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ANIMAL PRODUCTION |
,1
* Department of Animal Science, University of California, Davis, CA 95616; and
and
NSW Department of Primary Industries, Armidale, NSW, 2350 Australia
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: beef cattle • carcass characteristic • implant • maturity
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), feeding regimes (Ferrell and Jenkins, 1998), weaning age (Schoonmaker et al., 2004), implant status (Bruns et al., 2005), and harvest end-points (Hermesmeyer et al., 2000) have improved our understanding of animal performance and carcass characteristics.
It was previously believed (Hood and Allen, 1973; Cianzio et al., 1985; May et al., 1994) that intramuscular fat (%) deposition occurred mainly in the last 50 to 80 d of the finishing period. However, there is evidence (Sainz et al., 1995; Bruns et al., 2004) to suggest that intramuscular fat deposition begins as early as 4 to 12 mo of age. Genotype (i.e., marbling EPD) affects intramuscular fat deposition (Gwartney et al., 1996). Heritabilities for marbling of approximately 35% (Ríos Utrera and Van Vleck, 2004) indicate that management and environment account for 65% of the variability in marbling. The amount of intramuscular fat in the LM has a high phenotypic correlation with marbling but also with subcutaneous fat thickness (Sainz and Vernazza Paganini, 2004); therefore, highly marbled carcasses may also have increased fat trim losses.
The objectives of this study were 1) to determine the influence of biological type (early-moderate or late maturity), implant status, or both on carcass characteristics [HCW (kg), intramuscular fat (%), 12th-rib fat thickness (fat thickness; cm), LM area (cm2) and KPH (%)] at harvest; and 2) to develop a table of mean values of carcass characteristics across a range of harvest end-points based on the results of objective 1 to be used as inputs to an ongoing program for modeling beef cattle growth and carcass quality. Meta-analysis methodologies outlined by Normand (1999) and St-Pierre (2001) have been adopted to quantify the factors affecting carcass characteristics.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Study Sample
A search was conducted for articles describing beef cattle performance and carcass characteristics. Two databases were used: OVID (http://www.ovid.com) and CAB (http://www.cabi.org). The OVID search criteria included the terms growth rate, animal performance, and intramuscular fat. The search occurred on a monthly basis from 1 September 2002 to 30 June 2004. For the second database, CAB abstracts, the search criteria included the terms beef, steer, and carcass characteristics. The CAB data set limited the search to articles published between 1 January 1982 and 30 June 2004.
Data Selection
This study was part of an ongoing program for modeling beef cattle growth and carcass quality. Therefore, some of the criteria encompassed variables that may be required at a later stage. The inclusion criteria were publications in English reporting results on animal performance or carcass characteristics of beef cattle finishing in a feedlot and that reported intramuscular fat or marbling score, fat thickness, and ME intake (or information sufficient to calculate ME intake, such as initial and final weight, ADG, NEg, and NEm, days on feed, and DMI).
Data Collection
Data extracted from the papers included general information: author name, journal reference, date of publication, and number of animals in the experiment. The quantitative and qualitative details included breed, sex, production system (feedlot or pasture), details about growth promoting implants, days on feed, feed (composition, intake, NEg, NEm, ME, and ME intake, etc.), and carcass characteristics (carcass weight, dressing percent, LM area, marbling score, fat thickness, and KPH). Actual values were selected when available except when the adjusted fat thickness values were reported. If the study reported a number of results with different covariate adjustments, the initial weight was chosen. Codes were used to categorize the data as needed; unique codes were allocated to publications and additionally to experiments within the papers; and breeds were coded as follows: British, Continental, Adapted (Bos taurus breeds adapted to tropical conditions), British x Continental, British x Adapted, Continental x Adapted, British x Brahman, British x Continental x Adapted, or British x Continental x Brahman. The proportion of British breeding in crossbreds varied from 25 to 75%. Implant status (single or repeat applications) was coded as estrogenic, combination, or no implant. Individual publications, as well as the different experiments included in them, were individually coded and are referred to as source.
Calculations and Statistical Analysis
Diet energy: NEm, NEg, and ME contents were calculated using an Excel (Microsoft Inc., Seattle, WA) spreadsheet based on the inputs of final BW, initial BW, ADG, and DMI according to the methods outlined by Zinn and Shen (1998). In brief, a quadratic equation was solved for NEm; 2 solutions were given and the lowest value was discarded. The NEg was then calculated as NEg = (0.877 x NEm) – 0.41. Calculations were based on NRC (1996) equations.
In a number of papers, intramuscular fat was not reported but marbling score was. In these cases, marbling scores were transformed to a common measure. A linear equation (intramuscular fat = [0.0127 x marbling score] – 0.8043; R2 = 0.78), developed by Savell et al. (1986), was then used to predict the percentage of ether extractable fat (intramuscular fat, %) based on the following marbling score conversions: Practically devoid, 100–199; Traces, 200–299; Slight, 300–399; Small, 400–499; Modest, 500–599; Moderate, 600–699; Slightly abundant, 700–799; Moderately abundant, 800–899.
The pooled SE or SEM was generally reported. When the SE was not reported, the average SE across the publications was calculated and used as the SE. All data for carcass characteristics were analyzed using the MIXED procedure of SAS (SAS Inst. Inc., Cary, NC). A test for homogeneity of study means was undertaken with only breed as a fixed effect.
Preliminary Analysis.
A preliminary analysis of covariance on HCW (kg) was performed to determine the biological type for each of the breeds: early maturity (frame size 4), moderate maturity (frame size 5), and late maturity (frame size 6 or higher), in which the statistical model included the fixed-effect terms breed and implant status; fat thickness as a covariate; all 2-way interactions; and intercept-slope, fat thickness, and source (the subject in the MIXED procedure of SAS) as the random-effect terms. Source of publication was the experimental unit. The least squares means of HCW were grouped as: early maturity, HCW 
325 kg; moderate maturity, 325 kg < HCW < 335 kg; and late maturity, HCW 
335 kg based on mean fat thickness in the 1.0- to 1.5-cm range. Spearman correlations were used to assess correlations.
Studies.
Two studies using analysis of covariance were undertaken: study 1, type of implant, with fat thickness as the covariate and evaluating the effect of biological type and type of implant; and study 2, with final BW as the covariate and examining the effect of biological type and implant status (implanted vs. nonimplanted) on carcass characteristics. The analysis of covariance on each of the carcass characteristics was performed, and the statistical model included the fixed-effect terms biological type and implant status; covariate as mentioned above; all 2-way interactions; and intercept-slope, source (the subject in the mixed procedure of SAS), and fat thickness (study 1) or BW (study 2) as the random-effect terms. Source (i.e., publication) was the experimental unit.
All models were evaluated for the assumptions of normality and constant variance. A log transformation on KPH in the main analysis was performed to correct for the nonnormal distribution of the raw data; outliers were identified and deleted from the data set based on the Shapiro-Wilk test (P < 0.05) for normality. The test for normality used the UNIVARIATE procedure of SAS. A Levene’s test of the residuals (P < 0.05) was used to test the assumption of constant variance, and a weight statement was included to correct for the violation of constant variance. The weight, w2 = w1/w, was used, where w1 = inverse of the squared SE, and w = its mean value (St-Pierre, 2001). All random-effects models used REML for estimating variance components.
The initial analysis was conducted in a model with random slope and intercept effects, including a possible covariance between the slope and intercept (option UN, for an unstructured covariance, in PROC MIXED). This allowed for a test of significance for the estimate of the covariance of slope and intercept. This covariance parameter was declared significantly different from 0 at a probability of P < 0.10. A more liberal test than the traditional P = 0.05 was used because accurate estimations of variances and covariances require a considerable number of observations (St-Pierre, 2001). However, if the covariance parameter was not significantly different from 0, option VC, for a variance components structure in PROC MIXED, was used. Observations in the figures have been adjusted (Y values on the regression line + Residuals) to account for the multidimensional space created when the results are from a mixed model regression (St-Pierre, 2001).
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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for future development of fat deposition models and Robinson et al. (2001) for challenging the new models. Hence, the total number of unique reports was 49. Seven of the reports had 1 additional feedlot finishing experiment which gave a total of 55 unique reports that potentially could be used in this study. Thirteen of these were excluded: 3 had insufficient data available to calculate ME intake; 3 had very high intramuscular fat or KPH values at low BW; 2 had no breed type recorded; 1 had results at days on feed less than 20 d; 1 had a large variation in initial ages; 1 had no description of marbling score; 1 had results only on Holstein steers; and 1 did not report fat thickness, KPH, or LM area. In total 43 publications (Table 1) were considered eligible, for an inclusion rate of 78%. Several treatments within the sources were deleted from the data set: 1 adapted breed (source 8) because it was the only adapted breed in the study; several treatments in sources 22, 30, 32, and 33 because the treatments were altered at different periods during the finishing phase, therefore the control in each of the experiments was used because it was the only consistent treatment; 2 in source 12 because they were a combined treatment effect; and 3 treatments in source 21 because the last change in the diet to control the predicted gain was only fed for 27, 58, or 53 d, respectively.
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The HCW analysis with fat thickness as a covariate (P < 0.01) included fixed effect terms breed (P < 0.01) and implant status (P < 0.01) and the intercept-slope, fat thickness, and source as random variables where source was the subject in the SAS code. The estimate for intercept-slope covariance was statistically different from 0 (P = 0.03); therefore, the random parameters were correlated, so the covariance component was fitted to the model. Seven outliers were identified and removed from the data set, and the Levene’s test indicated a constant variance. The least squares means of HCW were 311, 281, 321, 327, 308, 327, 360, and 365 kg for British, British x Adapted, British x Brahman, British x Continental, British x Continental x Adapted, British x Continental x Brahman, Continental, and Continental x Adapted, respectively, with a mean fat thickness = 1.11 ± 0.02 cm and mean HCW = 325 ± 3.79 kg. Based on the biological criteria stated above, British, British x Adapted, British x Brahman, British x Continental x Adapted, and British x Continental x Brahman were classified as early maturity breeds; British x Continental as a moderate maturity breed; and Continental and Continental x Adapted as late maturity breeds. A preliminary analysis of carcass characteristics indicated that breeds with a Brahman influence behaved differently than the other breeds (e.g., least squares means of Brahman breeding vs. British breeding in the early maturity category had an intramuscular fat of 4.06 vs. 4.73%, P < 0.01). Therefore, steers with pure Brahman breeding were deleted from the data set, so that these results apply only to Bos taurus beef breeds. A single degree of freedom contrast at P = 0.05 level between biological types (early maturity, moderate maturity, or late maturity) as fixed effects indicated no significant differences between early maturity and moderate maturity (LM area, P = 0.06; fat thickness, P = 0.48; KPH, P = 0.11; and intramuscular fat, P = 0.07). Based on these results the early and moderate maturity levels were grouped together into an early-moderate maturity category.
Study 1.
Carcass characteristics were analyzed with biological type (early-moderate maturity or late maturity) and implant status (estrogenic, combination, no implant; repeats included) as fixed effect terms; fat thickness as the covariate and a 2-way interaction between implant status and fat thickness with the exception of HCW; and the intercept-slope, fat thickness, and source as random variables. Outliers were detected and removed for each of the carcass characteristics. For intramuscular fat, a weight statement was used as described above to correct for the violation of a nonconstant variance. The estimate for intercept-slope covariance was not statistically different from 0 for LM area (P = 0.11), KPH (P = 0.19), and intramuscular fat (P = 0.74); therefore, the random parameters were not correlated so a reduced model without a covariance component was fitted for these carcass characteristics. For HCW (P = 0.01), the model had a covariance component fitted. The results shown in Table 2 for the early-moderate maturity category at fat thickness = 1.11 cm indicate that compared with nonimplanted steers, application of estrogenic or combination implants increased HCW by 2.9 or 4.8%; increased LM area by 3.2 or 6.3%; decreased intramuscular fat 8.1 or 5.4%, respectively, and decreased KPH by 7.6% for estrogenic implants and increased KPH by 1.1% for combination implants.
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to 5 show the adjusted data and the statistical fits through the data. The effects of biological type and implant status on carcass characteristics at 25-kg increments from 550 to 600 kg are shown in Table 3.
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) so that comparisons can be made with the publications used in this study because the majority of the experiments used fat thickness as the predetermined end-point; and 2) with final BW as a covariate (study 2, Table 3) so that a table of mean values at different BW could be reported that will be used as reference values in an ongoing program for modeling beef cattle growth and carcass quality.
Publication bias is a major concern of the meta-analyst as to whether the collection of analyzed studies was selected using a biased mechanism. Meta-analysis studies often use a funnel plot (Normand, 1999), a scatter plot of sample size, or other measure of precision on the y-axis vs. the estimated effect size (Light and Pillemer, 1984) on the x-axis, to detect publication bias. Funnel plots were not utilized in this study because there was no common control across all studies to calculate the effect size. In this study, the reported variability and ranges of values, as shown in Figures 1 to 5, indicate that the sources represent an unbiased appraisal of the literature.
Traditionally the source would have been treated as a fixed effect. However, the MIXED model procedure provides a tool to analyze data with fixed and random effects. The effect of source (the variance between studies not accounted for by the other variables in the model) is random because the interest is not in the specific levels of the factors but rather to the larger set of all levels constituting the population (St-Pierre, 2001). Therefore, this analysis covers a broad inference space, which includes additional variance through the random sources and thus enables inference of future observations. Even so, interpretation of the results needs to be taken in context, and significant results might not necessarily describe the true underlying biology.
Differences were seen in both the studies between early-moderate and late maturity breed types, indicating that frame size does take account of much of the variation in carcass composition. These differences were not seen in all sources, e.g., Chase et al. (1998) and Paschal et al. (1995) did not show any differences between breeds. This meta-analysis has shown differences in carcass characteristics between early-moderate and late maturity across a large number of studies. Therefore, on average HCW increases by 62 kg for every 100 kg of additional BW, regardless of breed type or implant status. The same increase in BW is accomplished by a 0.1 or 0.2 cm increase in fat thickness in implanted vs. nonimplanted early-moderate maturity steers, respectively. Similarly, these same groups display 0.15 and 0.5% increases in KPH and intramuscular fat over the same BW range. Interactions between harvest BW and implant status on carcass characteristics are less pronounced for late maturity cattle.
Results for implant status in both analyses (Tables 2 and 3) indicate that implant status had significant effects on carcass quality. These differences were not uniform across sources. Hunt et al. (1991) did not detect any differences between implant groups. Hermesmeyer et al. (2000) reported some differences in their data; for animals fed on an ad libitum basis, KPH and marbling score only differed between the nonimplanted control group steers that received an implant containing 24 mg of estradiol and 120 mg of trenbolone acetate but not between controls and steers that received an implant containing 28 mg of estradiol benzoate and 200 mg of trenbolone acetate. Some differences with LM area were detected, but the results were not consistent; and in the restricted intake groups there were no differences in marbling score and KPH (Hermesmeyer et al., 2000). Samber et al. (1996) did not detect differences in HCW, KPH, or fat thickness; however, they did report that some implants produced fatter carcasses than others. Two treatments in the study increased LM area and animals receiving repeat implants had fewer Choice and Prime carcasses than the nonimplanted group. Differences in HCW and LM area were detected by several sources (Johnson et al., 1996; Woodward and Fernandez, 1999; Bruns et al., 2005). When fat thickness was not the harvest end-point, differences in fat thickness were detected by Woodward and Fernandez (1999), who reported 0.60 vs. 0.90 cm for implanted and nonimplanted steers, respectively.
Anabolic implants (estrogenic or combination with single or repeat applications) can affect the deposition of intramuscular fat (Platter et al., 2003). Not only are repeat applications important, but also the time of implanting can have an impact. Bruns et al. (2005) reported increased HCW by 3.2 and 3.8%, LM area by 0.9 and 1.2%, and decreased fat thickness by 3.7 and 7.5%, whereas marbling (i.e., intramuscular fat) decreased by 8.0 and 5.1% for early vs. delayed implanting, respectively, as compared with nonimplanted controls. The Bruns et al. (2005) study suggests that deposition of intramuscular fat is sensitive to anabolic growth promotants administered during early periods of growth. The mechanisms behind either implanting strategy (single or repeat) or time of implanting still need to be elucidated.
As indicated above, caution must be applied when interpreting the results (Figures 1 to 5, study 2) in relationship to the biology, but the equations do have merit in predicting carcass characteristics within the confidence limits of the data. Several equations predicting carcass characteristics have previously been reported (Perry and Fox, 1997; Bruns et al., 2005). Bruns et al. (2005) showed a linear relationship between empty BW and intramuscular fat based on serial slaughter data (d 0, 57, and 140). The intramuscular fat values in this study at 600 kg of BW are in agreement with the Bruns et al. (2005) study at 506 kg of empty BW. Differences in slope and intercept exist between this study and that of Bruns et al. (2005) due to differences in design. The Bruns et al. (2005) study was based on serial slaughter data and therefore presents a detailed picture of the increase in intramuscular fat that occurred during the feeding period within their experiment. By contrast, this study analyzed the relationship of intramuscular fat with BW at harvest, after variable feeding periods in a range of biological types and management strategies across a large number of studies.
Fat deposition is complex and does not follow a simple relationship. Days on feed are important for fat deposition, but the correlations between final BW and days on feed (–0.26 and 0.35 for implanted early and late maturing cattle, respectively) confound the results if both terms are in the statistical model. The F values for the intramuscular fat analysis suggest that final BW (F = 92.39) as opposed to days on feed (F = 63.56) be used as the covariate, and these results were similar across all carcass characteristics. Biological type and implant status do take account of much of the variation. This meta-analysis adds additional and more recent knowledge of carcass compositional data to that reported by Owens et al. (1995) and the NRC (1996). The traditional equations for water, fat, and protein accretion (NRC, 1996) relate the contents of chemical components and energy with empty BW. This study has extended these concepts to include carcass composition, and the results show that variation can be accounted for by biological type and implant status (Figures 2 to 5). Future work is needed and includes the following: 1) development of relationships between body composition measurements (e.g., fat thickness, KPH, and intramuscular fat % and density) to represent total body fat as 4 fat depots: intermuscular, visceral, subcutaneous, and intramuscular fat; and 2) incorporate the effects of biological type and implant status into the fat deposition model of Sainz and Hasting (2000) as part of an ongoing program for modeling beef cattle growth and carcass quality.
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Footnotes |
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2 Corresponding author: rdsainz{at}ucdavis.edu
Received for publication March 23, 2006. Accepted for publication June 14, 2006.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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