The first thing to consider is: what will be the end use of this hemp? If you are looking to harvest seeds, you will need different equipment and hemp varieties than if you are looking to harvest stalks for fiber. It turns out that the optimum time to harvest hemp for fiber is well before the optimum time to harvest hemp for seeds. So it is probably best to decide on one goal or the other before you plant a field. It is possible to harvest both seeds and stalks, however.
If harvesting hemp for fiber on a smaller plot, a well-maintained sickle-bar mower or hay swather may be used to cut the stalks. The stalks are cut and left in the field and allowed to rot slightly to begin separating the fibers from the stalk. This process is called “retting”. After retting, a baler may be used to bale the hemp stalks, at which point the stalks are ready for storage, drying, and sale.
If harvesting hemp for seed, a combine may be used, although this can be a challenge. It is recommended to raise the blade a meter or higher, but even then the long fibers of the hemp plant can cause wear and tear on the machine by winding through moving parts.
There are other, more specialized hemp farming machines available as well. I will leave you with the link to a video that shows an example of a hemp harvest.