Yarrow Production

Can you please give me some info on growing Yarrow as a cash crop. I need sowing, harvesting and water requirements. We live in southeast Montana. We have some well drained smaller fields that we would like to use for herbs. We are dryland with brown prairie soil. We are interested in crops that can be direct seeded. We can possibly water our crops using a water truck during the early stages if we are too dry for them to get a good hold.

You have the right idea, but the wrong crop. Selecting something which grows as a native plant is always a good idea. Unfortunately, the Yarrow Flowerhead market is quite limited, and may require special equipment (not yet invented) to be competitive.
In your region, it is not uncommon to find native stands up to 300 acres (especially on overgrazed pastures) up next to forest lines. I think the entire domestic market needs is less than 400 acres, with most being imported from India at a landed price of less than US$1.40/lb. It is still harvested by hand in Europe, with inexpensive labor sources.
There are other pharmaceutical root crops which I could recommend for a yarrow-type habitat. That would include bloodroot, wild indigo, and mandrake. You had mentioned brown prairie soils, also not best suited for yarrow. These soils might be better suited for such crops as chamomile and sesame. The chamomile also needs a flowerhead harvester (not yet developed).

Additional comments from Conrad Richter:

Rick Miller has answered this from the point of view of growing yarrow as a dried flower. But there are other markets for this plant. For example, Richters has been getting inquiries about growing it for the essential oil market, and also for the bulk medicinal dried herb market. Yarrow has some potential as a mosquito repellent (a Scandanavian study showed that it is as effective in repelling mosquitoes as DEET). Although these markets may be limited, they may have some potential.
There is little information at this stage on the agronomy of the crop available. For the most part, growers will have to experiment with everything from seeding rates to harvesting.