A Quantitative and Molecular Evaluation of Bovine Respiratory Disease, Growth Traits, and Carcass Traits in Crossbred Steers

The first objective of the two studies presented herein evaluated on farm sources of variation and correlated effects contributing to BRD incidence in a population of crossbred steers sent to the feedlot from 2010-2013. Analyses revealed that incidence of BRD was lowly positively correlated to birth weight (BW) and lowly negatively correlated to weaning weight (WW), hip height (HH), sire breed, site, and birth year. These results indicate that the traits analyzed herein are not precursors for BRD in the current population. However, since little is still known about the correlation between genetic predisposition to BRD and animal performance, further studies should be conducted in the future. The second study presented herein evaluated single nucleotide polymorphisms (SNP) located on previously described QTL regions of Bos Taurus Autosome 6 and Bos Taurus Autosome 20 for potential associations with growth traits, carcass traits, and incidence of BRD in crossbred steers. Growth traits analyzed included birth weight (BW), weaning weight (WW), and hip height (HH). Carcass traits evaluated included hot carcass weight (HCW), yield grade (YG), marbling score (MS), ribeye area (REA), and back fat thickness (BF). Along with growth and carcass traits, incidence of BRD was also evaluated in the current population. Genotyping analyses identified fourteen unique SNP located on BTA 6 and eleven unique SNP located on BTA 20 that were significantly (P \u3c 0.05) associated with the aforementioned traits. However, no markers on either BTA 6 or BTA 20 were identified as significantly associated with BF. These results indicate that there may be a higher genetic predisposition to BRD than previously thought. However, before their incorporation into MAS programs, additional SNP located on BTA 6 and BTA 20 should be genotyped, along with other QTL regions spanning the genome. Additionally, a larger population of crossbred steers should be utilized to further validate the results herein