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Effects of divergent selection for serum IGF-I concentration on performance, feed efficiency and ultrasound measures of carcass composition traits in Angus bulls and heifers

P. A. Lancaster^*, G. E. Carstens^*, F. R. B. Ribeiro^*, M. E. Davis, J. G. Lyons^* and T. H. Welsh, Jr.^*

^* Department of Animal Science, Texas A&M University, College Station, 77843 Department of Animal Sciences, The Ohio State University, Columbus, 43210

g-carstens{at}tamu.edu

Abstract

Angus bulls and heifers from lines divergently selected for serum insulin-like growth factor-I (IGF-I) concentration were used to evaluate the effects of IGF-I selection line on growth performance and feed efficiency in 2 studies. In study 1, bulls (low line n = 9; high line n = 8; initial BW = 367.1 ± 22.9 kg) and heifers (low line n = 9; high line n = 13; initial BW = 286.4 ± 28.6 kg) were adapted to a roughage-based diet (ME = 1.95 Mcal/kg DM) for 24 d and fed individually for 77 d using Calan gate feeders. In study 2, bulls (low line n = 15; high line n = 12; initial BW = 297.5 ± 34.4 kg) and heifers (low line n = 9; high line n = 20; initial BW = 256.0 ± 25.1 kg) were adapted to a grain-based diet (ME = 2.85 Mcal/kg DM) for 32 d and fed individually for 70 d using Calan gate feeders. Blood samples were collected at weaning and at the start and end of each study and serum IGF-I concentration was determined. Residual feed intake (RFI) was calculated, within study, as the residual from the linear regression of DMI on mid-test BW^0.75 (MBW), ADG, gender, gender by MBW and gender by ADG. In study 1, calves from the low IGF-I selection line had similar initial and final BW and ADG, compared to calves from the high IGF-I selection line. In addition, DMI and feed conversion ratio (FCR) were similar between IGF-I selection lines, however, calves from the low IGF-I selection line tended (P < 0.10) to have lower RFI than calves from the high IGF-I selection line (-0.26 vs. 0.24 ± 0.31 kg/d). In study 2, IGF-I selection line had no influence on performance or feed efficiency traits. However, there was a tendency (P = 0.15) for an IGF-I selection line by gender interaction for RFI. Bulls from the low IGF-I selection line had numerically lower RFI than those from the high IGF-I selection line, whereas, in heifers IGF-I selection line had no effect on RFI. In studies 1 and 2, weaning and initial IGF-I concentrations were not correlated with either FCR or RFI. However, regression analysis revealed a gender by IGF-I concentration interaction for initial IGF-I concentration in study 1 and weaning IGF-I concentration in study 2 such that the regression coefficient was positive for bulls and negative for heifers. These data suggest that genetic selection for postweaning serum IGF-I concentration had minimal effect on RFI in beef cattle.

Key Words: beef cattle • carcass composition • divergent selection • IGF-I • residual feed intake

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