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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
* Equipe Systèmes de Production;
and
Croissance et Métabolisme du Muscle; and
Nutriments et Métabolismes, INRA UR1213 Herbivores, Site de Theix, F-63122 Saint-Genès-Champanelle, France
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: beef • breed • cattle • maturity • meta-analysis • muscle • sex
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Variation in beef quality between and within animals is partly attributed to genetic and environmental production factors that affect the muscle characteristics. To manipulate beef quality for economic advantage it is necessary to understand how factors such as muscle type, breed, and sex influence the muscle characteristics in the growing animal.
Experiments have identified that muscle characteristics such as contractile fiber cross-sectional area, metabolic enzyme activity, collagen content and solubility, and lipid content change as cattle mature (Jurie et al., 1995a; Wegner et al., 2000). Differences in muscle characteristics also occur between muscle types (Jurie et al., 1995b; Von Seggern et al., 2005) and sexes (Picard et al., 1995). Few differences in muscle characteristics are believed to exist between cattle breeds raised under similar production circumstances (Jurie et al., 2005).
Experiments to investigate muscle characteristics in growing animals have considered only a limited number of factors and normally use small numbers of animals. A meta-analysis summarizes many experiments and considers many factor levels, increasing the likelihood that inferences reflect the population. McPhee et al. (2006) used a meta-analytic approach to assess factors affecting carcass characteristics in steers. A similar approach would be valuable for studying muscle characteristics.
The objective of this study was to perform a meta-analysis to 1) establish the effect of degree of maturity with different muscle types, breeds, or sexes on 11 muscle characteristics in French cattle and 2) rank the factors for the extent they influence changes in muscle characteristics with maturity. This will aid the construction of a dynamic model of muscle development.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Study Sample
During a period from 1986 to 2006, several experiments were conducted by personnel of the Herbivore Research Unit of the Institut National de la Recherche Agronomique (INRA) in France to investigate muscle characteristics in French cattle. A database was created to compile the data from these experiments and to allow the relationship between degree of maturity and muscle characteristics to be explored in a meta-analysis. Access to the original results from the experiments was available so the database contained measurements of muscle characteristics from individual muscle samples of individual animals. As such, this meta-analysis used the measurements from individual animal muscles of each experiment rather than treatment means, which are commonly used in meta-analytic studies when only published results are available.
Database Collection and Coding for Meta-Analysis
For the meta-analysis, the quantitative and qualitative details extracted from the experimental results included the muscle from which the sample was obtained; the sex, breed, and identity number of the animal from which the muscle sample was obtained; the BW of the animal when muscle samples were obtained; name of the experiment in which the sampled animal was included; and measurements for the muscle characteristics. Codes were used to distinguish the data according the animal’s breed, sex, and muscle type as needed. Unique codes were given to each experiment.
Muscle Characteristics and Factors Considered in Meta-Analysis
Eleven muscle characteristics that were considered the most likely to explain differences in beef quality were evaluated in the meta-analysis (Table 1). These characteristics, which were the dependent variables in the meta-analyses, were the mean muscle fiber cross-sectional area (CSA; µm2), percentage of slow oxidative fibers (%SO), fast oxidative-glycolytic fibers (%FOG), and fast glycolytic fibers (%FG), activity of isocitrate dehydrogenase (ICDH; µmol·min–1·g–1), and lactate dehydrogenase (LDH; µmol·min–1·g–1) in fresh muscle, concentration of insoluble (CINS; µg of OH-proline·mg–1) and total (TCOL; µg of OH-proline·mg–1) collagen in dry muscle, and concentration of phospholipid (PL; mg·g–1), triglyceride (TG; mg·g–1), and total lipid (TLIP; mg·g–1) in fresh muscle
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). The mature BW used to calculate the DoM for different breeds and sexes are given in Table 2. A DoM >1 is possible when a sampled animal has a BW greater than the standard mature BW for its breed and sex (as defined in Table 2). Degree of maturity was treated as a continuous, quantitative variable to allow analysis of the range of DoM values in the database.
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Sexes and Breeds
The database (Table 1
) and the meta-analysis considered 3 sexes and 5 breeds. The sexes were categorized as bulls, cows, and steers. The breeds included in the database were Aubrac, Charolais, Limousin, Montbéliard, and Salers. Two experiments used steers from crossbreeding of Charolais and Salers cattle.
Data Investigation and Statistical Analyses
Preliminary Data Investigation
A test for homogeneity of experiment means was performed. For all muscle characteristics, there was a difference (P < 0.001) between the means in each experiment. This identified the need for a random-effects model with the experiment as a random variable to account for between-experiment variation. The Levene’s test was used to assess the variance between fixed-effect groups. When the test indicated unequal variance, it was accommodated for by incorporating the Satterthwaite’s approximation into the statistical analyses (Spilke et al., 2005). Outlying values were identified from a PROC UNIVARIATE analysis of the data. Graphical investigation indicated that these outlying values were not measurement errors and it was decided to leave them in the analyses, as they were possibly representative of the population. Data investigation also identified a difference in the DoM between bulls and cows. Most bulls were at a DoM <1, whereas most cows were at a DoM >0.85. This is a characteristic of French beef production systems in which cows are slaughtered for beef production at a much later age than bulls but usually before 10 yr of age to retain carcass value. Consequently, when analyzing the effect of muscle type and breed, the analyses were carried out on the data of bulls and cows separately and the effect of sex considered bulls versus steers (effect of castration)
Statistical Analysis of Muscle Characteristics with Degree of Maturity in Different Muscles, Breeds, and Sexes. Three sets of analyses were carried out. The first set compared the effect of each muscle (LT, ST, and TB) on the muscle characteristics. The analysis was carried out on data from bulls and then repeated with data from cows. A further 2 sets of analyses used data from the ST muscle only to assess the effect of breed (in bulls and cows separately) and castration (steers vs. bulls).
Muscle characteristics were analyzed using mixed models (PROC MIXED, SAS Inst. Inc., Cary, NC) using REML for estimating the variance components. In this manner, the fixed-effect terms of the statistical model were muscle type (set 1), breed (set 2), or castration (set 3) with DoM as the covariate. The initial model tested for cubic, quadratic, and linear effects with DoM and their possible interaction with muscle type (set 1), breed (set 2), or castration (set 3). The higher order terms and interactions that were not significant were sequentially removed from the model and analysis repeated. Analyses incorporated the experiment from which the data originated as the random effect. The statistical model initially included the random intercept, linear, quadratic, and cubic effects as appropriate for the model utilized and a possible covariance between these (option UN). The covariance parameter was regarded as significantly different from zero at a probability of P < 0.07, which is more lenient than the usual 0.05 level because accurate estimations of variance and covariance require a substantial number of observations (St-Pierre, 2001). If the covariance parameter was not correlated, the UN option was removed and analyses repeated. Nonsignificant random effects were removed sequentially from the statistical model and analyses repeated as required. To account for the multidimensional space that is created when results are from a mixed model regression, the observations in the figures have been adjusted (Y values on the regression line plus residuals) as recommended by St-Pierre (2001).
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The final database contained measurements of muscle characteristics from 2,642 individual muscle samples of 33 experiments. More measurements were available for fiber and enzyme characteristics than for collagen and lipid characteristics (Table 1). The ST muscle accounted for 1,134 samples, LT for 1,026 samples, and TB for 482 samples. The number of samples from bulls, steers, and cows were 1,437, 726, and 479, respectively. The Charolais breed contributed 987 samples; Limousin, 850; Montbéliard, 326; Salers, 323; Aubrac, 126; and Charolais x Salers, 30.
Effect of Degree of Maturity on Muscle Characteristics
Cross-Sectional Fiber Area.
The CSA increased quadratically with DoM in bulls (P < 0.001; Figure 1) and linear changes were observed in cows (Figure 1). Mean CSA was greater in ST muscle than in LT and TB muscles in bulls (P < 0.05; Table 3) and cows (P < 0.05; Table 4). The muscles of the bulls had different linear and quadratic coefficients when regressing CSA against DoM (P < 0.001; Table 3). The greatest increase in CSA was seen in the ST muscle of bulls (Figure 1). The linear coefficients indicate that the CSA continued to increase in the LT and TB muscle but decreased slightly in the ST muscle of cows (Table 4; Figure 1). Mean CSA was greater in Charolais compared with Aubrac and Montbéliard bulls (P < 0.05; Table 5) and greater in Limousin cows compared with other cow breeds (P < 0.05; Table 6). The bull breeds differed in their linear coefficients (P < 0.01) but not quadratic coefficients (Table 5). The linear coefficient was greatest with Aubrac and least in Montbéliard bulls. Compared with steers, the bulls had a greater linear coefficient (P < 0.01) and a greater mean CSA (P < 0.05; Table 7).
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and 4). For the relationship of %SO with DoM, the linear and quadratic coefficients were different between the muscle types of bulls (P < 0.05; Table 3), and the linear coefficient was different between muscle types of cows (P < 0.05; Table 4). Mean %SO was greater in Montbéliard bulls and Aubrac and Salers cows compared with the other breeds (P < 0.05; Tables 5 and 6). There was a linear relationship of %SO with DoM when considering just ST muscle, and linear coefficients were different between bull breeds (P < 0.05; Table 5). The %SO did not differ between steers and bulls (Table 7).
The ST and TB muscles had a greater mean %FOG compared with the LT muscle in both bulls and cows (P < 0.05; Tables 3 and 4). For the relationship of %FOG with DoM in bulls, there was a difference between the muscles for the linear (P < 0.001), quadratic (P < 0.05), and cubic (P < 0.05) coefficients (Table 3). Mean %FOG was greatest in Aubrac and Salers bulls and least in Montbéliard bulls (P < 0.05; Table 5). In cows, the Limousin had a decreased mean %FOG compared with the other cow breeds (P < 0.05; Table 6). When comparing breeds using just the ST muscle, the %FOG gave a quadratic relationship with DoM. The linear coefficient for this relationship was negative and different between the bull breeds (P < 0.01; Table 6). Mean %FOG was not different between bulls and steers however, regressing %FOG against DoM gave different linear (P < 0.001) and quadratic (P < 0.01) coefficients for bulls and steers (Table 7).
The ST muscle had a greater mean %FG compared with the LT and TB in bulls and cows (P < 0.05; Tables 3 and 4). When regressing %FG with DoM, the linear coefficient differed between muscle types of bulls (P < 0.001; Table 3). There was no difference in the mean %FG between bull breeds but Limousin cows had a greater mean %FG compared with the other cow breeds (P < 0.05; Table 6). When comparing bull breeds using just the ST muscle, the %FG gave a quadratic relationship with DoM. The linear coefficient for this relationship was different between the bull breeds (P < 0.001; Table 5). The mean %FG was greater in bulls than steers. The quadratic relationship of FOG% in the ST muscle with DoM gave different linear (P < 0.001) and quadratic (P < 0.001) coefficients for bulls and steers (Table 7).
Activities of Isocitrate Dehydrogenase and Lactate Dehydrogenase
In bulls, ICDH activity showed a quadratic (Figure 3) and LDH activity a cubic (Figure 4) relationship with DoM but there was no change for these enzyme activities with increasing DoM in cows.
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and 4). For the relationship of ICDH activity with DoM in bulls, there was a difference between the muscles for the linear and quadratic coefficients (P < 0.001; Table 3). Mean ICDH activity was greater in Montbéliard bulls compared with Limousin bulls (P < 0.05; Table 5) with other bull breeds intermediate. Charolais cows had less mean ICDH activity compared with the other cow breeds (P < 0.05; Table 6). The linear and quadratic coefficients for the relationship of ICDH activity with DoM were different between the bull breeds (P < 0.001; Table 5) but the same between bulls and steers. Mean ICDH activity was greater in bulls compared with steers (P < 0.05; Table 7).
The ST muscle had a greater mean LDH activity compared with the LT and TB muscles in both bulls and cows (P < 0.05; Tables 3 and 4). There was a difference between the muscle types in bulls for the linear coefficient of the relationship of LDH activity with DoM (P < 0.05; Table 3). Mean LDH activity was not different between the bull breeds. In the cows, mean LDH activity was greatest in the muscle of Limousin and least in the muscle of Charolais and Salers (P < 0.05; Table 6). When regressing LDH activity with DoM, the linear (P < 0.001) and quadratic (P < 0.01) coefficients were different between the bull breeds (Table 5). Bulls had less mean LDH activity than steers (P < 0.05; Table 7) but linear, quadratic, and cubic coefficients for the relationship with DoM were the same between sexes.
Insoluble and Total Collagen Concentration
The CINS and TCOL concentration in the muscle of both bulls and cows showed linear changes with increasing DoM (Tables 3 and 4). In both bulls and cows, the mean CINS concentration in dry muscle was greatest in the ST muscle and least in the LT muscle (P < 0.05; Tables 3 and 4). In the bulls, the linear coefficient was different between the muscles (P < 0.001; Table 3). The linear coefficient was positive for LT muscle, negative for ST muscle, and close to zero for TB muscle. Mean CINS concentration was greater in the muscle of Salers bulls and greater in Salers and Charolais cows compared with other breeds (Tables 5 and 6). The linear coefficient when regressing CINS concentration against DoM differed for different bull (P < 0.001) and cow breeds (P < 0.01). The Charolais bulls and Aubrac cows had a positive linear coefficient but it was negative for other bull and cow breeds (Table 5 and 6). Mean CINS concentration and the relationship of CINS concentration with DoM did not differ between bulls and steers (Table 7).
Mean TCOL concentration was greatest in ST muscle and least in LT muscle in both bulls and cows (P < 0.05; Tables 3 and 4). For the relationship of TCOL concentration in the muscle with DoM in bulls, the linear coefficient was negative and the value differed for each muscle type (P < 0.05; Table 3). In cows, the linear coefficient was positive for LT muscle and negative for ST and TB muscles (P < 0.05; Table 4). Mean TCOL concentration was greater in Charolais and Salers bulls and cows compared with Aubrac and Limousin (P < 0.05; Tables 5 and 6). When TCOL concentration was regressed with DoM the linear coefficient differed between the bull (P < 0.01; Table 5) and cow breeds (P < 0.05; Table 6). The linear coefficient was negative for all except the Aubrac cows. Mean TCOL and linear coefficients for the relationship of TCOL concentration with DoM did not differ between bulls and steers.
Phospholipid, Triglyceride, and Total Lipid Concentration
Concentrations of PL, TG, and TLIP in the muscle developed linearly with DoM in bulls (Table 3; Figure 5) but did not change in cows (Table 4; Figure 5).
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and 4). For the relationship of PL concentration with DoM in bulls, the linear coefficient did not differ between the muscles or breeds. In bulls the linear coefficient for the relationship of PL with DoM was negative but in steers it was positive (P < 0.01; Table 7).
Mean TG concentration was greatest in the LT muscle and least in ST muscle for both bulls and cows (P < 0.05; Tables 3 and 4). The linear coefficients were different between the muscles for the relationship of TG with DoM in bulls (P < 0.01; Table 3). The linear coefficient was negative in the ST muscle and greater in the LT muscle than the TB muscle. Mean TG concentration and linear coefficients for the relationship with DoM did not differ between bull breeds. In cows, Aubrac and Charolais breeds had a greater mean TG concentration compared with Limousin (P < 0.05; Table 6). Mean TG was greater in steers than in bulls. The linear coefficient for the relationship of TG with DoM for steers was large and positive compared with the negative coefficient with bulls (P < 0.01; Table 7).
Mean TLIP concentration was greatest in LT muscle and least in ST muscle in both bulls and cows (P < 0.05; Tables 3 and 4). Regression of TLIP concentration with DoM for bulls gave positive linear coefficients for all muscles, indicating increasing TLIP concentration with DoM. The increase was greatest for LT and lowest for ST muscle (P < 0.05; Figure 5). Mean TLIP concentration and its development with DoM did not differ between bull breeds. Mean TLIP was greater for the Aubrac and Charolais cows compared with Limousin cows (P < 0.05; Table 6). Steers had a greater mean TLIP compared with bulls. There was a greater increase in muscle TLIP concentration in steers than bulls (P < 0.01; Table 7).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The objective of this study was to quantitatively review the development of 11 muscle characteristics with increasing animal maturity in different muscle types, breeds, and sexes of French cattle. Degree of maturity was used so that the muscle characteristics for the different animal types could be compared at a similar physiological state. The use of original measurements, as in this meta-analysis, is considered advantageous because the data are not limited to that of published means. This removes the possibility of publication bias, which occurs when only selective results from a study are published or when subjective judgments are made when selecting publications for a meta-analysis (Finney, 1995).
By defining the experiment from which data were obtained as a random effect, the regression analysis adjusts for the within- and between-experiment variation allowing the true variation due to the effects of animal maturity, muscle type, breed, and sex to be considered across all experiments. Therefore, a broader inference space is utilized as the focus moves from the experimental level to a larger set of levels constituting the population. This increases the likelihood that inferences reflect the population and future observations (St-Pierre, 2001). The ability to detect the true effects of the factors (maturity, muscle, breed, and sex) is aided further by the measures from individual samples being determined in the same laboratory using the same procedures. Thus, no variation is added from different laboratory techniques. More data were available for the fiber and enzyme characteristics than for the collagen and lipid characteristics. This implies that statistical power is greater for inference with the fiber and enzyme results compared with the collagen and lipid results.
Development of Muscle Characteristics in Different Muscles of Cattle
The greatest changes in the muscle characteristics with increasing DoM were seen in the immature bulls. This indicates that the majority of change in muscle characteristics occurs between birth and maturity. The lack of change in the muscle characteristics of cows reflects the steady state in muscle composition that is reached in mature cattle. Jurie et al. (2006), using mature cows aged 4 to 9 yr, also found no changes in muscle characteristics with age. Thus, at a DoM >0.8, there appears to be less change in muscle characteristics and a plateau is reached. These changes in muscle characteristics correspond to the growth pattern in beef cattle where BW changes most rapidly in young animals with growth rate slowing toward maturity. Interactions of the muscle type, breed, or castration with DoM, DoM2, or DoM3 indicate that the development of muscle characteristics differs between the various muscles, breeds, or sexes being considered. The regression analysis can be used to evaluate how the muscle characteristics are changing with DoM. A linear relationship indicates a constant increase or decrease with increasing DoM. A quadratic or cubic relationship indicates that at greater DoM, the increase or decrease of muscle characteristics is not proportional to that at less DoM.
Development of muscle characteristics with DoM is unique for each muscle, and mature cattle have established noticeable differences in their characteristics between muscles. Typically, postnatal growth is associated with an increased muscle fiber size because the number of muscle fibers is fixed at birth (Wegner et al., 2000). Hypertrophy of the muscle fibers explains the increase in CSA with increasing DoM in young bulls. The small increase in CSA in the cows represents reduced protein deposition in the muscles of older animals reaching their adult BW (Owens et al., 1993). In accordance with the means in this study, previous research has indicated that in bulls, the LT and TB muscles have a smaller CSA than ST muscle (Jurie et al., 2005). The CSA in LT and ST muscles increases at less DoM but then declines at greater DoM as indicated by the positive linear coefficients and negative quadratic coefficients. The greater linear coefficient for the ST indicates that the increase in CSA is greater for ST muscle compared with LT. The TB muscle is opposite in its CSA development with a decline in CSA at early DoM followed by an increase at later DoM. This confirms the muscle-specific allometry observed by Brandstetter et al. (1998).
Jurie et al. (2006) showed that the ST muscle of cows was more glycolytic and the TB muscle was more oxidative, which is consistent with the bull and cow results in this meta-analysis. The literature indicates that glycolytic fiber percentage and glycolytic enzyme activity increase up to approximately 12 to 16 mo of age (Jurie et al., 1995a) after which they decline with a change toward an increasingly oxidative fiber composition and activity (Jurie et al., 2005). Puberty occurs in cattle at a DoM of approximately 0.5 (Smith et al., 1976). The DoM near puberty represents a period when an inflection point is reached in the quadratic and cubic relationships established for the fiber type percentages in this meta-analysis. In parallel to the decreasing proportion of oxidative fibers and increasing proportion of glycolytic muscle fibers before puberty, the activity of the oxidative enzyme ICDH decreased and the activity of LDH (a glycolytic enzyme) increased. The dynamic nature of the fiber composition and metabolic activity with DoM reflects energy requirements of the muscle during development and occurs in response to substrate supply and to chemical, hormonal, and neural signals (Hocquette et al., 1998; Oddy et al., 2001). The changes observed in this meta-analysis reflect the use of glycolytic metabolism to supply energy during periods of rapid growth before puberty (Brandstetter et al., 1998). Changes in the developmental rate of metabolic enzyme activity and fiber percentage occur at the same DoM for all muscles but the magnitude of the change differs between muscles because of their different anatomical locations and different requirements for nutrients and energy in response to physiological function (Lefaucheur and Gerrard, 2000; Oddy et al., 2001). The greatest changes in metabolic enzyme activity were seen in the ST muscle, which may reflect the greater energy requirement of this muscle due to its role in animal movement.
Previous research indicates that the ST muscle has the greatest collagen content and the LT had the least collagen content (Jurie et al., 2005, 2006; Von Seggern et al., 2005), which was also found with the results from bulls and cows in this meta-analysis. Aging of intramuscular collagen has been noted to increase its thermal stability and reduce its solubility with cooking (Nishimura et al., 1996). This meta-analysis found that in young bulls, CINS increased in the LT muscle and decreased in ST, with little change in the TB muscle with increasing animal maturity. This suggests that only the LT muscle is susceptible to reductions in collagen solubility as the animal grows. Nishimura et al. (1996) studied collagen concentration in 7-mo-old bovine fetuses through to 36-mo-old steers and indicated that the majority of change in collagen concentration and solubility in the muscle occurred before 6 mo of age; after that time, the total and soluble collagen decreased only fractionally. Given this, the major changes in collagen characteristics are not likely to be observed in this study as collagen data could not be obtained for animals with low DoM.
In accordance with the means in our study, the literature shows that LT muscle has a greater lipid concentration compared with ST muscle, and TB muscle is intermediate (Jurie et al., 2005; Von Seggern et al., 2005). The increased TLIP in the muscles of bulls reflects increased fat deposition as protein deposition declines toward maturity (Robelin, 1986; Owens et al., 1993; Hocquette et al., 1998). The plateau in lipid concentration of cows can be attributed to the reduced rate of fat deposition observed in older animals (Owens et al., 1993). Furthermore, cull cows for beef production in France are slaughtered at a similar body condition and this probably equalized the muscle lipid concentration across the cows. Phospholipids are structural lipids found in muscle cell membranes and the small decline in PL with DoM in bulls is likely to be a result of other muscle cell components increasing at a greater rate. Triglycerides are the major component of total lipid; hence, the changes in TG tended to mimic the changes in TLIP.
Influence of Breed on Muscle Characteristics
Few differences in muscle characteristics were observed between Aubrac, Charolais, Limousin, and Salers bulls at 15 to 24 mo of age by Jurie et al. (2005). In this meta-analysis, a greater number of animals and an additional dairy breed were considered and the muscle characteristics were found to differ between breeds.
Breeds differ in their BW at maturity and the length of time to reach maturity. Dairy breeds such as Holstein and Montbéliard tend to be early maturing, whereas beef breeds such as Charolais and Limousin are later maturing. The rustic breeds, Aubrac and Salers, are used primarily for beef production although they have less mature BW and are earlier maturing than the beef breeds. Use of DoM allows for comparison between breeds with differing mature BW and rate of maturity. Differences in muscle characteristics between breeds are likely to be a consequence of metabolic and physiological differences in the breeds that have evolved in response to production purposes. The smaller linear coefficient indicates that there was a smaller increase in CSA with DoM in Montbéliard bulls compared with other breeds. Furthermore, the dairy and rustic breeds tended to have a smaller mean CSA and greater oxidative fiber type and enzymatic activity. Glycolytic metabolism is used to provide energy for muscle growth, and fiber percentages are altered at different rates between the breeds to meet energy requirements (Hocquette et al., 1998; Oddy et al., 2001). The fiber proportions and metabolic enzyme activities of the dairy and rustic breeds are likely to be due to these breeds having less propensity toward protein deposition and lean muscle growth compared with other breeds, and, therefore, requiring less muscle fiber hypertrophy and less demand for glycolytic metabolism.
The breeds differed in rates of change in CINS and TCOL, but this is likely because of changes in collagen that were not proportional to breed-dependent changes in other muscle components resulting in dilution or concentration of collagen in the muscle with increasing maturity (Gerrard et al., 1987). Salers bulls and cows had the greatest CINS and TCOL compared with other breeds. Earlier maturing breeds tend to deposit more collagen with a greater insoluble proportion (Campo et al., 2000), which may account partly for the greater CINS in Salers in this study.
The concentration of lipid in the muscle and its development with DoM was not different between the bull breeds. This indicates that maturity level is a strong driver of lipid development in different cattle breeds. Although the mean TG and TLIP were different between cow breeds, the maximum difference was only 0.3% of the muscle mass, which may not lead to noticeable differences in meat quality.
Influence of Sex on Muscle Characteristics
Different muscle characteristics between bulls and steers are likely a result of differences in gonadal hormones, notably testosterone, in animals after puberty. Testosterone is a driver for rapid BW gain and metabolism that promotes lean muscle growth as opposed to fat deposition (Seideman et al., 1982). This probably promoted the greater mean CSA and greater increase in CSA with DoM in bulls than steers. Testosterone has a role in controlling changes in muscle fibers (Seideman and Crouse, 1986), which may have been responsible for the greater mean %FG and the smaller changes in the respective cubic and quadratic relationship of %FOG and %FG with DoM that were observed in bulls compared with steers. Despite the greater proportion of glycolytic fibers, LDH activity was less and oxidative activity greater in bulls. Oxidative metabolic activity utilizes fatty acids as an energy source (Ashmore, 1974) and is likely to be the link to the reduced lipid concentration and deposition in bulls compared with steers. Testosterone has a stimulating effect on collagen synthesis (Gerrard et al., 1987) although this was not evident in our study, which considered many breeds.
Hypotheses for Model Development
This study indicates that muscle characteristics are not likely to follow a linear development with increasing maturity. Nonlinear analysis and modeling would provide better predictive equations that are more useful for decision-making. The muscle characteristics of each muscle develop following the same general pattern with increasing maturity; however, rates at which the muscle characteristics change differ among muscles. When regressing the muscle characteristics against DoM, the coefficient values and means differed between muscles, indicating that muscle type accounts for the largest proportion of the variation in muscle characteristics. For modeling, this suggests that the same nonlinear equation can be used for all muscles but each muscle will have to be modeled separately. Within a muscle, the breeds and sexes also differed in their coefficients and mean values; therefore, breed and sex have a role in determining muscle characteristics and meat quality. Amplitude and rates of change in the muscle characteristics with different sexes and breeds could be modeled by fitting different parameter values for the different breeds and sexes within a muscle. To minimize the number of parameters that need to be fitted, the meta-analysis indicates that breeds could be grouped. The dairy and rustic breeds differ in their muscle characteristics, in particular fiber composition and enzyme activity, compared with the beef breeds so it might be possible to group into beef and nonbeef breeds. Similarly, cow and steer data can be grouped so that parameters are fitted for male and nonmale animals because changes in muscle characteristics with different sexes appear to be strongly influenced by testosterone production. To improve the functionality for decision-making, it is envisioned that the effect of DoM on muscle characteristics will be combined with a growth model.
1 Corresponding author: nicola.schreurs{at}yahoo.co.nz
Received for publication January 18, 2008. Accepted for publication July 1, 2008.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), semitendinosus (ST, 
), or triceps brachii (TB, 
) muscle is regressed with the degree of maturity of the animal.
