• By the IGS Science Team
  1. A new definition of contemporary groups based on the age of the dam.

Regardless of how users designate contemporary groups (CG), all calves born from first-calf dams will
be placed into a separate CG from calves out of mature cows. Given the vast majority of producers actually manage this age group separately, it is reasonable to define their calves as their own CG. Handling these as separate CG will reduce the environmental noise caused by different management strategies and biological constraints
for this age group.

  1. Milk modelling updates.

The magnitude, and even direction, of the correlation between weaning weight direct and milk, has been long debated in scientific circles. In fact, there is a wide range of estimates that exist in the scientific literature. Given that, the science team developed a model that assumes milk and weaning weight direct are independent (i.e., genetic correlation of 0). In addition, with some of the other proposed updates, it was discovered that the evaluation solved more effectively when genomics was removed for Milk EPDs. In light of this discovery, the IGS Milk EPD will not use genomic information for the time being.

  1. Different variances for different sexes.

Males usually have a higher growth potential than females simply due to gender. As a consequence, the variation associated with their weights also tends to be greater. This difference in the amount of variation between the sexes are set to a male scale in the up-dated growth trait predictions.

  1. New DNA Marker subset.

As the number of genotyped animals has increased, so has our ability to estimate marker effects and identify subsets that are more predictive. Relative to growth traits, a new (and larger) subset of markers has been identified to add more accuracy to EPD.

Results of Updates to Growth Model

With these proposed changes, a considerable amount of work went into testing if the new models improved growth trait predictions. One of the most common procedures for evaluating updates to EPD systems is
to exclude a certain portion of the phenotypes available, run the evaluation, and compare the correlation of the

EPD from two systems to the phenotypes that were removed from the evaluation (higher correlation is bet- ter). For these updates, this procedure was used where all animals born after 2018 were excluded from the evaluation system and then comparisons between the current growth trait EPDs and the updated EPDs were made to this phenotypic information. The results for each of the analyses are presented in the following table.

The results in the table above show the evaluation updates had higher correlations to phenotypes than the previous growth trait models. This equates to more precise EPDs for Birth, Weaning, and Yearling Weight.

An additional trait that is evaluated with the growth analysis is the Milk EPD. A Milk EPD represents the genetic difference in calf weaning weight based on the maternal environment provided by the dam. Due to the nature of this trait being the maternal component of weaning weight, a different validation strategy must be used to evaluate the updated predictions. To evaluate the updated Milk predictions an expected weaning weight for the excluded animals was formed using the following equation: Predicted Weaning Weight = Calf WW EBV + Dam’s Milk EBV

This predicted weaning weight was then correlated with the excluded weaning weight phenotypes. Again, the updated predictions of Milk had higher correlations compared to the previous Milk EPDs (0.42 vs. 0.39, respectively). These results show that the updated predictions more precisely predict the weaning weight of an animal than the currently published evaluation.

Breeders may notice reranking of animals with the release of the growth trait updates, mostly for weaning, yearling and maternal weaning weight and Milk. While the change may be unsettling, the end results by every measure have shown an improvement in the precision of the growth trait predictions.

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About Shorthorn

The Shorthorn breed is truly unique, from their distinctive roan colour, to their rich breed history and most importantly, the wonderful community of breeders, there is nothing else quite like a Shorthorn.

The key to the Shorthorn breeds advantage lies in their balanced genetic profile, driven by market participation that has been developed and refined, after more than 200 years of genetic selection under Australian conditions.

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