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ANIMAL GENETICS |
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* Department of Animal Sciences, The Ohio State University, Columbus 43210-1095;
and
Department of Animal Science, University of Florida, Gainesville 32611-0901
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Key Words: beef cattle • genetic parameter • growth • insulin-like growth factor • selection
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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; Murphy et al., 1987). Elimination of circulating IGF-I by deletion of the IGF-I gene in mice results in severe growth retardation (Sjogren et al., 1999). Virtually no growth hormone-induced growth was observed in IGF-I –/– mice, a finding that supports the view that IGF-I directly mediates the effects of growth hormone (Wang et al., 1999). The Cre/loxP re-combination system was used to delete the IGF-I gene in the livers of mice (Sjogren et al., 1999; Yakar et al., 1999). The authors reported a 75% reduction in IGF-I concentration, but little effect on postnatal body growth, and concluded that IGF-I influences body growth primarily in an autocrine/paracrine manner, rather than in an endocrine manner. However, Davis and Simmen (1997) found direct additive genetic correlations of circulating IGF-I with BW and BW gains that ranged from –0.21 to –0.54 and averaged –0.38 in lines of beef cattle divergently selected for serum IGF-I concentration. Studies of the relationship between growth and IGF-I concentration conducted at Colorado State University in the 1980s appeared to indicate that IGF-I was more reflective of growth that had already occurred than predictive of subsequent growth (J. S. Brinks, Colorado State University, Fort Collins, personal communication). Therefore, the objective of the current study was to calculate genetic, phenotypic, and environmental correlations of IGF-I with preweaning and post-weaning growth during specified time periods in Angus cattle divergently selected for postweaning serum IGF-I concentration to determine if IGF-I is more highly correlated with growth that has already occurred or with future growth and to further elucidate the role of endocrine IGF-I. |
MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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; Davis and Simmen, 1997). In brief, the 4 bull calves with the greatest and the 4 with the lowest residuals (adjusted for age of calf and age of dam) for IGF-I concentrations were saved each year for breeding within the respective selection lines. Selection was based on the mean IGF-I concentration of 3 blood samples (taken at d 28, 42, and 56 of the 140-d postweaning test). Yearling bulls were used for breeding to minimize the generation interval. Approximately 8 cows were culled from each line each year (based on physical unsoundness, reproductive failure, and oldest age) and replaced with approximately 8 pregnant heifers with the greatest or lowest residuals for serum IGF-I concentrations. Selection of heifers was based on the mean IGF-I concentration of 3 blood samples collected at the same time as for the bulls. All available heifers were bred, and selections were made among heifers that conceived.
Mating Scheme
Selected heifers within the high IGF-I line were stratified such that 1 of the 4 heifers with the greatest IGF-I concentrations, 1 of the 4 heifers with the next greatest concentrations, and so on, were assigned to each bull. The same mating procedure was followed in the low line using males and females with the lowest IGF-I concentrations. This mating scheme was intended to increase the probability of producing at least 1 replacement bull and 2 replacement heifers from each sire each year. Cows 2 yr of age and older were randomly assigned to bulls for mating. To minimize the increase in inbreeding, half-sib and closer matings were avoided for heifers and cows.
Management Scheme
Spring-born calves were reared by their dams without creep feed until weaning at approximately 7 mo of age. After weaning, bull calves were given ad libitum access to a cornsoybean meal-based concentrate diet, plus grass hay in large round bales. Heifers born from spring 1989 through fall 1993 were given ad libitum access to NPN (feed grade urea)-treated corn silage, in addition to grass hay in large round bales. Heifers born in spring 1994 and later were fed a corn-soybean meal diet designed to result in postweaning BW gains of approximately 0.75 kg/d. Throughout the study, bulls and heifers were allowed 2 to 3 wk to adjust to the postweaning diet and then were fed for a 140-d test period. Bulls were fed in a 3-sided barn with adjoining exercise lots located at EARS. Heifers born from spring 1989 through fall 1993 were fed in a drylot with access to an enclosed barn located at the North Appalachian Experimental Watershed, Coshocton, OH. Heifers born in spring 1994 and later were fed in a 3-sided barn with adjoining exercise lots located at EARS.
Fall-born calves were weaned at approximately 140 d of age and then fed a corn-soybean meal diet formulated to yield BW gains of 0.9 kg/d, plus grass hay, in drylot for 112 d. Fall-born calves were weighed near the end of the 112-d growing period. This weight was used as a weaning weight in the data analysis so that the weight of the fall-born calves was taken at a similar age as for the spring-born calves. After the 112-d growing period, bull calves remained at EARS and were managed in the same manner as spring-born bulls. Heifer calves born during fall 1993 and previously were transported to the North Appalachian Experimental Watershed and managed in the same manner as spring-born heifers. Heifers born during fall 1994 and later remained at EARS and were fed the same diet as spring-born heifers. Average on-test age of all spring- and fall-born calves combined was 251 d (SD = 19 d; Table 1).
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Collection of Blood Samples
Approximately 25 mL of blood was collected into sterile 16 x 150-mm glass tubes via jugular venipuncture of each animal. The blood was allowed to clot for 24 h at 4 ° C. Serum was obtained by centrifugation (1,800 x g for 20 min) and frozen at –20 ° C until it was assayed.
Determination of IGF-I Concentrations
The RIA for IGF-I concentrations was performed at the University of Florida using antiserum raised against human IGF-I in rabbits (UBK487), following procedures described by Bishop et al. (1989). There was no interassay CV for this assay because all samples from the different collection times (i.e., d 28, 42, and 56) were run in a single assay. The raw data from the assays were lost, and, therefore, it was not possible to calculate the interassay CV. Bishop (1991) reported intra- and interassay CV of 10.2 and 14.3%, respectively, using data from the early years of the IGF-I selection experiment.
Statistical Analysis
Data were analyzed using an animal model with a set of multiple-trait, derivative-free, REML computer programs written by Boldman et al. (1993). Pedigrees of base population animals were traced back 3 generations to create the numerator relationship matrix. A total of 2,674 animals were included in the A–1 matrix.
First, IGF-I concentrations at the various time points, BW, and BW gains were 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 model for these analyses included fixed effects of birth year of calf (1989 through 2000), season of birth (spring vs. fall), IGF-I selection line (high vs. low), sex of calf (bull vs. heifer), and age of dam (2, 3, 4, 5 to 9, 
10 yr), and random animal, maternal genetic, and maternal permanent environmental effects, as well as a linear covariate for age of calf (omitted in analysis of birth weight). Maternal genetic and maternal permanent environmental effects were deleted from the final model if determined by likelihood ratio tests to be unimportant sources of variation for a given trait. Second, IGF-I concentrations at the various time points were paired with BW and BW gains in a series of bivariate analyses using the reduced model for each trait to estimate the correlations between the direct and maternal components of the traits, as well as the environmental and phenotypic correlations among the traits.
In the univariate and bivariate analyses, convergence was defined as the point where the variance of the simplex was less than 10–9. "Cold restarts" of the multiple-trait, derivative-free REML programs were performed using the converged values to ensure that the log likelihood was the global, and not a local, maximum. Restarts were performed until –2 times the log likelihoods used in the simplex search algorithm did not change to the second decimal place from one restart to the next. Approximate SE were derived for the genetic correlations using the formula of Falconer and Mackay (1996).
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Estimates of heritabilities obtained for serum IGF-I concentrations are shown in Table 2. Direct heritability for IGF-I concentrations at d 28, 42, and 56 of the post-weaning test and for mean IGF-I concentrations was 0.44 ± 0.07, 0.51 ± 0.08, 0.42 ± 0.07, and 0.52 ± 0.08, respectively. These estimates agree well with those obtained by Davis and Simmen (1997) using data from earlier years of the same selection experiment, with the exception that the estimate at d 56 was lower than the previous estimate of 0.71 ± 0.16. The estimates are somewhat larger than the values of 0.31 ± 0.18 observed by Herd et al. (1995) at birth, 0.32 ± 0.06 by Johnston et al. (2001) at 8 to 10 mo of age, 0.34 ± 0.09 and 0.43 ± 0.12 by Johnston et al. (2002) at 9 and 22 mo, respectively, in cattle fed at 2 different locations in Australia, and 0.32 ± 0.06 at weaning (average age = 201 d) and 0.30 ± 0.07 during the postweaning period (average age = 310 d) by Moore et al. (2005) in Angus seedstock cattle fed on pasture in Australia. Results from the present and previous studies provide ample evidence that serum IGF-I concentration is moderately to highly heritable in beef cattle.
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for BW ranged from 0.32 ± 0.07 for birth weight to 0.52 ± 0.08 for weight at d 84 of the postweaning test (Table 2). Estimates of for gain from birth to weaning, weaning to d 0, and d 0 to 140 of the postweaning period were 0.47 ± 0.07, 0.52 ± 0.08, and 0.45 ± 0.09, respectively. These estimates of direct heritability of BW and BW gains generally agree well with average estimates from many studies as summarized by Woldehawariat et al. (1977), Mohiuddin (1993), and Koots et al. (1994a). Postweaning BW gains by 14-or 28-d periods were less heritable than weight gain during the entire 140-d postweaning period, likely due to sampling effects and additional background "noise" during the short time intervals. Results of the current study indicated that the direct heritability of IGF-I was similar to that of weaning weight, postweaning weight, and pre- and postweaning gain. Therefore, it should be possible to rapidly change serum IGF-I concentrations in beef cattle via selection.
Estimates of 
were 0.17 ± 0.06, 0.20 ± 0.06, 0.10 ± 0.05, and 0.19 ± 0.06 for IGF-I concentrations at d 28, 42, and 56 of the postweaning test, and for mean IGF-I concentrations, respectively (Table 2). Although these estimates are somewhat larger than those reported by Davis and Simmen (1997), they still indicate that postweaning IGF-I concentration is determined more by the genetic characteristics of the calf than by those of the dam. Estimates of maternal heritabilities of weight traits varied from 0.06 ± 0.04 for off-test weight to 0.17 ± 0.16 for weaning weight. Mohiuddin (1993) and Koots et al. (1994a) also found low maternal heritabilities for birth, weaning, and yearling weight. In the current study, estimates of were 0.18 for gain from birth to weaning, 0.15 for gain from weaning to d 0 of the postweaning test, and 
0.11 for weight gain during various portions of the postweaning test.
In agreement with the results of Davis and Simmen (1997), the proportion of phenotypic variance due to permanent environmental effects of the dam (c2) was essentially zero for all measures of IGF-I (Table 2). Estimates of c2 were near zero for birth weight, 0.22 ± 0.04 for weaning weight, 0.21 ± 0.04 for on-test weight, and then declined throughout the postweaning period to a low of 0.10 ± 0.03 for off-test weight. In his review, Mohiuddin (1993) found that estimates of c2 averaged 0.03, 0.07, and 0.03 for birth, weaning, and yearling weight, respectively.
Correlations between direct and maternal genetic effects were large and negative for all measures of IGF-I (ram – 0.89) and for all weight (ram – 0.68) and gain (ram – 0.71) traits other than birth weight (Table 2). Mohiuddin (1993) reported that ram averaged – 0.35, – 0.15, and – 0.26 for birth, weaning, and yearling weight, respectively, in the studies that he summarized. Koots et al. (1994b) found that ram averaged – 0.27 for birth weight and –0.30 for weaning weight. The large direct-maternal correlations in the current study may have been due, in part, to the relatively small maternal variances, which might have been a result of the selection criterion employed (i.e., serum IGF-I concentrations during the postweaning period). The direct-maternal covariances and the proportions of total phenotypic variance accounted for by these covariances are shown in Table 2. The covariance between direct and maternal genetic effects accounted for less than 30% of the total variance in each trait, even though the values for ram were large. The large direct-maternal correlations may also have been an artifact of the divergent selection for IGF-I.
Genetic, phenotypic, and environmental correlations among IGF-I measurements taken at d 28, 42, and 56 of the postweaning period, and mean IGF-I, are shown in Table 3. Genetic correlations among the d 28, 42, and 56 IGF-I concentrations were 0.87 ± 0.03, 0.89 ± 0.02, and 0.86 ± 0.03, respectively, indicating that many of the same genetic mechanisms were involved in determining serum IGF-I concentrations at these 3 time points. This result is not surprising given that the 3 measurements were taken 2 wk apart. Davis and Simmen (1997) reported genetic correlations of 1.0, 0.91, and 0.99 among IGF-I measurements at these same time points. Moore et al. (2005) found a genetic correlation of 1.0 ± 0.04 between IGF-I concentrations measured at weaning (average age = 201 d) and during the postweaning period (average age = 310 d) in Australian Angus seedstock cattle fed on pasture. In the current study, environmental correlations among the d 28, 42, and 56 IGF-I concentrationss were 0.71, 0.54, and 0.66, respectively. Phenotypic correlations among the IGF-I measurements were intermediate to the genetic and environmental correlations. Genetic, environmental, and phenotypic correlations of IGF-I at d 28, 42, and 56 with the mean IGF-I concentration for each calf were large, as expected, due to the part-whole relationship between the mean and its components, and agreed closely with the correlations presented by Davis and Simmen (1997). Given the large genetic correlations observed in the current study and by Davis and Simmen (1997) and Moore et al. (2005), as well as the magnitude of the heritability of IGF-I (Table 2), a single IGF-I measurement should be sufficient for selection purposes, rather than using the average of multiple measurements for selection.
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. Genetic correlations of birth weight with serum IGF-I concentrations at d 28, 42, and 56 of the postweaning period and with mean IGF-I concentrations were –0.30 ± 0.12, –0.19 ± 0.13, –0.33 ± 0.12, and –0.24 ± 0.12, respectively. These correlations were of the same sign, but lower in magnitude, than those derived using data from earlier years of the same selection experiment (Davis and Simmen, 1997). Genetic correlations of weaning weight with IGF-I concentrations at d 28, 42, and 56 and with mean IGF-I were 0.15 ± 0.11, 0.12 ± 0.11, 0.11 ± 0.11, and 0.11 ± 0.10, respectively. Genetic correlations of the various IGF-I measurements with postweaning BW ranged from 0.07 ± 0.12 to 0.32 ± 0.11 and generally demonstrated an increasing trend from d 0 to 140 of the postweaning test. These results are in contrast to those of Davis and Simmen (1997) who reported that genetic correlations of IGF-I concentrations with weaning weight, postweaning BW, and postweaning weight gain ranged from –0.21 to –0.54 and averaged –0.38. Results might have differed between the study of Davis and Simmen (1997) and the current study because of sampling effects or differences in statistical models used to generate parameter estimates. In addition, genetic correlations are strongly influenced by gene frequencies (Bohren et al., 1966; Falconer and Mackay, 1996), which might have changed over time within the IGF-I selection lines because of selection applied or random drift. Estimates of genetic correlations between circulating concentrations of IGF-I and BW at different ages have also varied in the literature. Johnston et al. (2001) found a genetic correlation of 0.11 ± 0.14 between IGF-I and postweaning live weight. On the other hand, Johnston et al. (2002) observed genetic correlations of –0.25 ± 0.25 and 0.03 ± 0.14 between IGF-I concentrations and midtest weight in groups of cattle fed at 2 different locations in Australia. Moore et al. (2005) reported direct genetic correlations of IGF-I with birth weight, 200-d weight, and 400-d weight of –0.22 ± 0.08, –0.17 ± 0.09, and –0.10 ± 0.14, respectively.
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). Genetic correlations between the direct component of IGF-I and maternal components of birth weight and 200-d weight were 0.15 ± 0.13 and 0.31 ± 0.11, respectively, in the study by Moore et al. (2005). The authors therefore suggested that selection for lower IGF-I concentrations might result in decreased maternal weaning weight. On the other hand, Herd et al. (2002) found evidence of a negative relationship between plasma IGF-I concentrations measured at the end of the post-weaning test and the maternal components of birth weight and weaning weight, indicating that selection against IGF-I should lead to improved maternal nutrition to the calf before and after birth. The correlation between the additive genetic effect for BW at various ages and maternal genetic effect for IGF-I was negative for all trait combinations and averaged –0.11, –0.14, –0.05, –0.16, –0.25, –0.27, –0.32, –0.32, and –0.39 for birth and weaning weight, and for weight at d 0, 28, 42, 56, 84, 112, and 140, respectively, implying that genes that result in increased BW also result in decreased maternal effects for serum IGF-I concentrations.
Correlations between the maternal genetic effects for IGF-I concentrations and weight traits were generally positive for all trait combinations other than for the combination of birth weight and IGF-I (Table 4) and averaged 0.13, 0.08, 0.19, 0.24, 0.25, 0.32, 0.37, and 0.43 for weaning weight and for weight at d 0, 28, 42, 56, 84, 112, and 140, respectively, indicating that genes that result in increased maternal effects for IGF-I concentrations also result in increased maternal effects for weaning and postweaning BW. This relationship strengthened with age and was greatest at the conclusion of the postweaning period.
Environmental correlations between IGF-I measurements and weight traits were generally positive, but small, averaging 0.01, 0.02, 0.09, 0.09, 0.13, 0.12, 0.12, 0.09, and 0.08 for birth weight, weaning weight, and weight at d 0, 28, 42, 56, 84, 112, and 140, respectively (Table 4). Therefore, a slight tendency existed for environmental improvements that resulted in increased serum IGF-I concentrations to also result in increased BW. Numerous studies have shown that diet, including energy and protein content, influences plasma and serum IGF-I concentrations in cattle (e.g., Breier et al., 1986; Anderson et al., 1988; Houseknecht et al., 1988). In fact, concentration of circulating IGF-I provides an objective indicator of nutritional status in beef cattle (Roberts et al., 1997, 2005).
Phenotypic correlations of IGF-I concentrations at d 28, 42, and 56 of the postweaning period and of mean IGF-I concentrations with birth weight were –0.07, –0.09, –0.08, and –0.08, respectively (Table 4). These correlations were of the same sign, but slightly smaller in magnitude, compared with the values reported by Davis and Simmen (1997). The average phenotypic correlation of IGF-I concentrations with weaning weight was 0.07. Average phenotypic correlations of IGF-I with postweaning BW ranged from 0.10 for on-test weight (i.e., weight at d 0) to 0.18 for weight at d 84 of the postweaning period. Therefore, only a weak phenotypic association was observed between endocrine IGF-I and BW from birth to the conclusion of the 140-d postweaning test. These results were consistent with those of Davis and Simmen (1997). Moore et al. (2005) found phenotypic correlations of the direct component of IGF-I with the direct components of birth weight, 200-d weight, and 400-d weight of –0.10, 0.06, and 0.16, respectively, which agree closely with the phenotypic correlations from our study.
Genetic, environmental, and phenotypic correlations of serum IGF-I concentrations with weight gain during various time intervals are shown in Table 5. Additive genetic correlations of IGF-I concentrations with BW gains were positive during all intervals, except between weaning and the beginning of the postweaning test, during which time the calves were making the transition from nursing their dams to a high concentrate diet, and from d 84 to 112 of the postweaning period. Averaged across the 4 measures of IGF-I, the genetic correlation of IGF-I concentrations with weight gain from birth to weaning was 0.14, whereas the correlation with gain from d 0 to 140 of the postweaning test was 0.29. These results are in contrast to those of Davis and Simmen (1997) who obtained genetic correlations between IGF-I concentrations and postweaning gain that averaged –0.37 using data from earlier years of the same selection experiment. Johnston et al. (2001) reported a genetic correlation of –0.25 ± 0.20 between IGF-I concentrations and finishing average daily gain in temperate breeds of cattle in Australia. In addition, Johnston et al. (2002) obtained genetic correlations of –0.23 ± 0.32 and –0.20 ± 0.17 between IGF-I concentrations and ADG over postweaning tests conducted on 2 different groups of Australian cattle. Herd et al. (2002) also found evidence of a negative relationship between plasma IGF-I concentrations measured at the conclusion of a 70-d postweaning test and ADG during the test. Genetic correlations between IGF-I concentrations and ADG have also varied in sign and magnitude in pigs (e.g., see review by Bunter et al., 2002), perhaps because of sampling effects or perhaps because of the growth phase (phases of lean vs. fat accretion) during which the animals were performance-tested (Hermesch et al., 2001).
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). Values for rA1M2 within the various segments of the postweaning period varied in sign and generally were small in magnitude. The correlation between the additive genetic effect for weight gain and the maternal genetic effect for IGF-I tended to be negative for all time intervals except from weaning to d 0 and from d 84 to 112 of the postweaning test. When weight gain for the entire 140-d postweaning test was considered, the value for correlation between additive genetic effect for trait 2 (performance trait) and maternal effect for trait 1 (IGF-I; rA2M1), averaged over the 4 measures of IGF-I, was –0.37.
Correlations between the maternal genetic effects for IGF-I concentrations and weight gain from birth to weaning and from d 0 to 140 of the postweaning test averaged 0.15 and 0.32, respectively (Table 5). Values for correlation between maternal effects of traits 1 and 2 were positive for the first half of the postweaning test and negative for the last half of the test.
Weak positive environmental (rE1E2 < 0.15) and phe-notypic (rP1P2 < 0.20) relationships were observed between IGF-I concentrations and BW gains (Table 5). Davis and Simmen (1997) reported an average environmental correlation of 0.15 and an average phenotypic correlation of 0.02 between IGF-I concentrations and postweaning gain using data from earlier years of this study. Johnston et al. (2001) reported phenotypic correlations of 0.15 between IGF-I concentrations and post-weaning live weight and –0.01 between IGF-I and finishing average daily gain. Small, but significant, simple correlations were observed between IGF-I concentrations measured weekly and ADG from birth to weaning in male (r = 0.21; P < 0.001) and female (r = 0.12; P < 0.05) Hereford calves (Govoni et al., 2003).
One of the objectives of the current study was to calculate genetic, phenotypic, and environmental correlations of IGF-I with preweaning and postweaning growth during specified time periods to determine if IGF-I is more highly correlated with growth that has already occurred or with future growth, and to further elucidate the role of endocrine IGF-I. The largest genetic and phenotypic correlations between IGF-I concentrations at d 28, 42, and 56 of the postweaning period and weight gain occurred during the first 28 d of the 140-d postweaning test, possibly indicating that IGF-I was more reflective of past growth than indicative of future growth. However, genetic correlations of IGF-I concentrations with weight gain fluctuated too greatly over the various time intervals to allow definitive conclusions to be reached in this regard. Considerable random noise likely occurred within the shorter time frames and masked the true genetic relationships between IGF-I and weight gain.
Traditionally, feed efficiency in feedlot cattle has been measured as feed conversion ratio (i.e., feed intake:weight gain ratio) or its reciprocal. An alternative trait for expressing feed efficiency, first proposed by Koch et al. (1963), is residual feed intake (RFI), which is calculated as the difference between an animal’s actual feed intake and expected feed intake based on size and growth rate. The primary advantage of RFI is that selection for improved feed efficiency using this trait will reduce feed inputs without changing genetic potential for growth or liveweight. Identification of indicator traits that are predictive of RFI would be useful to reduce costs and improve accuracy of identifying cattle with superior RFI. Recent studies from Australia demonstrated that IGF-I is positively genetically correlated with feed efficiency and RFI (Johnston et al., 2002; Arthur et al., 2004; Moore et al., 2005). Therefore, selection should be directed toward reduced serum or plasma IGF-I concentrations to produce cattle with reduced RFI values (i.e., more efficient cattle). The small genotypic and phenotypic correlations of serum IGF-I concentrations with BW and BW gains obtained in the current study are desirable from the standpoint that selection for lower IGF-I concentrations, with the aim of improving RFI of a herd of cattle, could be practiced without significantly impacting growth rate or live-weight. In 2004 the Australian genetic improvement program known as BREEDPLAN began using both RFI test data and IGF-I measurements to estimate RFI breeding values for the Angus breed in Australia (Arthur et al., 2004).
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IMPLICATIONS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Footnotes |
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2 This experiment was a contributing project to North Central Regional Project NC-1010, "Interpreting Cattle Genomic Data: Biology, Applications, and Outreach."
3 The authors wish to thank W. D. Shriver, J. D. Wells, and F. J. Michel for their excellent technical assistance.
5 Current address: Department of Physiology and Biophysics, University of Arkansas for Medical Sciences and Senior Investigator in Developmental Biology, Arkansas Children’s Nutrition Center, Little Rock 72202.
4 Corresponding author: davis.28{at}osu.edu
Received for publication October 4, 2005. Accepted for publication February 15, 2006.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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