It came to our attention that some bulls PTAs for rear udder width (RUW) in breeds other than Holstein had larger than normal changes.
The reason for these results is new genetic correlation estimates were implemented in December. In the August run, using genetic parameters obtained in 2003, rear udder height (RUH) had the highest correlation (0.85) with RUW and was therefore used to fill missing values for RUW. The new genetic correlation estimates available, showed a decrease to 0.67 in genetic correlation of RUH with RUW. Since now dairy form had the highest correlation (0.75) with RUW, it was used as the substitute trait instead of RUH. Holsteins continue to use RUH as the substitute trait because the correlation with RUW is very high (0.92). Genomic PTAs have not been computed for RUW because RUW is not exchanged in multi-trait across country evaluation (MACE), except for Brown Swiss. Instead, genomic PTAs for RUW are computed from the most correlated trait, which is now dairy form in breeds other than Holstein.
Here are some examples for some bulls:
As you can see above, Dairy form and RUH do not change substantially. On the contrary, consequence of the change in correlations, RUW does change for single bulls - although population-wise mean and standard deviation of RUW has only minor variation. Even if actual numbers in the above example seem different, both solutions are correct. The update in genetic parameters is required to better reflect the data currently used for genetic and genomic evaluations. Using Dairy Form rather than RUH makes RUW estimation more accurate, as new correlations between Dairy Form and RUW are higher. It is highly important to underline that this change is not affecting only a specific bull or group of bulls. This modification affects all the population similarly, for all the bulls on all breeds other than Holstein. Also note that this procedure to fill missing traits has been used for about 20 years.
We acknowledge there is room for improvement for these traits. RUW PTAs from MACE could be used for Brown Swiss (the only breed with MACE evaluation for RUW), or missing PTAs for foreign bulls could be filled before rather than after the genomic evaluation, or Interbull could include RUW in MACE for the other breeds. However, all these solutions require research and development and cannot be applied in a very short time. Note also that in the December run, Interbull added 10 more MACE conformation traits for Brown Swiss that are not scored in the United States and are not evaluated in other breeds.