ASSOCIATION OF GENETIC PRODUCTION INDEXES WITH PRODUCTION OUTCOMES OF FEEDLOT STEERS
Genetic knowledge is increasingly utilized within the beef seedstock sector, yet uncertainty remains on how to maximize these inputs at the commercial level. The objectives of this study were to evaluate the effect of sorting natural and conventionally marketed fed beef cattle according to a genetic prediction index on performance and carcass traits, and determine the association of genetic indices to economically important production outcomes. The study hypothesis was that sorting beef cattle by genetic index would not influence the outcome of economically important traits. Two cattle sources were utilized, Angus and Angus × Hereford steers that did not receive growth promoting technologies or antimicrobials to comply with a natural marketing program (NAT; n=98; initial BW = 362 ± 11 kg) and Angus steers that were fed conventionally (CONV; n=81; initial BW = 364 ± 11 kg). Cattle were assigned to pens within source and stratified according to a feeder advantage (FA1) index from least to greatest by pen (NAT=10 steers/pen and 9 pens/treatment; CONV=10 or 11 steers/pen and 7 pens/treatment). All cattle were managed similarly and fed the same basal diet with the exception that NAT diets did not contain antimicrobial (monensin or tylosin) or growth promoting technologies (implants or beta-agonist). Individual cattle were harvested upon achieving a target shrunk (4%) BW of 635 kg. Genomic indices for WW and gain increased quadratically (P < 0.01) as the FA1 index increased for both cattle types. For conventionally finished cattle, genomic indices for heifer pregnancy (P ≤ 0.02) decreased whereas milk (P < 0.01), gain (P < 0.01), carcass weight (P = 0.03), marbling (P < 0.01), and ribeye area (P = 0.02) increased as the FA1 index increased. Daily dry matter feed offered (DDMFO) increased quadratically (P < 0.01) as FA1 index increased for both NAT and CONV. Within the NAT cattle, FA1 index was correlated to DOF (r = -0.39; P < 0.001), DDMFO (r = 0.35; P < 0.01), SUMDMO (r = -0.32; P < 0.01), day 0 BW (r = 0.30; P < 0.01), and ADG (r = 0.23; P < 0.05). Among CONV cattle, FA1 index was positively correlated (r = 0.44; P < 0.001) to DDMFO. The FA1 index score means did not differ (P = 0.08) amongst USDA QG’s, or USDA YG’s (P = 0.25). Numerically, NAT cattle in FA group 75 to 82 were greatest in HCW (P < 0.01), dressed yield (P < 0.01), and total carcasses value (P < 0.01). Net return carcass basis (P = 0.02) was numerically greatest for FA group 32 to 41.