circulating leptin is stronger with subcutaneous fat level than with intramuscular fat level.
The genetic parameter estimates for the growth, ultrasound and carcass traits presented in the study were moderate to high and were generally comparable to previously reported values (Bertrand et al., 2001; Devitt and Wilton, 2001; Crews et al., 2003). The phenotypic relationships of serum leptin concentration with ultrasound and carcass LM area observed in the current study are conflicting with the results of Wegner et al. (2001), who observed that plasma leptin concentration was unrelated to LM area in crossbred Wagyu cattle. However, Berg et al. (2003) observed a negative correlation (r = –0.33) between LM area and serum leptin in pigs. Geary et al. (2003) showed no significant phenotypic relationships of LM area with serum leptin concentration in 1 study consisting of a group of crossbred composite steers composed predominantly of Angus cattle, but showed a negative relationship (r = –0.45) between LM area and serum leptin in a separate group consisting of steers and heifer progeny of the composite breed used in the first study. Additionally, Minton et al. (1998) showed a positive relationship (r = 0.32) between LM area and serum leptin concentration. These contrasting correlations between LM area and serum leptin seem to be highly reflective of the published correlations of LM area and marbling score or subcutaneous fat thickness in cattle (Bertrand et al., 2001; Devitt and Wilton, 2001; Crews et al., 2003).
The current study showed positive or no phenotypic relationships between serum leptin concentration and daily feed intake, BW, and RFI of beef cattle, but not with ADG; the corresponding genetic correlations were negative. The generally low to moderate genetic correlations observed between serum leptin and measures of BW, feed intake, and efficiency of gain, despite the moderate to strong genetic correlations, may imply correspondingly strong positive environmental correlations of serum leptin with these traits. The phenotypic relationships of serum leptin with DMI and ADG in the current study are similar to the findings of Richardson et al. (2004). These authors observed a significant phenotypic correlation (r = 0.31) between serum leptin concentration and RFI in Angus cattle but no correlations with ADG. Richardson et al. (2004) also observed positive correlations of serum leptin with DMI and FCR, though the latter correlations were not statistically significant, possibly due to the limited number of steers used in the study.
A recent study by Brown et al. (2004) in growing Braunvieh-sired crossbred steers (n = 169) and Bonsmara bulls (n = 62) did not show any phenotypic correlations between serum leptin concentration with growth, feed intake, or RFI. The positive relationships of serum leptin with BW observed in the current study are consistent with evidence in pigs (Berg et al., 2003) and cattle (Liefers et al., 2002; León et al., 2004). Other evidence in cattle (Garcia et al., 2002) shows that BW is highly correlated (r = 0.85) to serum leptin concentration. A study in humans (Larsson et al., 1998) showed that subjects with high serum leptin (38.2 ± 8.0 ng/mL) compared with low serum leptin (7.0 ± 1.7) had greater BW (74.9 ± 8.4 vs. 60.4 ± 5.4 kg), greater body mass index (27.9 ± 2.7 vs. 21.9 ± 2.0 kg m–2), but significantly reduced habitual energy intake (1,838 ± 424 vs. 2,311 ± 669 kcal/d). The negative genetic correlation between DMI and serum leptin in this study are therefore consistent with the above findings in humans.
The results of this study imply that the relationship between serum leptin concentration and body fatness may be stronger than the relationship of serum leptin with feed intake. Delavaud et al. (2000) also observed that the relationship between circulating leptin concentrations and the day-to-day variation in nutritional status in cattle may be very low when compared with the relationships of circulating leptin with the long-term effect on adipose tissue mass. The molecular mechanisms regulating leptin production remain to be fully understood. However, the stronger relationship between serum leptin and body fatness may partly be due to the fact that several transcription factors that are essential to the differentiation of adipocytes also positively regulate the leptin gene promoter (Miller et al., 1996; Mason et al., 1998). Indeed, polymorphisms in the bovine leptin promoter have been shown to have strong associations with serum leptin concentration as well as body fatness (Liefers et al., 2003; Nkrumah et al., 2005).
The weak relationship between serum leptin and feed intake may also be due to the fact that the role of leptin in intake regulation is much more central than peripheral, and the saturable transport of leptin into the brain across the blood-brain barrier may be a rate-limiting step with respect to the central role of leptin (Caro et al., 1996). Indeed, doses of leptin that have no impact on food intake regulation when administered peripherally have been shown to considerably reduce food intake when centrally administered (Friedman, 1998).
The results of the current study indicated a stronger phenotypic and genetic relationship between serum leptin concentration and measures of ultrasound and carcass merit compared with correlations with performance and efficiency of gain. Serum leptin concentration can easily be incorporated into appropriate selection programs to favorably improve the carcass merit of beef cattle. Further studies using relatively larger numbers of animals are required to clearly define the phenotypic and especially the genetic associations between endocrine leptin and measures of feed intake, BW, growth, efficiency of gain, and carcass merit of cattle. It may also be interesting to determine the genetic and phenotypic relationships between serum leptin and other hormones involved in body energy homeostasis, and how these changes relate to changes in body composition during the finishing phase of the feedlot period.
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Footnotes |
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2 Corresponding author: stephen.moore{at}ualberta.ca
Received for publication November 20, 2006. Accepted for publication April 26, 2007.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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ates human ob gene expression. Proc. Natl. Acad. Sci. USA 93:5507–5511.This article has been cited by other articles:
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