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,5
* Department of Animal Sciences, The Ohio State University, Columbus 43210-1095 and
and
Department of Animal Science, University of Florida, Gainesville 32611-0901
4 Correspondence— phone: 614-292-4984; fax: 614-292-7116; E-mail: Davis.28{at}osu.edu.
A divergent selection experiment for serum IGF-I concentration began at the Eastern Ohio Resource Development Center 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. Following weaning, bull and heifer calves were fed in drylot for a 140-d period. Real-time ultrasound measurements of backfat thickness and longissimus muscle area were taken on d 56 and 140 of the postweaning test. Only ultrasound data from calves born from fall 1995 through spring 1999 were included in the analysis. At the time of this study, IGF-I measurements were available for 1,521 bull and heifer calves, and ultrasound data were available for 636 bull and heifer calves. Data were analyzed by multiple-trait, derivative-free, restricted maximum likelihood methods. Estimates of direct heritability for IGF-I concentration at d 28, 42, and 56 of the postweaning period, and for mean IGF-I concentration were 0.26 ± 0.07, 0.32 ± 0.08, 0.26 ± 0.07, and 0.32 ± 0.08, respectively. Direct heritabilities for ultrasound estimates of backfat thickness ranged from 0.17 ± 0.11 to 0.28 ± 0.12, whereas direct heritabilities for longissimus muscle area ranged from 0.20 ± 0.10 to 0.36 ± 0.12, depending on the time of measurement and the covariate used for adjustment (age vs. weight). Direct genetic correlations of IGF-I concentrations with backfat thickness at d 56 and 140 and with longissiumus muscle area at d 56 and 140 averaged 0.02, 0.20, -0.08, and 0.23, respectively, when age was used as the covariate for both IGF-I and ultrasound measurements. Corresponding genetic correlations when age was used as the covariate for IGF-I and weight was used as the covariate for ultrasound measurements were 0.05, -0.07, -0.22, and -0.04, respectively. Therefore, the positive associations of serum IGF-I concentration with backfat thickness and longissimus muscle area at d 140 seem to have been partially mediated by weight. Results of this study do not indicate strong associations of serum IGF-I concentration with fat thickness or muscling of bulls and heifers during the postweaning feedlot period.
Key Words: Beef Cattle • Body Composition • Genetic Parameters • Insulin-Like Growth Factor • Ultrasound • Selection
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