Estimated US bull PTA's for component percentages are determined by first estimating the yield PTA's and then combining them with breed averages. The breed averages are determined by the cows born in the base year. Official calculations are done with averages recomputed with each run.
The equation to apply is
PTAC% = [(BAC + PTAC)/(BAM + PTAM) - (BAC/BAM)]* 100
where BAC is the breed average for the component, BAM is the breed average for milk, PTAC is the PTA for the component, PTAM is PTA milk, and PTAC% is expressed to two decimal places (.xx).
For example, if milk and protein PTA's for a Holstein bull are +1500 and +50, the resulting estimated PTAP% is +.01.
The above equation can be simplified to
PTAC% = (PTAC*100 - PTAM*X) / (BAM + PTAM)
where X = 100*BAC/BAM and BAM, BAC, PTAM, and PTAC are as defined above. X is approximately 3.66 for calculating Holstein PTAF% and 3.15 for PTAP%.