P.J. LeMahieu1, E.S. Oplinger2, and D.H. Putnam3
1Agrecol, 4906 Femrite Drive, Madison, WI 53716.
Kenaf (Hibiscus cannabinus L.) is a fiber plant native to east-central Africa where it has been grown for several thousand years for food and fiber. It is a common wild plant of tropical and subtropical Africa and Asia. It has been a source of textile fiber for such products as rope, twine, bagging and rugs. Kenaf is a promising source of raw material fiber for pulp, paper and other fiber products, and has been introduced since WWII in China, USSR, Thailand, South Africa, Egypt, Mexico and Cuba.
Research in the United States to use the kenaf bast (outer bark) fibers for rope began in the 1940's when jute imports from Asia were interrupted by World War II. In the 1950's, the Agricultural Research Service (ARS) of the U.S. Department of Agriculture screened more than 500 plant species as potential fiber sources for pulp and paper manufacturing. As a result, kenaf was selected as the most promising nonwood fiber plant for this use. Continued research resulted in the development of high yielding, anthracnose resistant varieties. Today, research and development continues, primarily in Texas, Oklahoma, Mississippi and Southeastern U.S.A., with emphasis on development for newsprint manufacture.
Kenaf has a unique combination of long bast and short core fibers which makes it suitable for a range of paper and cardboard products. Scientists at the ARS have tested several kenaf pulping techniques, with the pulps being used to make several grades of paper including newsprint, bond, coating raw stock and surfaced sized. Results have been positive, particularly in terms of paper quality, durability, print quality and ink absorption.
Commercialization of kenaf for newsprint manufacturing is in its final stages. Commercial scale newsprint runs were conducted by the private sector in California, Texas and Florida. Newspapers made from kenaf pulp have been shown to be brighter and better looking, with better ink laydown, reduced ruboff, richer color photo reproduction and good print contrast. Quality analyses showed kenaf newsprint to have superior tear, tensile and burst ratings. Additionally, kenaf newsprint manufacturing requires less energy and chemicals for processing, an important advantage, both economically and environmentally.
The top leafy portion of the kenaf plant is not useful for pulping. Therefore, this part of the plant would be useful as forage if harvest equipment could be practically adapted to a dual collection operation.
Florida researchers found that immature plants at a height of 6 ft contained up to 20% protein. The kenaf ensiled successfully, both alone and with corn, and the silage was acceptable to heifers. They also found that kenaf leaf, dried and ground into a meal, had a greater crude protein digestibility than that of alfalfa meal. The amino acid composition of kenaf leaves is similar to that of alfalfa.
Oklahoma research showed that kenaf leaf and petiole (non-stalk) portions of the plant were readily consumed by lambs and contained low fiber and high N concentrations. Analysis of the leafy kenaf material showed values of 8.7% NDF, 3.5% ADF and 34.0% CP. Contrasted to this, total plant composition levels were 42.9%, 32.6% and 17.1% respectively. Clearly, the leaf and petiole portion of the plant contains the majority of the digestible nutrients.
Where kenaf is grown in home gardens for fiber, the more tender upper leaves and shoots are sometimes eaten either raw or cooked.
Kenaf is a member of the mallow (Malvaceae) family, with okra and cotton as relatives. Kenaf plants grown in dense stands are largely unbranched and grow to a height of 8 to 14 ft and under certain conditions will reach 20 ft. The stem's outer bark contains the long soft bast fibers which are useful for cordage and textiles. Bast fibers make up 20 to 25% of the stem on a dry weight basis. Beneath the bark, a thick cylinder of short woody fibers surrounds a narrow central core of soft pith.
Stem color of most varieties is green, but there are several red-stemmed and purple-stemmed accessions. Leaf shape varies considerably. While the first few leaves of kenaf seedlings are not lobed, some varieties develop post-juvenile leaves that are very deeply lobed. The root system is very extensive, with a deep tap root and widespreading lateral roots.
Kenaf plants produce large cream-colored flowers only after day length reaches approximately 12.5 hours in the fall. Flower production is indeterminate. Kenaf is primarily self-fertile, but is considered an often cross-pollinated crop. Seeds are dark grayish-brown, flattened triangular shaped, 5 to 6 mm long. There are roughly 15,000 to 20,000 seeds/pound.
The kenaf plant is said to have a wider range of adaptation to climates and soils than any other fiber plant in commercial production. Kenaf yields have been highest in regions with high temperatures, a long growing season and abundant soil moisture. It is quite sensitive to cool temperatures and grows slowly when temperatures are below 50°F.
Kenaf has performed well in the Gulf Coast region and the far southeastern United States. Yields in the Midwest U.S. have been high at times, but inconsistent. Length of frost-free season and availability of solar radiation may limit economic production of kenaf fibers in northern locations. Variety development for tolerance to cool air and soil temperatures could greatly expand kenaf's area of productive adaptation.
Kenaf is adapted to a wide range of soil types, but performs best on the heavier, well drained, fertile soils. Kenaf does not perform well on soils with severe drainage problems. Prolonged periods of standing water, particularly during the seedling stage, can severely inhibit growth.
Kenaf seeds are relatively small and require good seed-soil contact for germination. Therefore, a fine, firm, well-prepared seedbed is necessary. Given the success of raised-bed kenaf production in Texas, ridge-till planting may be an effective option in northern areas.
Recommended planting dates are similar to those for soybeans. Warm, moist soils after danger of a killing frost has passed are the ideal planting conditions. Planting too early often results in poor emergence and slow, non-competitive growth. Planting too late will often result in reduced yield potential due to reduced solar radiation availability.
The preferred plant population, row width and planting method may vary according to production region, growing conditions and cultivar used. More research is needed to determine interactions among these factors (particularly cultivar/population interactions) and to determine yield improvement potential through modification of these factors.
A harvest plant population of 75,000 to 100,000 plants/acre is generally recommended. However, as the crop is moved north, narrower rows and a population range of 100,000 to 120,000 plants/acre may be more desirable. Plants in stands that are too dense for the cultivar or seasonal growing conditions tend to be short, spindly and weak-stemmed. Plants in stands that are too sparse produce lateral branches that are too heavy. In both cases lodging is inevitable. Row spacing decisions should consider probable weed problems and control measures, harvest method and plant population goal.
Seed should be planted less than 1 in. deep if the soil moisture and seedbed texture are suitable. Kenaf can emerge from a depth of 2.5 in. under the most favorable conditions. The importance of high quality seed (germination over 80%), and equipment that gives uniform seed placement and good seed-soil contact cannot be overemphasized.
Kenaf's response to added fertilizers depends on soil nutrient levels, cropping history and other environmental and management factors. A range of fertility responses have been reported. In general, added nitrogen has increased kenaf yields. Some important considerations:
In the U.S., the varieties used most extensively are those developed by ARS researchers in Florida - 'Everglades 41' and 'Everglades 71'. Both varieties are resistant to anthracnose. Since their development in the 1960's, there has been little variety development activity, although the ARS is initiating new breeding efforts. Genetic improvements for adaptation to northern environments may be feasible.
In the southern U.S., kenaf emerges and grows so rapidly that it competes effectively with weeds. In cooler climates and with earlier planting dates, cultural and/or chemical weed control measures are more important. One weed species which is especially competitive with kenaf is velvetleaf, a relative of kenaf. At the seedling stage, velvet leaf and kenaf are very similar in appearance and rate of growth. Fields with high populations of this weed are not recommended for kenaf production.
Satisfactory weed control and crop tolerance have been demonstrated for many of the common preplant-incorporated, preemergence and postemergence herbicides. Special-Local-Need labeling, leading to full registration, is being pursued in some states.
Cultivation and other postemergence tillage practices such as rotary hoeing can be effective in controlling weeds. In the absence of herbicide registration for kenaf, and particularly in cooler climates which inhibit rapid early plant growth, mechanical weed control should be used.
Growers and researchers have found kenaf to be resistant to most plant diseases. Anthracnose is perhaps the most serious potential disease problem. The variety development work done by the ARS in Florida was very effective in breeding and selecting for anthracnose resistance. Damping-off is a moderate concern during seedling stages and seed treatments are being tested and registered for use.
Nematodes are viewed in some areas as the most serious constraint to kenaf production. In cotton growing areas, the root-knot nematode/fusarium wilt complex is expected to limit yield potential for both cotton and kenaf, and will create crop rotation challenges due to the common susceptibility of the two crops.
There has been little economic damage to kenaf by insects in experimental production fields. Potential insect problems could arise in the early stages of seedling emergence and development. However, the kenaf plant tolerates a fairly high population of chewing and sucking insects, and since the production emphasis is biomass rather than root, seed, fruit or flower, the required level of insect protection for kenaf may be much less than for most commercial crops.
Several harvest and storage methods have been tested. The preferred system will likely vary from one area to another because of differences in climate and milling requirements. Until recently, the most feasible method appeared to be chopping the green or air-dried plants with a forage chopper. The green material can be stored anaerobically like silage and the air-dried material can be piled or loosely stacked or baled.
The most recent innovation has been the development of an 8-row harvest machine which cuts the stalks and lays them down for drying in the field. The dried stalks are gathered, shredded at the field and transported to the fiber mill's storage area.
Yields in research plots have varied widely, from 2.5 ton/acre at Rosemount, Minnesota, to 15 ton/acre at College Station, Texas. One-acre blocks at Fort Gibson, Oklahoma yielded nearly 10 ton/acre in 1988. Commercial scale production in Texas has produced dry-weight yields of 7.5 ton/acre under irrigation and 6.0 tons/acre on dryland.
Fiber production field trials in the southeast U.S. have shown that kenaf can yield three to five times more fiber/acre/year than southern pine. With U.S. annual consumption of newsprint at over 12 million tons (1988), and with 60% of that volume imported at a cost of approximately $4.5 billion, the further commercialization of kenaf as a source of paper pulp would appear promising. However, successful commercialization will be dependent upon local cost comparisons which will consider economies of scale, transportation costs and local processor demand.
Successful introduction of any new crop depends on establishment of markets for the raw and processed crop materials, in concert with development of production areas. There appear to be significant markets for kenaf fibers in the manufacture of pulp, paper and paperboard products, and as synthetic fiber substitutes. Projections show that by the year 2000, over one million acres of kenaf would be needed annually to supply just the increase in U.S. newsprint demand over 1990 levels, without reducing the volume of newsprint imports or the consumption of wood fibers.
Production of kenaf as an industrial raw material will necessarily be localized in the same region as processing facilities. The unprocessed crop is too bulky to be transported great distances. As with many processed food crops, the actual price that producers receive for raw product will be determined by contract negotiation between them and the processors. That price must consider production costs, the comparable risks and profits of producing conventional crops and the comparable prices paid by potential customers for traditional fiber supplies. A producer would also need to consider kenaf's impact on government programs and its effects on crop rotations and on productivity of other crops in a rotation.