Presently in Canada, hemp is almost exclusively grown for grain, while fibre bearing stems are considered a nuisance of crop production. Rapid development of various industries (construction, automotive, textile) demanding high volumes of suitable, quality fibre is presently changing perception of hemp as a grain only crop.
Hemp will soon be grown as a dual purpose (grain and fibre) or even fibre only crop (i.e. textile applications); therefore, different harvesting and post-harvest handling methods of hemp stems are recommended for different applications of hemp fibre. (Refer to Fibre Production Section)
Hemp grain growers, who are not interested in fibre production, prefer short stature varieties that generate a small volume of stock biomass that has to pass through the combine and a smaller amount of straw residues that need to be handled. The strong bast fibres of hemp make it necessary to get rid of the straw and stubble before the next crop. The dry fibres become very tough and wrap tightly on bearings, shafts, etc. of tillage and seeding equipment.
Hemp, like any plant, removes nutrients from the soil and some of these nutrients remain in the stalk. If the remaining stalk, after harvest, is left in the field and subjected to rain the natural process of retting the nutrients from the stalk can be turned back into the soil through incorporation.
Different, region-specific approaches to the handling of post-combine residues to clean up the field for the next crop are used.
Incorporation of the fibre especially from shorter hemp varieities is being done in a number of ways.
Many farmers elect to clean up very tough and slowly decomposing hemp residues by burning of fields in the fall. As fibre markets develop, this need will be reduced. If there is sufficient time for the stalks to dry down, the fields can be harrowed and burned as a method of clean-up. Most fields are left to over-winter as the stalks just above the ground are fragile and break off easily when harrowed or rolled to flatten. If the stalks are close to the ground, the field will burn fast and hot. Ensure there is a good fire guard around the field and watch the wind. Producers will often have water in the field as a precaution. Some hemp stalks are large and piles that were made by harrowing can burn and smolder for a long period of time. Ensure the piles are extinguished before leaving the field to avoid the chance that embers could blow into a neighbouring field or river if the wind changes.
Farmers have been innovative in finding ways to get rid of the fibre, for which there are no ready markets. Spring burning is used to get rid of the fibre residue.
Standing hemp stalks do not burn well. By flattening stalks the field will burn black getting rid of all of the fibres. In the spring the stalk just above the ground is weak and will break off easily. A land roller is effective to flatten all of the stalks on the ground.
Figure viii- Rolling of post-harvest residues
A fire guard is made around the field with harrows dragging the outside round towards the center of the field. Harrows tilted back are also used to flatten the stalks in the spring before burning.
A rotary rake or Tedder rake has been tried as a method of windrowing hemp fibre for baling. The hemp stalks are left standing over winter. In the spring, the field is first rolled with a land roller. The fibre in the first few centimeters at the bottom of the stalk does not have strong bast fibre in it. Over winter, this portion of the plant deteriorates. On a dry day, the stalks break off easily when the roller goes over them, compressing them. The rotary rake is able to easily tear off the remaining stalks and windrow the fibre for baling.
The rake disturbs and incorporates a significant amount of soil into the swath. This is too much for most processors, so it is not recommended unless a specific market will accept the product. The Tedder rake is a high maintenance machine.
Figure ix – The Tedder rake handling hemp residues