A seeding machine, such as a row crop planter, is described which is adapted to switch between two or more seed varieties as the machine traverses a field. The control system uses a programmed quantity of seed representing a number of seeds in the seed meter that need to be substantially consumed once the flow of a first seed variety is stopped before introducing a second seed variety to minimize seed mixing. The seed quantity can be determined by a calibration process or published from the manufacturer or third parties. The seed quantity can also be part of a seeding prescription that includes assignment of where each seed variety is to be planted in a field. The seed quantity and the distance traveled to empty the meter can be used to optimize the planting operation including the machine direction which can also be part of the prescription.