Evaluation of fixed sources of variation and estimation of genetic parameters for incidence of bovine respiratory disease in preweaned calves and feedlot cattle.

The primary objective of this study was to estimate variance components and heritability of bovine respiratory disease (BRD) incidence in beef calves before weaning and during the finishing phase. The second objective was to investigate the impact of BRD incidence and treatment frequency on performance and carcass traits. Bovine respiratory disease is the biggest and most costly health challenge facing the cattle industry. The 2 populations used consisted of 1,519 preweaned calves and 3,277 head of feedlot cattle. The incidence rate of BRD in preweaned calves was 11.39%, and among treated cattle, 82.1% were treated once, 13.9% were treated twice, and 4.0% were treated 3 times or more. The incidence of BRD (P = 0.35) and the number of treatments (P = 0.77) had no significant effect on weaning BW. Heritability estimates of the entire preweaned population for BRD resistance and number of treatments were 0.11 +/- 0.06 and 0.08 +/- 0.05, respectively. The genetic correlation estimates for BRD incidence with weaning BW and birth BW were low (-0.02 +/- 0.32 and 0.07 +/- 0.27, respectively). The same estimate for the number of BRD treatments with weaning BW and birth BW was 0.25 +/- 0.35 and 0.30 +/- 0.27, respectively. The observed BRD incidence rate for feedlot cattle was observed at 9.43%. Incidence of BRD significantly (P < 0.01) decreased overall and acclimation ADG by 0.06 +/- 0.01 kg/d and 0.28 +/- 0.03 kg/d, respectively. Carcass traits were also significantly (P < 0.05) affected by BRD incidence; untreated cattle had a 9.1 +/- 1.7-kg heavier HCW. Results were similar in the analysis of treatment frequency. The heritability estimate of BRD incidence and the number of treatments were 0.07 +/- 0.04 and 0.02 +/- 0.03, respectively. Estimates of genetic correlations of BRD incidence with production traits were -0.63 +/- 0.22 for acclimation ADG, -0.04 +/- 0.23 for on-test ADG, -0.31 +/- 0.21 for overall ADG, -0.39 +/- 0.21 for final BW, -0.22 +/- 0.22 for HCW, -0.03 +/- 0.22 for LM area, 0.24 +/- 0.25 for fat, and -0.43 +/- 0.20 for marbling score. Similar results for the number of treatments and production traits were -1.00 +/- 0.68 for acclimation ADG, -0.04 +/- 0.39 for on-test ADG, -0.47 +/- 0.41 for overall ADG, -0.66 +/- 0.40 for final BW, -0.58 +/- 0.45 for HCW, -0.12 +/- 0.38 for LM area, 0.42 +/- 0.50 for fat, and -0.32 +/- 0.37 for marbling score. Because of the high economic cost associated with BRD incidence, even these modest estimates for heritability of BRD resistance should be considered for incorporation into beef cattle breeding programs.