New $BULL and QUEEN Selection Indexes

July 23rd, 2019 by shelbys@brahman.org

On August 15th, 2019, ABBA will be adding multi-trait selection indexes to its EPD portfolio.  These indexes came from extensive work by the ABBA Research and Breed Improvement Committee (commonly known as the Performance Committee) and renowned geneticist, John Genho.

An Expected Progeny Difference (EPD) is the prediction of how future progeny of an animal will perform; specifically, EPDs predict the genetic transmitting ability of an animal to their offspring.  EPDs are expressed as units of measure for the trait, plus or minus the average of the animal’s group.

While EPDs are a valuable tool, they have two major deficiencies.

  1. First, because they are expressed in the units of the trait, it is difficult to compare EPDs from different traits to each other. If we could either increase the average weaning weight of a herd by one pound, or increase the average marbling score of the herd by one point, which should we do? Which has more value? In the end, this is an economic question that breeders must consider with the relative value of the two traits.
  2. A second drawback to EPDs is they focus on traits individually. The animals we are trying to improve are a biological system; when we change metabolism, growth, mature size, carcass quality, docility or any other of the host of traits we can select for, there are repercussions. Some of these repercussions are expected, ie. make the cattle weigh more and they will get taller, but others are unexpected and these can more of an unintended consequence.

 

Economic selection indexes overcome these two drawbacks to single trait EPDs by considering the relative economic value of each trait in a holistic rather than segmented approach. The resulting tool  allows breeders to analyze multiple traits for a specific breeding objective through a single value. As a bonus, index values are expressed in the units of dollars, meaning animals with higher values have a greater potential to be profitable. This makes indexes more user-friendly than single trait EPDs – everyone knows a higher dollar value is better!

The $BULL Index is a terminal index focused on selling animals through grid pricing with carcass weight weighted most heavily of all traits. There is a considerable positive correlation to frame score and all weight traits, meaning animals selected using this index will tend to be larger framed, growthier animals. This index identifies cattle which have low birth weight (BW), good Calving Ease (CE) and good spread from BW to weaning weight (WW).  The $BULL index also includes scrotal circumference and docility with moderate carcass influence on the index.  The goal of this index is to identify cattle which would work well to produce bulls which could be marketed easily to commercial or purebred breeders.

The QUEEN index is a maternal index which focuses on a more balanced approach to cow selection.  Frame score, milk, and back fat all have no correlation to the index, meaning that selecting replacement heifers for this index will not increase the mature size of the herd or have an impact milk production. Scrotal circumference, docility, and calving ease are weighted more heavily than in the $BULL index since these traits are indicators of puberty and manageability. In summary, the QUEEN index is an indication of ideal cattle to be raised for replacement heifers.

When looking at selection indexes, it is more important to consider the correlations between the index and the traits than the weightings/coefficients of the index. As an analogy, we could consider the component parts of a pickup truck, studying its suspension, motor, transmission, etc. or we could just consider how fast it drives and how much it can pull. The weightings and coefficients are the component parts of how an index is built and are important details, but the correlations determine the rate at which we will make changes to animals.

Often traits which have no weighting in the index have a correlation and, at times, certain traits have a positive correlation with an index but a negative weighting (or vice versa); this is because of the genetic correlations between the traits. Remember, we have a biological system of many traits, not a collection of individual traits. For instance, carcass weight and calving ease are both included in the $BULL and QUEEN indexes with a positive weighting on carcass weight (ie. more pounds equals more money) and a positive weighting on calving ease (more unassisted births is a good thing). However, the biological reality is that these two traits work in opposition to each other (larger calves are not born as easily). The result of this tug of war is that BW has a positive correlation to the $BULL index but has virtually no correlation to the QUEEN index. An important part of the development of these indexes was studying their correlations to each trait to ensure that selection was going in the intended direction.

ABBA is proud to offer these selection indexes to Brahman breeders as additional tools to evaluate cattle.  This information will be published on www.brahman.org for future reference.