Optimum machinery management involves deciding how to operate, maintain, schedule and replace machinery in order to maximise after-tax profit. This paper focuses on machinery replacement as this has probably been the most complex aspect for South African farmers to manage over the last decade, due mainly to significant changes in the macroeconomy. Trends show that the tractor fleet on commercial South African farms has aged and shrunk since the 1980's. Current replacement cycles are now ten to twelve years, almost double the average cycle of the early 1980's. Economic factors affecting these trends include rising real tractor prices (partly due to the Atlantis Diesel Engine project), a falling Rand exchange rate, variable annual crop incomes, changing tax legislation and fluctuating, but generally higher, interest rates. Technological advances in tractor manufacture and more efficient machinery planning, operating and scheduling have also extended optimum replacement time.