Department of Animal Science
First Advisor
Matthew L. Spangler
Second Advisor
Ronald M. Lewis
Third Advisor
Larry A. Kuehn
Date of this Version
12-2023
Citation
A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science
Major: Animal Science
Under the supervision of Professor Matthew L. Spangler
Lincoln, Nebraska, December 2023
Abstract
This thesis presents two projects that investigate the impacts of breeding objectives and indexes-in-retrospect within the beef cattle industry. The first project synthesized how changes due to planning horizon, breeding system, and sale endpoint in the economic breeding goal of cattle influence the relative emphasis of traits as well as the re-ranking of selection candidates. A total of three breeding systems that represent straightbred and crossbred cattle were used along with six planning horizons (2, 5, 10, 20, 30, and 50 yrs.) for both a weaning and slaughter point of sale. The combination of these different breeding objective attributes resulted in 36 economic selection indexes that were applied to a group of 27,123 selection candidates. The perturbing of planning horizon and sale endpoint resulted in the largest changes in relative emphasis of traits in the goal. As planning horizon increased, the relative emphasis of traits shifted from profit drivers (weaning and hot carcass weight) to traits that support cow longevity within the herd (stayability and mature weight). Re-ranking of selection candidates found that the most impactful component of the breeding objective was the chosen sale endpoint followed by planning horizon. Within sale endpoints, the greatest re-ranking occurred between shorter (2, 5, and 10 yrs.) and longer (20, 30, and 50 yrs.) planning horizons. The second project focused on the selection that had already been achieved across ten major U.S. beef cattle breeds using an index-in-retrospect approach. The index-in-retrospect methodology allowed for an approximation of the underlying selection index that led to the observed genetic changes. The standardized selection differentials and realized trait emphasis depict the amount and kind of selection applied to traits found in published genetic trends for each of the respective breeds. Across many breeds, the trait that received the most selection emphasis was calving ease (direct and maternal). In addition, weaning and yearling weight were other traits that were selected for while carcass traits varied from breed to breed. Using current economic parameters, economically optimized indexes suggest that hot carcass weight would have the greatest relative emphasis except in cases where stayability was available as part of the selection criteria. Given these economically optimized indexes, response to selection determined that hot carcass weight, weaning weight, yearling weight, mature weight, and stayability would change the most, in units of selection intensity, from selection on the index.
Advisor: Matthew L. Spangler
Comments
Copyright 2023, Hunter F. Valasek