GENETIC PARAMETER ESTIMATES FOR SERUM INSULIN-LIKE GROWTH-FACTOR-I CONCENTRATION AND PERFORMANCE TRAITS IN ANGUS BEEF-CATTLE

Data for this study were obtained from an experiment involving diverge nt selection for blood serum IGF-I concentration in beef cattle. Multi ple trait derivative-free REML procedures were used to obtain genetic parameter estimates for IGF-I concentration at d 28, 42, and 56 of the postweaning period and for mean IGF-I concentration, as well as for w eights and gains. Included in the analysis were 1,563 animals in the A (-1) matrix, 731 of which had valid records for mean IGF-I concentrati on. Direct heritabilities (h(d)(2)) were .42 +/- .13, .53 +/- .15, .71 +/- .16, and .48 +/- .13 for IGF-I at d 28, 42, and 56 of the postwea ning period and for mean IGF-I, respectively. Heritability of maternal genetic effects (h(m)(2)) ranged from .02 to .12, whereas the proport ion of the total variance due to the maternal permanent environmental tal effect (c(2)) was essentially zero for all measures of IGF-I. Gene tic correlations of IGF-I with weaning and postweaning weights and wit h postweaning weight gain ranged from -.21 to -.54 and averaged -.38. The environmental correlation between IGF-I and performance traits var ied from .10 to .35 and averaged .22. Phenotypic correlations of IGF-I concentrations with weaning weight and postweaning weights and gains ranged from -.01 to .12 and averaged .04. Estimates of h(d)(2) indicat e that it should be possible to change IGF-I concentration in beef cat tle via selection. Negative genetic correlations imply that, if the go al is to make genetic improvement in weaning weights, postweaning weig hts and (or) postweaning gain in beef cattle, selection should be for decreased postweaning serum IGF-I concentration.