3241 1 Technical contributions of A.R. Rieke, C. M. Powell, J. A. Sterle, and M. D. Woltmann are gratefully acknowledged by the authors. Research supported by a grant from the Missouri Pork Producers Association. Contribution from the Missouri Agric. Exp. Sta. journal series no. 12,317. 2 Mention of a trade name or proprietary product does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products that may also be suitable. 3 Present address: Roman L. Hruska Meat Animal Research Center, P.O. Box 166, Clay Center, NE 68933. 4 Present address: Osborne Industries, Inc., P.O. Box 388, Osborne, KS 67473. Received May 10, 1995. Accepted July 14, 1995. Relationships of Serum Insulin-Like Growth Factor I Concentrations to Growth, Composition, and Reproductive Traits of Swine 1,2 W. R. Lamberson*, T. J. Safranski* ,3 , R. O. Bates* ,4 , D. H. Keisler*, and R. L. Matteri † *Department of Animal Sciences, University of Missouri, Columbia 65211 and † ARS, USDA, Columbia, Missouri 65211 ABSTRACT: Insulin-like growth factor I (IGF-I) is a peptide hormone that has been shown to be involved in metabolic regulation of growth and reproduction in livestock species. The objectives of this study were to quantify concentrations of IGF-I in growing pigs and determine whether IGF-I concentration can be used as a predictor of growth, composition, and reproductive traits. Forty male and 60 female pigs, divided equally between two locations, were weighed and bled at 3-wk intervals from 6 to 21 wk of age. At each sampling, two blood samples were collected via jugular venipuncture at an interval of at least 1 h. Serum was separated and IGF-I concentration determined via RIA. Pigs were weighed at each sampling date. Backfat and longissimus muscle area were measured with the use of B-mode ultrasound and adjusted to 100 kg. Age at puberty and first-parity litter size were measured on gilts. Effects of age, sex, location, and pig within sex × location on log-transformed IGF-I concen- trations were determined by analyzing data as a split- plot. Performance traits were fitted to a model including the effects of IGF-I concentration, sex, location, and interactions. The IGF-I concentrations increased ( P < .05) from 3 to 18 wk of age before dropping at 21 wk of age. Concentrations increased more rapidly in males than in females and differed significantly between sexes from 12 to 21 wk of age. Repeatability of IGF-I concentration was .29 ± .02; IGF-I concentrations of samples collected at 6 wk were not correlated with those at later ages. Correlations between IGF-I concentrations of samples at later ages ranged from .27 to .51. Heritability of IGF-I concentra- tion was .27 ± .07. There was a tendency for weight to be affected by a sex × age interaction ( P = .09). Weight of boars exceeded weight of gilts only at 21 wk (111.4 ± 1.1 vs 107.1 ± .8 kg). Regressions of weight on IGF-I concentrations were positive at all ages but greatest at 6 wk. The IGF-I concentration did not affect backfat thickness, longissimus area, percentage of lean, age at puberty, or litter size. Key Words: Pigs, Insulin-Like Growth Factor, Growth, Composition, Reproduction J. Anim. Sci. 1995. 73:3241–3245 Introduction Within-herd selection is an important tool for improvement of livestock populations. Growth and compositional traits have been found to be moderately to highly heritable and are expected to respond to selection (Stewart and Schinckel, 1990). Phenotypic trends have suggested that improvement has occurred at rates less than those desired and expected (McLa- ren et al., 1990). Genetic change has been shown to be in good agreement with that predicted based on observed selection differentials. The cause for poor response has thus been shown to be low selection differentials (David et al., 1985). Low selection differentials may result from selection emphasis being divided among many traits or failure by producers to measure all animals. The desire to limit the number of males kept intact until performance traits can be measured may contribute to the latter. Development of a criterion that is measurable early in life and is related to future growth and compositional perfor- mance would aid in selection. Insulin-like growth factor I is a hormone involved in regulation of growth (Daughaday and Rotwein, 1989). It has previously been shown in mice to be a selection criterion with Published December 11, 2014