HANDLING FIBER AFTER GRAIN PRODUCTION
Dual Crop – Grain and Fiber
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 can be grown as a dual purpose (grain and fiber), fiber only crop (i.e. textile applications) and even a tri-crop was successful in Texas in 2020 (CBD, grain, and fiber); therefore, different harvesting and post-harvest handling methods of hemp stems are recommended for different applications of hemp fiber. (Refer to Growing Hemp for Fiber Production Section).
Hemp grain growers, who are not interested in fiber 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 fibers of hemp make it necessary to get rid of the straw and stubble before the next crop. The dry fibers 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 fiber especially from shorter hemp varieties is being done in a number of ways.
- The fiber that comes out of the back of the combine is bound together in lumps. These lumps have been baled by some producers and then removed from the field. The stubble, if not too tall, can then be worked into the soil.
- A forage harvester has been used for chopping up the lumps from behind the combine, blowing out chopped fiber onto the field to be incorporated. Do not use the type of forage harvester that uses an auger to move the chopped product over to the blower as the auger will tend to wrap up.
- Most tandem discs will bury the fiber with some wrapping around the belt.
- High speed discs work well, as each disc is on its own arm, with no shaft, eliminating wrapping.
- The sooner it is incorporated after combining, the less problems, as the green fibre does not wrap like it does when it is dry. Once the fiber is in contact with the soil, the micro-organisms begin the breakdown of the fibre.
- Others leave the stubble stand all winter to catch snow and then seed a crop of peas or beans by broadcasting followed by discing. The field can be heavily harrowed a week later to control weeds and level the ground. After the harvest, the hemp fiber is broken down and can be worked in with ease.
- Disk seeding equipment has been used to seed directly into the stubble the next spring. The stubble will generally be knocked over with a harrow or roller before seeding.
- Forages like sweet clover have been broadcast on the field after seeding the hemp. The clover establishes and grows the next summer. By the next fall the hemp fiber has broken down gently reducing the number of problems with equipment wrapping. This technique works best for shorter varieties.
Some growers preserve post-grain harvest straw in anticipation for upcoming market opportunities for the fiber. This approach involves baling the straw right after combining, which means that the retting process (see fiber retting section) is often omitted. If grain was combined with tables elevated to about 2-3 feet above the ground, the post combine tall stubble could be cut with a discbine (with lowest possible conditioner pressure) or swather (swath width must be narrower than baler width) and subsequently baled or handled in a way described above (burning, raking).
Some existing hemp fiber buyers require retted hemp for their applications. In such instances, grain type (or eventually dual purpose) varieties could be winter retted, which implies leaving hemp straw residues on the ground until the spring, followed by baling when the straw is sufficiently dry (12-16 %).
Time of Fiber Harvest
Time of harvest of hemp as a fiber crop will depend on specifications provided by fiber contractors.
As a rule of thumb, textile application crops need to be cut when the male plants (or flowers in monoecious varieties) are shedding pollen, prior to seed setting on female plants (flowers). At this developmental stage, bast fiber is not heavily lignified, which allows for obtaining smooth, high quality fabrics. Delayed harvesting should be avoided as it results in developing coarser fibers, not suitable for certain textile applications. After harvest for textile applications, hemp stalks have to be retted. Note that under this management scenario a grower will not get income from hemp grain, but fair price should be negotiated with the buyer for high quality fiber.
There are two types of bast fibers:
- Primary bast fibers – make up 70-90% of the bast. They are characterized by long length (up to 50 mm), high cellulose (50-70%) and low lignin (about 7%) content. These fibers are the most valuable part of the stalk.
- Secondary bast fibers – comprise the remaining 10-30% of the bast fibers. They are shorter (approximately 2 mm), more lignified and hence of lower value for some applications (i.e. textile). Low stand density of hemp crop favours development of this type of bast fiber.
Hurd is the short fiber (about 0.5 mm) found in the inner woody core of hemp stalks. Hurd accounts for 70-80% of the stalk and typically contains 20-30% lignin. For millennia, hemp was grown for bast fiber, primarily for textile applications, while hurd was considered a waste by-product of bast production.
In addition to genotypic differences, bast fiber content in the stems can be modified by seeding density. Higher bast fiber content is found in heavy seeded fields (40-60 kg/ha) that form a dense canopy of slender, unbranched plants (Figure iii), producing high yields of superior quality bast fiber.
Owed to their different physical properties and chemical composition, bast and hurd fibers are suitable for a wide range of industrial applications including bio-composites, construction materials, textiles, insulation, bedding, paper production, ropes and twines and many others.
Until recently, major breeding efforts of the fiber usage type varieties were focused on increasing bast fiber content of the stem, mainly for textile industries. Revival of hemp production during the last decade resulted in developing applications for hurd as well, with building products being one of the chief materials driving hemp for fiber expansion. Biofeedstock needs for eco-friendly composites and plastics will further increase hemp fiber demand.
Research is being conducted to optimize winter retting methods for different fiber applications. In the areas experiencing sufficiently long periods of warm fall weather with occasional rain events, allows the retting process to be completed (see fiber retting section), the baling could be done before winter’s arrival. This approach leaves more time in the spring to prepare fields for new crop production.
Some hurd buyers prefer white fiber for their applications that can be derived from unretted (or only slightly retted) stalks; therefore, the stalks should be baled shortly after harvest, as soon as the stems are sufficiently dry (12-16%). Baling stems too wet may result in continuation of the retting process in the bale, or even worse, may cause rotting and hence severe loss of fiber quality/yield. To avoid contaminated hurd fiber and reduced performance in commercial products buyers demand weed-free bales.
For the time being, hurd originating from combined stalks are also accepted by processors because of very limited availability of designated fiber cultivars. As the fiber industry matures, a premium price will be obtained for hemp managed for a specific application.
Bast fiber buyers’, other than for textiles, (i.e., for biocomposites, insulation, etc.) uses do not always require hemp to be cut at the commencement of flowering. For their applications, lignified stems derived from hemp grown for the grain often deliver sufficient fiber performance. Unless specified by the end fiber user, retting may not be absolutely necessary.