Growing Beef Newsletter
May 2025, Volume 15, Issue 11
How selection pressure could influence the effectiveness of selection indexes
Randie Culbertson, ISU extension cow-calf specialist
Selection indexes combine multiple expected progeny differences (EPD) into a single value, allowing for the simultaneous selection of multiple economically relevant traits. Simply put, it is the economic merit of an animal as a parent. Utilizing an index can help simplify selection when buying bulls by prioritizing a single index value versus multiple individual EPDs and are designed to assist producers in achieving targeted breeding objectives to improve overall profitability. An economic index is the combination of economically relevant EPDs with economic emphasis placed on each EPD. The amount of emphasis for these traits is dictated by the magnitude of the economic contribution associated with each EPD. Most breed associations publish economic indexes and use economic values derived from breed wide estimates. From a population standpoint, the breed wide estimations provided produce a powerful tool for multi-trait selection for increased profitability. However, the downside to an index is it does not account for individual production goals, operation resources, or the herds own performance. Herds historically selected for a specific trait or breeding objective may have pitfalls that a breed wide index can’t account for.
To illustrate this, data from the Iowa State University McNay Memorial Research and Demonstration Farm’s registered Angus herd was used to generate a customized index using iGENDEC. iGENDEC is a web-based tool that allows the user to develop customized selection indexes aligned to an individual's operation. The ISU McNay Research herd has been selected for marbling, with approximately 30+ percent of fed calves qualifying as prime, yet the hot carcass weights tend to be lower than average. Terminal and all-purpose indexes published by breed associations have a large economic emphasis on marbling. However, this may not be the most appropriate for the McNay herd given the already high level of marbling.
An output from the iGENDEC customized index is the relative emphasis on traits and refers to the impact of each trait has on the overall index. For this herd, the relative emphasis for marbling was low at 5.7% while hot carcass weight was 21.9%. Since the McNay herd has been selected for marbling for decades, marbling has less emphasis in the index compared to hot carcass weight. A second customized index was developed using carcass data with slightly larger cattle at slaughter but lower average marbling scores. The relative emphasis for marbling in the second index increased to 12.2% while hot carcass weight dropped to 18.8% illustrating how the impact of these traits have changed due to the characteristics of the herd.
The practicality of developing operation and herd-specific indexes is not feasible for most operations. Indexes published by breed associations are still powerful tools for improving the profitability of a herd at a genetic level, but lack to ability to account for individual herd strengths and pitfalls. Tools such as iGENDEC were developed to address the shortcomings of breed-wide indexes by generating customized indexes, but it is recommended that users have a strong comprehension of beef cattle genetics and selection index theory. You can learn more about iGENDEC at igendec.beefimprovement.org/ or reach out to the Iowa Beef Center.
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