M. E. Davis1
The Ohio State University
Department of Animal Sciences
R. C. M. Simmen
Department of Animal Science
University of Florida
1 For more information, contact at: The Ohio State University, 2027 Coffey Road, 221 Plumb Hall, Columbus, OH 43210; 614-292-4984; fax 614-292-2929; e-mail: davis.28@osu.edu.
Divergent selection for blood serum insulin-like growth factor I (IGF-I) concentration has been ongoing at The Ohio State University's Eastern Ohio Resource Development Center (EORDC) since 1989 using 100 spring-calving (50 high-line and 50 low-line) and 100 fall-calving (50 high-line and 50 low-line) purebred Angus cows. Following weaning, bull and heifer calves are fed in drylot for a 140-day postweaning period. At the conclusion of the postweaning period, bulls not selected for breeding are slaughtered and carcass data are collected at a commercial packing plant. At the time of this analysis, IGF-I measurements were available for 945 bull and heifer calves, and carcass data were available for 343 bulls. A set of Multiple Trait, Derivative-Free, Restricted Maximum Likelihood (MTDFREML) computer programs were used for data analysis. Estimates of direct heritability for IGF-I concentration at days 28, 42, and 56, of the postweaning period and for mean IGF-I concentration were 0.42, 0.73, 0.42, and 0.50, respectively. Direct heritabilities for carcass traits ranged from 0.25 (for marbling score) to 0.59 (for yield grade). Maternal heritability and the proportion of phenotypic variance due to permanent environmental effect of dam were small (i.e., < 0.22) for IGF-I concentrations and for carcass traits. Additive genetic correlations of IGF-I concentrations with different variables were: backfat thickness, -0.20; ribeye area, 0.04; kidney, pelvic, and heart fat percentage (KPH), -0.99; hot carcass weight, 0.31; marbling score, 0.33; quality grade, 0.29, and yield grade, -0.06. Therefore, bulls with higher blood serum IGF-I concentrations had heavier carcasses, along with lower backfat thickness and KPH, but higher marbling scores and quality grades. Serum IGF-I concentration may be a useful selection criterion when efforts are directed toward improvement of carcass characteristics of beef cattle.
Use of physiological indicators of genetic merit may increase rates of genetic change in livestock via reduced generation intervals, increased selection differentials, an/or increased accuracy of selection (Kiddy, 1979; Woolliams and Smith, 1988; Blair et al., 1990). Serum IGF-I concentration may be useful as a physiological indicator trait in selection programs designed to improve carcass characteristics of beef cattle. Anderson et al. (1988) reported that IGF-I was negatively correlated with percentage of carcass fat, carcass fat accretion rate, total carcass fat, and fat thickness, and that it was positively correlated with percentage of carcass protein in Simmental crossbred bulls. Hayden et al. (1993) observed that plasma concentrations of IGF-I were highly correlated with empty body fat accretion, empty body weight gain, and protein deposition in compensating steers. In Angus beef cattle, Bishop et al. (1989) found that phenotypic correlations of serum IGF-I concentrations with carcass characteristics were low, but favorable for the last two 28-day periods of a 140-day postweaning performance test. Little is known about the genetic relationships of IGF-I with carcass characteristics. If IGF-I is moderately to highly heritable and has moderate to high genetic correlations with carcass traits, it may be used in a selection program to increase rates of genetic change for such traits. Therefore, the objectives of this study were to estimate heritabilities for various measures of blood serum IGF-I concentration and to estimate genetic, phenotypic, and environmental correlations of IGF-I with carcass characteristics of beef cattle.
Data for this study were taken from an experiment involving divergent selection for IGF-I that was initiated in 1989 using 100 spring-calving (50 high-line and 50 low-line) and 100 fall-calving (50 high-line and 50 low-line) purebred Angus cows with unknown IGF-I levels located at The Ohio State University's Eastern Ohio Resource Development Center (EORDC), Belle Valley, OH. Cows from the initial base population were randomly assigned to the selection lines.
Selection procedures for this experiment have been described previously (Davis et al., 1995). Each year, the four bull calves with the highest, and the four bulls with the lowest blood serum IGF-I concentrations (adjusted for age of calf and age of dam) are saved for breeding within the respective selection lines. Selection is based on the average of three IGF-I samples (taken at days 28, 42, and 56 of the 140-day postweaning test). Selected bulls are used for breeding as yearlings and then sold.
Approximately eight cows are culled from each line each year (based upon physical unsoundness, reproductive failure, and oldest age) and replaced with approximately eight pregnant heifers having the highest or lowest blood serum IGF-I concentrations (adjusted for age of calf and age of dam). All available heifers are bred and selections are made among heifers that conceive. Selection of heifers is based upon the average of three IGF-I samples collected at the same time as for bulls.
Spring-born calves were reared by their dams without creep feed until weaning at approximately seven months of age. Following weaning, bull calves were given ad libitum access to a corn-soybean meal based diet plus five pounds of grass hay per head per day. Heifers were fed a corn-soybean meal diet intended to yield postweaning gains of approximately 1.65 lbs./day. Bulls and heifers were fed in separate three-sided barns with adjoining exercise lots located at EORDC.
Fall-born calves were weaned at an average age of 140 days and then fed a corn-soybean meal diet formulated to yield gains of approximately 2.0 lbs./day, plus grass hay, in drylot for 112 days. Following the 112-day growing period, bull and heifer calves remained at EORDC and were managed in the same manner as spring-born bulls and heifers.
At the conclusion of the 140-day postweaning performance test, bulls not saved for breeding were slaughtered at a commercial packing plant located in Columbus, OH. Bulls were approximately 12 to 14 months of age at slaughter. Carcass data (hot carcass weight, backfat thickness, ribeye area, kidney pelvic and heart fat percentage (KPH), marbling score, quality grade, and yield grade) were collected by meat scientists from The Ohio State University Department of Animal Sciences. Numbers of observations available for this study were 905, 836, and 913, for IGF-I concentration on days 28, 42, and 56, respectively, of the postweaning period, 945 for mean IGF-I concentration (i.e., the mean of the IGF-I concentration at day 28, 42, and 56 for each calf), and 343 for all carcass traits except marbling score, for which 318 observations were available.
Approximately 25 mL of blood was collected in sterile 16 mm x 150 mm glass tubes via jugular puncture of each animal. The blood was allowed to clot for 24 hours at 4°C. Serum was obtained by centrifugation (1,800 x g for 20 minutes) and frozen at-20°C until it was assayed.
The RIA for IGF-I was performed in the laboratory of R. C. M. Simmen at the University of Florida using procedures described by Bishop et al. (1989).
Data were analyzed using a set of Multiple Trait, Derivative-Free, Restricted Maximum Likelihood (MTDFREML) computer programs written by Boldman et al. (1993). Pedigrees of base population animals were traced back three generations to create the numerator relationship matrix. Included in the analysis were 1,910 animals in the A inverse matrix, 945 of which had valid records for mean IGF-I, and 343 of which had valid carcass data records. First, each trait was analyzed singly to obtain estimates of direct heritability (hd2), maternal heritability (hm2), the proportion of phenotypic variance due to permanent environmental effects of dam (c2), and the correlation between direct genetic and maternal genetic effects (ram). The statistical model for this analysis included fixed effects of birth year of calf (1989, 1990, 1991, 1992, 1993, 1994, 1995), season of birth (spring vs. fall), IGF-I selection line (high vs. low), sex of calf (bull vs. heifer; omitted in analysis of carcass traits), age of dam (2, 3, 4, 5 to 9, > 10 year), random animal, maternal genetic, maternal permanent environmental effects, and a covariate for age of calf. Maternal and permanent environmental effects were deleted from the final model because they accounted for only a small proportion of the total variability in IGF-I concentrations and carcass traits.
Secondly, IGF-I concentration on days 28, 42, and 56, of the postweaning test, as well as mean IGF-I concentration, were paired with each of the carcass traits in a series of bivariate analyses to estimate the additive genetic (rA1A2), environmental (rE1E2), and phenotypic (rP1P2) correlations among the traits.
Simple means, standard deviations, and coefficients of variation for serum IGF-I concentrations and carcass traits are shown in Table 1.
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Table 1. Means, Standard Deviations (SD), and Coefficients of Variation (CV) for Serum IGF-I Concentrations and Carcass Traits. | ||||||
|---|---|---|---|---|---|---|
| Trait | N | Mean | SD | CV,% | Min. Value | Max. Value |
| IGF28, ng/mLa | 905 | 204.3 | 144.9 | 71 | 4.2 | 790.2 |
| IGF42, ng/mL | 836 | 233.1 | 157.1 | 67 | 17.2 | 978.0 |
| IGF56, ng/mL | 913 | 220.8 | 157.3 | 71 | 3.4 | 829.2 |
| Mean IGF-I, ng/mLb | 945 | 206.8 | 150.8 | 73 | 4.1 | 826.9 |
| Backfat thickness, inches | 343 | 0.37 | 0.13 | 34 | 0.05 | 0.8 |
| Ribeye area, inches2 | 343 | 12.35 | 1.26 | 10 | 7.70 | 17.1 |
| KPH,% | 343 | 2.4 | 0.6 | 24 | 1.0 | 4.0 |
| Hot carcass weight, lb. | 343 | 618.2 | 76.3 | 12 | 345.9 | 817.9 |
| Marbling scorec | 318 | 5.1 | 0.8 | 15 | 3.1 | 7.8 |
| Quality graded | 343 | 11.1 | 1.4 | 13 | 7.0 | 14.0 |
| Yield grade | 343 | 2.3 | 0.5 | 23 | 0.4 | 4.0 |
| a IGF28, IGF42, and IGF56 represent serum IGF-I concentration at day 28, 42, and 56, respectively, of the postweaning test. b Mean IGF-I is the average of serum IGF-I measurements for a given calf. c A score of 3.0 to 3.9 = traces, 4.0 to 4.9 = slight, 5.0 to 5.9 = small, 6.0 to 6.9 = modest, 7.0 to 7.9 = moderate, etc. d A score of 9 = select-, 10 = select o, 11 = select+, 12 = choice-, 13 = choice o, and 14 = choice+. | ||||||
Heritabilities and direct-maternal correlations obtained for blood serum IGF-I concentration and for carcass traits are shown in Table 2. Direct heritability for IGF-I at day 28, 42, and 56, of the postweaning test, and for mean IGF-I was 0.42, 0.73, 0.42, and 0.50, respectively. Direct heritabilities for carcass traits at an age-constant endpoint ranged from 0.25 (for marbling score) to 0.59 (for yield grade). These heritability estimates agree reasonably well with those of Rouse and Wilson (1997) who reported estimates of 0.30, 0.37, 0.26, and 0.25, for hot carcass weight, marbling score, ribeye area, and fat thickness, respectively, for the Angus breed. The estimates also agree well with those of Koots et al. (1994) who reported average heritabilities from a large number of studies to be 0.23, 0.38, and 0.42, for carcass weight, marbling score, and ribeye area, respectively.
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Table 2. Parameter Estimatesa for Serum IGF-I Concentrations and Carcass Traits Derived from Full Animal Model (Age-Constant Endpoint). | ||||
|---|---|---|---|---|
| Trait | h2d | h2m | c2 | ram |
| IGF28b | 0.42 | 0.06 | 0.00 | - 0.78 |
| IGF42b | 0.73 | 0.20 | 0.00 | - 0.87 |
| IGF56b | 0.42 | 0.17 | 0.00 | - 0.80 |
| Mean IGF-Ic | 0.50 | 0.16 | 0.00 | - 0.76 |
| Backfat thickness | 0.42 | 0.00 | 0.02 | 0.93 |
| Ribeye area | 0.32 | 0.10 | 0.12 | -1.00 |
| KPH | 0.29 | 0.16 | 0.00 | -1.00 |
| Hot carcass weight | 0.29 | 0.02 | 0.22 | -1.00 |
| Marbling score | 0.25 | 0.04 | 0.19 | -1.00 |
| Quality grade | 0.54 | 0.07 | 0.14 | -1.00 |
| Yield grade | 0.59 | 0.01 | 0.00 | -1.00 |
| a hd = heritability (direct effect); hm = maternal heritability; c2 = proportion of phenotypic variance due to permanent environmental effect of dam; ram = correlation between direct and maternal effects. b IGF28, IGF42, and IGF56 represent serum IGF-I concentration at day 28, 42, and 56, respectively, of the postweaning test. c Mean IGF-I is the average of serum IGF-I measurements for a given calf. | ||||
Maternal heritability and the proportion of phenotypic variance due to permanent environmental effect of dam were generally small (i.e., < 0.22) for IGF-I concentrations and for the carcass traits. The correlation between direct and maternal genetic effects was -0.78, -0.87, -0.80, and -0.76, for IGF-I concentration at day 28, 42, and 56, of the postweaning period, and for mean IGF-I, respectively. Estimates of ram converged to -1.0 for all of the carcass traits except backfat thickness for which ram = 0.93.
Because estimates of hm2 and c2 were small, a second analysis was performed in which maternal genetic effects and permanent environmental effects of dam were omitted from the model. Estimates of hd2 for carcass traits obtained in this analysis are shown in Table 3. Heritability estimates for direct effects obtained with the reduced model were generally similar to those obtained with the full animal model, except that the heritability of KPH was zero when the reduced model was employed.
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Table 3. Heritability of Direct Effects (hd ) for Carcass Traits (Age-Constant Endpoint). | |
|---|---|
| Trait | h2d |
| Backfat thickness | 0.45 |
| Ribeye area | 0.30 |
| KPH | 0.00 |
| Hot carcass weight | 0.41 |
| Marbling score | 0.40 |
| Quality grade | 0.43 |
| Yield grade | 0.49 |
Additive genetic correlations of IGF-I concentrations with backfat thickness, ribeye area, KPH, hot carcass weight, marbling score, quality grade, and yield grade averaged, -0.20, 0.04, -0.99, 0.31, 0.33, 0.29, and -0.06, respectively (Table 4), when carcass data were adjusted to an age-constant endpoint. These results indicate that bulls with higher blood serum IGF-I concentrations had heavier carcasses, along with lower backfat thickness and KPH, but higher marbling scores and quality grades. The additive genetic correlations of IGF-I concentrations with ribeye area and yield grade were near zero.
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Table 4. Genetic, Environmental, and Phenotypic Correlationsa of IGF-I Measurements with Carcass Traits (Age-Constant Endpoint). | ||||||||
|---|---|---|---|---|---|---|---|---|
| Carcass trait | ||||||||
| IGF-I measurement | Correlation | Backfat thickness | Ribeye area | KPH | Hot carcass weight | Marbling score | Quality grade | Yield grade |
| IGF28b | rA1A2 | -0.20 | 0.01 | -0.97 | 0.14 | 0.48 | 0.27 | -0.09 |
| rE1E2 | 0.12 | 0.09 | 0.17 | 0.11 | 0.04 | 0.21 | 0.11 | |
| rP1P2 | 0.02 | 0.07 | 0.14 | 0.12 | 0.16 | 0.22 | 0.04 | |
| IGF42 | rA1A2 | -0.14 | -0.03 | -0.98 | 0.26 | 0.32 | 0.19 | 0.00 |
| rE1E2 | 0.08 | 0.06 | 0.15 | -0.02 | 0.07 | 0.15 | 0.05 | |
| rP1P2 | 0.01 | 0.04 | 0.12 | 0.06 | 0.13 | 0.16 | 0.03 | |
| rA1A2 | -0.28 | 0.00 | -1.00 | 0.40 | 0.18 | 0.47 | -0.09 | |
| IGF56 | rE1E2 | 0.04 | 0.06 | 0.14 | -0.01 | 0.09 | 0.04 | 0.03 |
| rP1P2 | -0.03 | 0.05 | 0.12 | 0.07 | 0.10 | 0.11 | 0.00 | |
| rA1A2 | -0.18 | 0.18 | -0.99 | 0.43 | 0.32 | 0.21 | -0.07 | |
| Mean IGF-Ic | rE1E2 | 0.09 | 0.03 | 0.20 | -0.02 | 0.10 | 0.20 | 0.09 |
| rP1P2 | 0.01 | 0.07 | 0.16 | 0.11 | 0.15 | 0.19 | 0.04 | |
| a rA1A2 = additive genetic correlation between traits 1 and 2, rE1E2 = environmental correlation between traits 1 and 2, and rP1P2 = phenotypic correlation between traits 1 and 2. b IGF28, IGF42, and IGF56 represent serum IGF-I concentration at day 28, 42, and 56, respectively, of the postweaning test. c Mean IGF-I is the average of serum IGF-I measurements for a given calf. | ||||||||
Environmental correlations of the various IGF-I measurements with backfat thickness, ribeye area, KPH, hot carcass weight, marbling score, quality grade, and yield grade averaged, 0.08, 0.06, 0.17, 0.02, 0.08, 0.15, and 0.07, respectively (Table 4), indicating that environmental effects that resulted in increases in blood serum IGF-I concentration also tended to result in small increases in carcass trait values.
Phenotypic correlations of IGF-I measurements with backfat thickness, ribeye area, KPH, hot carcass weight, marbling score, quality grade, and yield grade, averaged 0.00, 0.06, 0.14, 0.09, 0.14, 0.17, and 0.03, respectively (Table 4), indicating small positive phenotypic associations of IGF-I with most carcass characteristics.
Three-hundred-forty-three purebred Angus bull calves were slaughtered at approximately 12 to 14 months of age. Direct heritability estimates for blood serum IGF-I concentration at day 28, 42, and 56, of the 140-day postweaning performance test, and for mean IGF-I concentration were 0.42, 0.73, 0.42, and 0.50, respectively, indicating that IGF-I concentration is a moderately to highly heritable trait that should respond to selection. Additive genetic correlations of IGF-I concentrations with backfat thickness, ribeye area, KPH, hot carcass weight, marbling score, quality grade, and yield grade averaged, -0.20, 0.04, -0.99, 0.31, 0.33, 0.29, and -0.06, respectively. These correlations indicate that bull calves with higher serum IGF-I concentrations had heavier carcasses, lower backfat thickness and KPH, and higher marbling scores and quality grades. Therefore, selection for blood serum IGF-I concentration would be expected to reduce internal and external fatness, while increasing degree of marbling and quality grade.
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