Predicted transmitting abilities (PTAs) for component percentages are estimated by combining yield PTAs with breed averages. If only yield evaluations from another country are available, then those evaluations must be converted to the U.S. scale first using Interbull conversion equations. The breed averages are determined by cows born in the base year and are recomputed with each official evaluation.
The equation to apply is
PTAC% = 100 × [(BAC + PTAC)/(BAM + PTAM) − (BAC/BAM)]
where PTAC% is the estimated PTA for component percentage expressed to 2 decimal places (.##), BAC is the breed average for component yield, PTAC is the PTA for component yield, BAM is the breed average for milk yield, and PTAM is the PTA for milk yield. For example, if PTA milk = +1,500 pounds and PTA protein = +50 pounds for a Holstein bull in December 2011 and the Holstein lactation averages were 26,007 pounds for milk and 784 pounds for protein, then PTA for protein percentage would be 100 × [(784 + 50)/(26,077 + 1,500) – (784/26,077)] = +0.02.
The above equation can be simplified to
PTAC% = [(100 × PTAC) − (F × PTAM)]/(BAM + PTAM)
where F = 100 × (BAC/BAM). The factor F for calculating Holstein PTA for component percentage was 3.65 for fat and 3.01 for protein in December 2011.