M.E. Davis and R.C.M. Simmen
Department of Animal Sciences
Data for this study were obtained from an experiment involving divergent selection for blood serum IGF-I concentration in beef cattle. Multiple trait derivative-free restricted maximum likelihood (REML) procedures were used to obtain genetic parameter estimates for IGF-I concentration on days 28, 42, and 56 of the postweaning period and for mean IGF-I concentration, as well as for weights at various ages and for postweaning weight gain. The statistical model included fixed effects of year, season, selection line, sex of calf, age of dam, random animal, maternal, permanent environmental effects, and a covariate for age of calf. Included in the analysis were 1,563 animals in the A-1 matrix, 731 of which had valid records for mean IGF-I concentration. Direct heritability (hd2) was 0.42, 0.53, 0.71, and 0.48 for IGF-I on days 28, 42, and 56 of the postweaning period, and for mean IGF-I, respectively. Heritability of maternal genetic effects (hm2 ) ranged from 0.02 to 0.12, whereas the proportion of the total variance due to the maternal permanent environmental effect (c2 ) was essentially zero for all measures of IGF-I. The correlation between direct and maternal effects ranged from -0.79 for IGF-I on days 42 to -1.00 for mean IGF-I. Genetic correlations of IGF-I with weaning and postweaning weights and with postweaning weight gain ranged from -0.21 to -0.54 and averaged -0.38. The environmental correlation between IGF-I and performance traits varied from 0.10 to 0.35 and averaged 0.22. Phenotypic correlations of IGF-I concentrations with weaning weight and postweaning weights and gains ranged from -0.01 to 0.12 and averaged 0.04. Estimates of hd2 indicated that it should be possible to change IGF-I concentration in beef cattle via selection. Negative genetic correlations implied that, if the goal is to make genetic improvement in weaning weights, postweaning weights, and/or postweaning gain in beef cattle, selection should be for decreased postweaning serum IGF-I concentration.
In the future, nutritional traits (e.g., serum levels of Ca, P, Mg, Na, K, Se, I, Co, and vitamins D and K) could be used in conjunction with physiological traits (e.g., IGF-I concentrations) in multiple trait analyses to more accurately predict the growth and developmental behavior of individual animals (Odenya et al., 1992). Use of physiological indicators of genetic merit may enhance rates of genetic change in livestock via reduced generation intervals, increased selection differentials, and/or increased accuracy of selection (Kiddy, 1979; Woolliams and Smith, 1988; Blair et al., 1990). Serum IGF-I concentration is a potentially useful physiological indicator trait in that it is secreted in a nonpulsatile manner and is phenotypically associated with weight and growth rate in cattle (Lund-Larsen et al., 1977; Bishop et al., 1989; Davis and Bishop, 1991; Park, 1993), as well as in other livestock species, including swine (Buonomo et al., 1987; Scanes et al., 1987; Spicer et al., 1992), sheep (Olsen et al., 1981; Roberts et al., 1990), and chickens (Huybrechts et al., 1985; Goddard et al., 1988). However, little is known about the genetic relationships of IGF-I with weight and growth rate. If IGF-I is moderately to highly heritable and has moderate to high genetic correlations with economic traits of interest, its use in a selection program may increase rates of genetic change. The objectives, therefore, were to estimate heritabilities for various measures of serum IGF-I concentration and to estimate genetic, phenotypic, and environmental correlations of IGF-I with weights and gains in beef cattle.
Selection Procedures. 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 in 1990 using 100 fall-calving (50 high-line and 50 low-line) purebred Angus cows located at the Eastern Ohio Resource Development Center (EORDC).
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 bull calves with the lowest IGF-I concentrations (adjusted for age of calf and age of dam) were saved for breeding within the respective selection lines. Selection was based on the average of three serum IGF-I samples (taken on days 28, 42, and 56 of the 140-day postweaning test). Yearling bulls were used for breeding in order to minimize the generation interval. Selected bulls were used for breeding as yearlings and then sold.
Approximately eight cows were culled from each line each year (based on physical unsoundness, failure to conceive in 2 consecutive years and oldest age) and replaced with approximately eight pregnant heifers having the highest or lowest serum IGF-I concentrations (adjusted for age of calf and age of dam). All available heifers were bred, and selections were made among heifers that conceived. Selection of heifers was based on the average of three postweaning serum IGF-I samples collected at the same time as for bulls.
Management Procedures. Spring-born calves were reared by their dams without creep feed until weaning at approximately 7 months of age. Following weaning, bull calves were given ad libitum access to a corn-soybean meal-based concentrate diet plus 5 pounds of grass hay/head/day. Heifers were fed a corn-soybean meal diet intended to yield postweaning gains of approximately 1.65 pounds/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 approximately 140 days and then fed a corn-soybean meal concentrate diet formulated to yield gains of approximately 2.0 pounds/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.
Data Collection. All bull and heifer calves were weighed at birth, weaning, the beginning of the postweaning performance test, and every 28 days thereafter until conclusion of the 140-day postweaning period. In addition, calves were weighed at day 42 of the postweaning test when one of the three blood samples was collected for each calf. Number of observations available for this study were 671, 603, and 679 for IGF-I concentration on days 28, 42, and 56, respectively, of the postweaning period, 731 for mean IGF-I concentration of each calf, and 731 for all performance traits except birth weight (n = 838) and weight at day 42 of the postweaning period (n = 603). Fewer observations were available at day 42, because weights and blood samples were not taken at that time for calves born in Spring 1989. In addition, serum samples for heifers born in Spring 1990 were damaged by a freezer malfunction, which necessitated resampling of the heifers on days 84, 98, and 112 of the postweaning period. On rare occasions, glass tubes were broken during centrifugation, which also contributed to differing numbers of observations for IGF-I concentration on days 28, 42, and 56. Average age of the calves included in this study at the beginning of the postweaning test period was 236 days for spring-born calves and 262 days for fall-born calves.
Serum Samples. Approximately 25 ml of blood was collected into sterile 16 mm x 150 mm glass tubes via jugular puncture of each animal. The blood was allowed to clot for 24 hours at 4oC. Serum was obtained by centrifugation (1,800 x g for 20 minutes) and frozen at -20oC until it was assayed.
Radioimmunoassay for Insulin-Like Growth Factor I. 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).
Statistical Analysis. Data were analyzed using a set of Multiple Trait, Derivative-Free, Restricted Maximum Likelihood (MTDFREML) computer programs written by Boldman et al. (1993). These programs were used to estimate (co)variance components using the animal model and derivative-free REML. Pedigrees of base population animals were traced back three generations to create the numerator relationship matrix. A total of 1,563 animals were included in the A-1 matrix. 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 effects (ram). The statistical model for this analysis included fixed effects of birth year of calf (1989, 1990, 1991, 1992, 1993, 1994), season of birth (spring vs fall), IGF-I selection line (high vs low), sex of calf (bull vs heifer), age of dam (2, 3, 4, 5 to 9, > 10), random animal, maternal genetic, maternal permanent environmental effects, and a covariate for age of calf (omitted in the analysis of birth weight).
IGF-I concentration on days 28, 42, and 56 of the postweaning test, as well as mean IGF-I concentration, were paired with birth weight, weaning weight, on-test weight, weight on days 28, 42, and 56 of the postweaning period, off-test weight, postweaning gain in a series of bivariate analyses to estimate the additive genetic correlation between traits 1 and 2 (rA1A2), the correlation between the additive genetic effect for trait 2 (IGF-I) and the maternal genetic effect for trait 1 (a weight or gain trait; rA2M1), the environmental correlation between traits 1 and 2 (rE1E2), and the phenotypic correlation between the two traits (rP1P2).
Estimates of hd2 were 0.42, 0.53, 0.71, and 0.48 for IGF-I concentration on days 28, 42, and 56 of the postweaning period and for mean IGF-I concentration, respectively (Table 1). These estimates were somewhat larger than those presented previously for mice (Blair et al., 1987, 1989; Baker et al., 1991), sheep (Morel et al., 1991) and cattle (Enns et al., 1991). On the other hand, Davis and Bishop (1991) reported heritability estimates of 0.25 ± 0.35, 0.62 ± 0.23, 0.68 ± 0.21, and 0.54 ± 0.27 for serum IGF-I concentration at 328, 354, 656, and 684 days of age, respectively, in identical twin crossbred heifers.
|Table 1. Estimates1 from (co)variance component analyses for serum IGF-I concentrations and performance traits.|
|IGF282||4,628||0.42||0.02||. . .||-0.97|
|IFG42||5,649||0.53||0.09||. . .||-0.79|
|IGF56||5,600||0.71||0.12||. . .||-0.86|
|Birth weight||22||0.48||0.20||. . .||-0.53|
|Day-42 weight||1,072||0.33||. . .||0.27||0.98|
|Postweaning gain||550||0.30||0.05||. . .||-1.00|
|1sigma p2 = phenotypic variance, hd2 = heritability (direct
hm2 = maternal heritability, c2 = proportion of phenotypic variance due to permanent environmental effect of dam,
ram = correlation between direct and maternal effects. 2IGF28, IGF42, and IGF56 = serum IGF-I concentration on days 28, 42, and 56, respectively, of the postweaning test.
3 Mean IGF-I = average of serum IGF-I measurements for a given calf.
Estimates of hd2 for weights and gains ranged from 0.28 for on-test weight to 0.49 for off-test weight (Table 1). These values agreed closely with average estimates from many studies as summarized by Woldehawariat et al. (1977) and agreed reasonably well with the average estimates presented in the reviews by Mohiuddin (1993) and Koots et al. (1994a). Results of this study indicated that the direct heritability of IGF-I is somewhat higher than that of birth weight, weaning weight, postweaning weight, and postweaning gain. Therefore, it should be possible to rapidly change serum IGF-I concentrations in beef cattle through selection.
Estimates of hm2 were 0.02, 0.09, 0.12, and 0.02 for IGF-I concentration on days 28, 42, and 56 of the postweaning period and for mean IGF-I concentration, respectively (Table 1). Maternal heritabilities were lower than direct heritabilities, indicating that postweaning IGF-I concentration was determined more by the genetics of the calf than by the genetics for maternal ability of the dam. Davis et al. (1995) found that age of dam effects were not important (P > 0.23) for any of the measures of IGF-I in these data. Age of dam effects that may have influenced serum IGF-I concentration during the preweaning period were largely attenuated by the onset of the postweaning period. No literature estimates for maternal heritability of IGF-I were found. Estimates of hm2 for weight and gain traits varied from zero for weight on day 42 of the postweaning test to 0.20 for birth weight. Mohiuddin (1993) and Koots et al. (1994a) also found low maternal heritabilities for birth, weaning, and yearling weight.
The proportion of phenotypic variance due to permanent environmental effects of the dam (c2) was essentially zero for all measures of postweaning IGF-I concentration (Table 1). Previous estimates of c2 were not found in the literature for IGF-I. Estimates of c2 were zero for postweaning gain and 0.14 for off-test weight but ranged from 0.23 to 0.27 for weights at other ages. Mohiuddin (1993) found that c2 averaged 0.03, 0.07, and 0.03 for birth, weaning, and yearling weight, respectively.
Correlations between direct and maternal effects were large and negative for all measures of IGF-I and for all weight and gain traits, other than weight on day 42 of the postweaning period (Table 1). No other estimates of ram were found in the literature for IGF-I. Mohiuddin (1993) found that ram averaged -0.35, 0-.15, and -0.26 for birth, weaning, and yearling weight, respectively. Koots et al. (1994b) reported that ram averaged -0.27 for birth weight and -0.30 for weaning weight.
Genetic, phenotypic, and environmental correlations among IGF-I measurements taken on days 28, 42, and 56 of the postweaning performance test are shown in Table 2. Genetic correlations among the three IGF-I concentrations were 1.0, 0.91, and 0.99, indicating that a high proportion of the genes involved in determining serum IGF-I concentrations on days 28, 42, and 56 were the same. This result is not surprising given the short time span between the three IGF-I measurements. Environmental correlations among the IGF-I measurements were not as large as the genetic correlations and averaged 0.56. Phenotypic correlations among the IGF-I concentrations were 0.69, 0.68, and 0.69.
|Table 2. Genetic, environmental, and phenotypic correlations1 among IGF-I measurements on days 28 (IGF28), 42 (IGF42), and 56 (IGF56) of the postweaning performance test.|
|IGF42||rA1A2||. . .||0.99|
|rE1E2||. . .||0.55|
|rP1P2||. . .||0.69|
|1 rA1A2 = additive genetic correlation between traits 1
rE1E2 = environmental correlation between traits 1 and 2.
rP1P2 = phenotypic correlation between traits 1 and 2.
Genetic, phenotypic, and environmental correlations of IGF-I with performance traits are shown in Table 3. The genetic correlation of birth weight with serum IGF-I concentration on days 28, 42, and 56 of the postweaning period and with mean IGF-I concentration were -0.79, -0.70, -0.77, and -0.75, respectively, indicating that many of the genes that resulted in heavier birth weights also resulted in lower postweaning serum IGF-I concentrations. Genetic correlations of the various IGF-I measurements with weaning weight, postweaning weights, and postweaning weight gain ranged from -0.21 to -0.54 and averaged -0.38. These results implied that, if the goal is to make genetic improvement in weaning weights, postweaning weights, and/or postweaning weight gain in beef cattle, selection should be for decreased blood serum IGF-I concentration during the postweaning period.
The correlation between the additive genetic effect for IGF-I and the maternal genetic effect for weights and gains was positive for all trait combinations examined other than for the combination of IGF-I concentration on day 42 of the postweaning test and weaning weight. Across all bivariate analyses, rA2M1 averaged 0.51, implying that many of the genes that resulted in increased serum IGF-I concentrations also resulted in increased maternal effects for weight and gain traits.
The environmental correlation between IGF-I measurements and performance traits ranged from 0.10 to 0.35 and averaged 0.22. It was not surprising that environmental improvements resulted in increased serum IGF-I values, as well as increased weights and gains. It is well known that diet, including energy and protein content, influences plasma and serum IGF-I concentration in cattle (Breier et al., 1986; Anderson et al., 1988; Houseknecht et al., 1988; Ronge et al., 1988; Elsasser et al., 1989; Hays et al., 1995). In addition, Sarko et al. (1994) concluded that weather effects, including temperature and humidity, contribute to variation in serum IGF-I levels in postweaning beef calves, possibly due to their influence on feed intake.
Phenotypic correlations of IGF-I concentration on days 28, 42, and 56 of the postweaning period and of mean IGF-I concentration with birth weight were -0.14, -0.19, -0.17, and -0.16, respectively. Therefore, increased birth weights were associated with slight phenotypic decreases in postweaning serum IGF-I concentrations. Phenotypic correlations of IGF-I concentrations with weaning weight and postweaning weights and gains ranged from -0.01 to 0.12 and averaged 0.04, indicating little phenotypic association of endocrine IGF-I with weights and gains.
Results of this study indicated that postweaning blood serum IGF-I concentration is highly heritable and has moderate to high negative genetic correlations with weight and gain traits that are of economic importance in beef cattle. Therefore, serum IGF-I concentration may be a useful physiological indicator trait that could be used to enhance rates of genetic change in beef cattle.
|Table 3. Genetic, environmental, and phenotypic correlations1 of IGF-I measurements with performance traits.|
|rA2M1||0.56||0.67||0.70||0.83||1.00||0.93||. . .||. . .|
|rP1P2||-0.14||0.07||0.06||0.11||0.12||0.09||0.05||. . .|
|rA2M1||0.27||-0.08||0.14||0.21||0.88||0.34||. . .||. . .|
|rP1P2||-0.19||0.06||0.03||0.06||0.05||0.05||. . .||0.03|
|rA2M1||0.48||0.23||0.27||0.47||0.93||0.50||. . .||. . .|
|rA2M1||0.40||0.28||0.32||0.50||0.94||0.56||. . .||. . .|
|1 rA1A2 = additive genetic correlation between traits 1 and 2, rA2M1 = correlation between additive genetic effect for trait 2 (IGF-I) and maternal
effect for trait 1 (performance trait), rE1E2 = environmental correlation between traits 1 and 2, rP1P2 = phenotypic correlation between traits 1 and 2.
2 IGF28, IGF42, and IGF56 = serum IGF-I concentration on days 28, 42, and 56, respectively, of the postweaning test.
3 Mean IGF-I = average of serum IGF-I measurements for a given calf.
The authors wish to thank R.M. McConnell, J.D. Wells, J.W. Karr, L.L. Mizer, and F.J. Michel for their excellent technical assistance.
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