RELATIONSHIPS OF SERUM INSULIN-LIKE GROWTH-FACTOR-I CONCENTRATIONS TOGROWTH, COMPOSITION, AND REPRODUCTIVE TRAITS OF SWINE

Insulin-like growth factor I (IGF-I) is a peptide hormone that has bee n shown to be involved in metabolic regulation of growth and reproduct ion in Livestock species. The objectives of this study were to quantif y concentrations of IGF-I in growing pigs and determine whether IGF-I concentration can be used as a predictor of growth, composition, and r eproductive traits. Forty male and 60 female pigs, divided equally bet ween 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 j ugular venipuncture at an interval. of at least 1 h. Serum was separat ed and IGF-I concentration determined via RIA. Rigs were weighed at ea ch sampling date. Backfat and longissimus muscle area were measured wi th 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 x location on log-transformed IGF-I concentrations were determined by analyzing data as a split-plot. Perf ormance traits were fitted to a model including the effects of IGF-I c oncentration, sex, location, and interactions. The IGF-I concentration s 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. Rep eatability of IGF-I concentration was .29 +/- .02; IGF-I concentration s of samples collected at 6 wk were not correlated with those at later ages. Correlations between IGF-I concentrations of samples at later a ges ranged from .27 to .51. Heritability of IGF-I concentration was .2 7 +/- .07. There was a tendency for weight to be affected by a sex x a ge interaction (P = .09). Weight of boars exceeded weight of gilts onl y at 21 wk (111.4 +/- 1.1 vs 107.1 +/- .8 kg). Regressions of weight o n 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.