Single Trait EPDs vs. EPD Indexes vs. Production Ratios
The numerous tools to aid in genetic selection allow for expedited progress toward breeding objectives. However, there are variations in how values are calculated and the units they are reported in. Single Trait Expected Progeny Differences (EPDs), EPD Indexes, and Production Ratios are typical of reported data and can provide guidance towards breeding objectives when applied as they are intended. While the bells and whistles vary across breed associations, the basics of each measurement can be applied across breeds and genetic reporting platforms.
Single Trait EPDs
These numbers are a prediction of how an animal will perform as it pertains to a single production trait. They can provide insight into the genetic potential of an animal based on their own performance, as well as previous performance contributions from the pedigree on both the sire and dam sides. The number is calculated assuming a 50% contribution from the dam and a 50% contribution from the sire. Examples of single trait EPDs include Birth Weight (BW), Calving Ease Direct (CED), Weaning Weight (WW), etc. After doing a systems analysis of an operation and concluding that there is potential for improvement for a single production trait, we can dial in on an individual number that allows for measured improvement in selection. For example, if there has been an increased incidence of dystocia and assisted births, and the goal is to not be a technician in the parturition process, high selection pressure can be placed on increasing the CED EPD value to have measured progress that aligns with the goals of the operation.
EPD Indexes are the result of a formula that combines single trait EPDs at different weights to typically reflect the economic return or loss of that combination, represented in a dollar value. The exact formula to calculate these indexes are part of the secret sauce, and therefore, it will not always be apparent how much weight is placed on each inclusion of single trait EPDs in the calculation. This also opens the door for potential assumptions that must be evaluated to determine how closely aligned they are with breeding and marketing objectives. When utilizing indexes, selection pressure is dispersed across multiple traits.
Examples would be HerdBuilder (HB; Red Angus), Maternal Weaned Calf Value ($M; Angus), and Mainstream Terminal Index ($MTI; Limousin). Expanding on HerdBuilder, the emphasis is placed on EPDs from conception to weaning (CED, CEM, WW, Milk, MW, HP, STAY) to evaluate an animal’s merit as a maternal genetic contribution reflected in a dollar value above or below breed average. Assumptions include “that bulls are mated to heifers and cows; replacement heifers are retained and all remaining progeny are marketed at weaning.” Knowing the assumptions behind each index are key in applying this selection tool to individual herds.
Production ratios are a measure of how an individual animal performs when compared to its peers raised in a similar environment. These are expressed as a percent of the group average. Ratios are created within a single herd and herd managers are responsible for selecting the animals included in a group, which can vary from herd to herd. For example, if a bull calf has a weaning weight ratio (WWR) of 104, he performed 4% above the average of the group. On the other side, if a bull calf has a WWR of 96, he performed 4% below the group average from birth to weaning.
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