In the northern part of the Corn Belt the hemp usually does not grow so tall and therefore can be handled more easily with machines. During the first World War hemp-harvesting machinery was developed. These harvesters (see above) in one operation cut an 8- or 9-foot swath and elevate the stalks to a quarter-circle platform where they are turned automatically and dropped or spread on the ground for retting. The butts of the stems all lie in the same direction and are relatively even. The thickness of the layer of stalks in the swath influences the speed and uniformity of the dew retting. Machines of this type, because of their labor economy, are recommended for use in the Northern States, where hemp can be safely spread for retting when harvested.
Hemp harvesters are usually owned by the hemp mills. They are rented to the individual farmers, who usually furnish the motive power and the labor to run the harvesters.
Retting
Retting is the partial rotting of the hemp stalk. It permits the fiber in the stalk to separate easily in long strands from the woody core. The fiber strands break if unretted stems are bent or broken.
In this country the usual practice is to ret hemp by allowing it to lie on the ground, where it is exposed to rain and dew. This method is called dew retting.
Dew retting is dependent upon dews and rains to furnish the moist conditions necessary for the growth of the molds that cause the retting. In warm, moist weather the retting may require 1 to 2 weeks, but usually 4 to 5 weeks is required for retting in Kentucky and Wisconsin. Hemp has remained spread under snow in Wisconsin until spring without serious injury, but more often hemp left under snow all winter is overretted and ruined.
If hemp stalks are lifted from the ground before they are sufficiently retted, the fiber will not separate easily from the woody hurds (small pieces of the woody core of the plant) in milling. However, if the retting is permitted to go too far, the fiber separates very readily from the core, but the adhesive substance between the individual fiber cells in the long strand breaks down and the fiber is weak. Hemp further overretted produces mostly short broken strands of fiber called tow fiber, which is less valuable than the long parallel strands of fiber called line fiber.