Holstein Plaza

The following information is based upon an article written by Bennet Cassell, professor emeritus at Virginia Tech and George Wiggans, research geneticist at USDA-AIPL, entitled, "Adjustments make all cow PTAs similar to those on genomically tested cows." Their complete article can be found online at www.holsteinusa.com, along with this news release.

Background – Why More Rescaling?
In April 2010, USDA introduced two new procedures to the national genetic evaluations, which generated a great deal of discussion and interest within the industry.

First, imputation of genotypes, cows with a number of genomically tested progeny (typically five or more) had their genotypes imputed. Meaning, if a cow had enough information available between her genomic-tested offspring and sire to be able to derive (or "impute") her genotype with a high level of accuracy, she received a genomic prediction.

The second change, which had a larger impact on cow PTAs, was USDA "rescaling" PTAs of cows with genomic information. Rescaling is the process of scaling down the difference between the cows' PTAs and her PAs (Parent Averages). The rescaling of PTAs, in 2010, was done only to animals with genomic predictions, which created the important problem that, since non-genomic animals had not had their PTAs rescaled, the PTAs of genomic-tested cattle could no longer be compared side-by-side with PTAs of non-genomic tested cattle.

What Will Happen in April 2011
Beginning with the April 2011 genetic evaluations, USDA-AIPL will adjust the PTAs on all Holstein, Jersey and Brown Swiss cows, whether genomic tested or not, to make all evaluations more directly comparable. It is important to note that ONLY PTAs for milk, fat (pounds and percent) and protein (pounds and percent) are affected. The improvements made to the PTAs on older cows will trickle down to the genetic evaluations of their progeny. This process will result in better estimates of parent averages.

Rescaling of cow PTAs begins with the within-year Parent Average. To do this, cows are broken down into 10 groups (for each birth year) based on their PA. PTAs on cows with a low PA are scaled up and PTAs on cows with a high PTA are scaled down. If the difference between a cow's PA and the average PA for the birth year is large, larger adjustments will be made. The correlation between the current PTA Milk and the adjusted PTA Milk is over 0.99, so changes in rank are expected to be small for almost all cows.

The main factor in determining the all-cow adjustment is how much an animal's PTA differs from its Parent Average. As a generalization:

· If the PTA is greater than the PA, the adjustment will cause the PTA to drop
· If the PTA equals the PA, then there is almost no adjustment
· If the PTA is less than the PA, the adjustment will cause the PTA to increase

Research has shown the adjusted PTAs of cows included in the genomic evaluations still tend to be slightly biased upwards. Therefore, the traditional PTAs of any Holstein cow that obtains genomic information will be subjected to a reduction of 221 pounds of milk, 8.8 pounds of fat and 6.4 pounds of protein. This represents the average difference between the overall combined PTA on genotyped cows and the PTA obtained directly from the genetic markers. This adjustment is much smaller than the one made in April 2010.