Holstein Association USA acted on this research and ranked bulls on an alternative index called the Commercial Herd Selection Criteria from 1990 through 1995. With the many traits available, the use of a profit function can help to account for all the incomes and expenses. Unfortunately, some costs are difficult to measure. The PTA approach is more accurate because heritabilities of traits differ, genetic correlations are not the same as phenotypic correlations, and all phenotypes are not available at the same time. In 1999, scientists in the S-284 Health Traits Research Group proposed that yield traits, health traits, and type traits all be combined using a lifetime profit function.
Profit functions provide a more direct method of assigning economic values.
To measure profit, estimates of actual incomes and expenses are needed. This multiplication makes the economic function a nonlinear function of the original traits. The linear function is much simpler to use and was correlated with the nonlinear function by .999. The expected prices for milk, fat, and protein were obtained from average U.S. prices over recent years. The profit function requires that PL be expressed in lactations instead of months.