There is little sound information about the impact of cow genetic selection programs on whole farm profit. In this paper aggregate industry data about dairy herds are analysed to identify trends in dairy herd genetic, production and reproductive performance. The genetic distribution within herds that would result over time from a long-term genetic selection program is simulated, and a representative whole-farm bioeconomic model is used to explore the impact of genetic change in a dairy herd on the profit of a representative case farm. Analysis of an industry herd recording database revealed an average annual rate of increase in the Balanced Production Index (BPI) of 7 units for the herd (2.9 and 10.1 for the bottom and top BPI quartiles) and 10.8 units for artificial insemination sires used within herds. Modelling these trends for herds, with an age-cohort range average of 43 units of BPI and 20 per cent cohort attrition rates, shows that the natural range between bottom and top BPI quartiles expands gradually but remains between 75–100 units in most herds across 50 years of selection. This finding of persistence in the distribution of genetic merit around the herd mean indicates that the common application of comparing performance of BPI quartiles within herds provides almost no insight into impacts of genetic selection on farm profit. Bioeconomic modelling revealed that the genetic gain of a herd achieving an annual rate of increase in herd BPI of around 10 units per year contributed an average of around $2,500 extra to farm profit per annum for a 250-cow herd representative farm. Applying more widely the findings and insights from modelling genetic gain in a representative pasture-based dairy farm suggests it is likely that that, on many, or even most, dairy farms, the gains in profit from cow genetic selection is likely to be modest in light of other investment opportunities and is thus not something to be focussed on to the effective exclusion of other more profitable resource uses. ‘Ignore the principle of equi-marginal returns to all inputs at your peril’ is sage farm management economics advice to those who believe that all animal farmers need do is to improve the genetic potential of their animals. Good advice to dairy farmers would be to: (i) have realistic expectations about the role of genetic gain in their business; (ii) evaluate returns from investment in herd genetics; and (iii) compare expected returns from investments into all limiting factors present on the farm.