Van der Werf, H. M. G., W. C. A. van Geel
and M. Wijlhuizen 1995. Agronomic research on hemp (Cannabis sativa L.) in
The Netherlands, 1987-1993. Journal of the International Hemp Association
A Dutch research programme on the crop physiology and agronomy of hemp was conducted from 1987 to 1993, the first three years being centered at the DLO Research Institute for Agrobiology and Soil Fertility (AB-DLO) in Wageningen (Meijer et al. 1995). The last four years of the programme were carried out at AB-DLO, at the Research Station for Arable Farming and Field Production of Vegetables in Lelystad and at Wageningen Agricultural University (Van der Werf 1994, Van der Werf & Van Geel 1994). This research was part of a national research programme investigating the feasibility of hemp as a raw material for paper pulp.
Within the European Union, the prices of many arable crops have fallen in recent years, and subsidies for food crops have been reduced in an attempt to combat production surpluses. As a result, crop rotation on arable farms in The Netherlands is increasingly restricted to a few crops which are relatively profitable (potato, sugar beet) or agronomically indispensible in the rotation (cereals). This short crop rotation has increased the incidence of diseases, in particular soil pathogens, and has lowered yields. It has also led to greater use of biocides, in particular of soil fumigants. This is a worrying development, as it is generally agreed that arable farming should become more sustainable, and use less biocides.
The identification and development of a 'new' crop, to be introduced into current rotations, might help solve this problem. However, the new crop candidate should be profitable, produce for a large non-food market, require little or no biocide and help reduce disease incidence in the current crop rotations. Hemp grown for paper pulp, was proposed as such a crop in the early 1980s (Du Bois 1982). Its yield was reported to be high and it was said to improve soil structure. Furthermore, hemp was claimed to suppress weeds effectively, and to be virtually free from diseases or pests. Finally, it was said that hemp stems would fetch high prices as raw material for the pulp market. A few years later, similar and even more fantastic claims were made by Herer (1985) in his book 'The Emperor Wears No Clothes'. A quote from Herer's book: "Depending on which US agricultural report is correct, an acre of full grown hemp plants can sustainably provide from four to 50 to 400 times the cellulose found in cornstalks, kenaf or sugar cane, the planet's next highest annual cellulose plants." From these claims it would seem that hemp is a wonder crop with nothing but advantages.
On 10 July 1843, Mr. Theophile Gautier, a french writer, wrote his weekly contribution to the newspaper La Presse. The first line of his contribution was most remarkable:
"For a long time, we had heard talk about
the marvellous effects brought about by hashish,
without believing much of it."
He then continued to report his personal findings in the matter. Gautier followed an empirical approach: he heard stories which seemed improbable, was sceptical, but investigated the facts. At the start of our research into the crop physiology of hemp, we were in the same position as Mr. Gautier: we had heard about the marvellous potential of fibre hemp, but we were quite sceptical about it. Like Mr. Gautier, we investigated the facts. This paper reviews the relevant literature and reports some of our major results.
The chemical composition of hemp stems
Hemp stems can be separated into two components: the stem tissues outside the vascular cambium (bark) and the stem tissues inside the vascular cambium (core). Bark and core differ in their chemical composition: Bedetti & Ciaralli (1976) reported 67% cellulose, 13% hemicellulose and 4% lignin in the bark of an Italian hemp cultivar harvested in October. Its core contained 38% cellulose, 31% hemicellulose and 18% lignin. Bosia (1976) presented similar results. In stems from a number of experiments conducted in The Netherlands, the mean cellulose content of bark was 64.8%, of core it was 34.5%. The mean hemicellulose and lignin contents were 7.7% and 4.3% in bark and 17.8% and 20.8% in core (Van der Werf et al. 1994a).
Fibre length and the contents of cellulose and lignin are important quality parameters of raw materials for paper. The strength of paper increases with fibre length. Cellulose content is important, because in chemical pulping the pulp yield corresponds to the cellulose content of the raw material. Since lignin is removed by environmentally unfriendly procedures, a low lignin content is desirable. In the bark, fibres are longer, cellulose content is higher, and lignin content is lower than in the core. As a result, bark is more valuable as a raw material for paper than core and the value of hemp stems therefore depends primarily on its bark content.
Figure 1. Location of sites mentioned in the text,
1 Valthermond, 2 Lelystad, 3 Randwijk
Pulp and paper
Paper was invented in China in the first century BC, with hemp bast fibre as one of its major components (Abel 1980). In 1913, world consumption of paper was 14 million ton, in 1950, it was 40 million ton and in 1989, 230 million ton (FAO 1991). However, hemp fibres have lost their former major role in pulp and paper making. In 1989, 92% of the virgin fibre used to manufacture paper came from wood, the remaining 8% was from annual crops or crop residues (FAO 1991).
During recent decades the pulp and paper industry has been criticized for its negative impact on the natural environment: deforestation or the replacement of old-growth forests by tree plantations (Postel & Ryan 1991), the emission of chemical waste, high energy use by pulp and paper mills, and the production of toxic and mutagenic waste products by chlorine bleaching (McDougall et al. 1993). Measures taken to tackle these problems include increased recycling of paper, more sustainable management of tree plantations and forests and a shift towards less harmful pulp and paper technologies.
A comeback for bast fibre crops as the raw material of paper may further contribute to the solution of some of these problems. Growing such annual crops on farmland obviously lessens the need to cut down forests. In addition, less energy is required to produce pulp from kenaf (Keto 1990) or hemp (S. J. Lips, DLO Agrotechnological Research Institute, personal communication 1994) compared to wood. Finally, the lignin content is lower in bast fibre crops such as kenaf and hemp than in wood, offering better opportunities for non-chlorine bleaching or the production of unbleached pulp (McDougall et al. 1993).
Potential yield and trial yields
Field experiments have shown that under average Dutch weather conditions, a hemp crop sown on April 15 and grown at a density of 64 plants per m2, will reach canopy closure on June 1 (Van der Werf 1994). Under favourable conditions (sufficient water and nutrients, no pests, diseases or other stresses), crop growth rates of 180 kg of above ground dry matter per ha per day have been found in hemp crops between canopy closure and flowering (Van der Werf et al. 1995). Similar or higher crop growth rates have been found for many other annual crops.
If a hemp crop which reached canopy closure on June 1 were harvested on September 8, a closed canopy would have been present for 100 days. Therefore, for a late cultivar which would flower after September 8, the increase in dry matter yield between June 1 and September 8 could be 0.18 x 100 = 18 ton of above ground dry matter per ha. At canopy closure about 2 ton of dry matter per ha is present, so the potential yield would be 18 + 2 = 20 t/ha. Since a non-flowering hemp crop contains 2 to 3 t/ha of leaf dry matter, potential stem yield would be 17 to 18 t/ha for late-flowering hemp in The Netherlands.
From 1987 to 1993, field trials were conducted on three sites in The Netherlands (Figure 1): Valthermond (peaty sand soil), Lelystad (light marine clay soil) and Randwijk (heavy river clay soil).
The crops were sown in April or early May and harvested in September or October. Plant densities at emergence were between 10 and 800 per M2, but in most experiments between 65 and 150 per m2. In most experiments, the crops were sprayed with fungicides at 14-day intervals during June, July and August to prevent the occurrence of fungal diseases. In these experiments, above ground dry matter of living plants at final harvest varied between 8 and 19 t/ha. Stem dry matter yields were 7-13 t/ha at Valthermond, 8-14 t/ha at Lelystad and 11-17 t/ha at Randwijk. At Randwijk yields were high, partly because one of the cultivars flowered very late, increasing its yield potential. Other factors probably also contributed to yield differences between locations: temperature (Randwijk is a 'warm' location), soil-borne diseases (more soil pathogens at Valthermond) and soil pH and soil water status (less favourable at Valthermond).
At all locations, observed yields generally were well below the potential yield of 17-18 t/ha of stem dry matter. Our research has revealed a number of causes for this discrepancy. These causes will be reviewed below.
Plant density and self-thinning
Recommended seeding rates for fibre hemp vary between 40 and 150 kg per ha, which correspond to plant densities shortly after emergence of about 200 to 750 plants per m2 (Dempsey 1975). Plant densities between 80 and 400 per m2 have been found to have little effect on stem dry matter yield (Jaranowska 1963, Jakobey 1965, Rynduch 1975, Marras & Spanu 1979, Meijer et al. 1995). The economically optimal plant densities of hemp grown for bast fibre are higher than the lowest plant density that gives maximum stem dry matter yield because plant density continues to improve stem quality when it no longer increases stem yield. With increasing plant density, bast fibre content in the stem tends to increase (Heuser 1927, Jakobey 1965) and the fineness of the bast fibres (an important aspect of fibre quality for spinning purposes) improves (Jakobey 1965).
In experiments conducted from 1987 to 1989, seeding rates were high (20-140 kg per ha) as recommended in the literature. However, at these densities more plants died before the end of the growing season than were reported from the experiments conducted in other European countries. Even at the lowest densities investigated, almost half the plants had died before harvest in September (Meijer et al. 1995). Further analysis of these results and of the results of plant density experiments conducted in 1991 and 1992 has revealed that the number of plants surviving at harvest depends on the dry matter yield of the crop (Van der Werf et al. 1995). At high plant densities, inter-plant competition may generate a size hierarchy, i.e. increase variability in the size of the individuals, with large plants suppressing smaller ones. Eventually, self-thinning may occur, if suppressed plants die. Self-thinning is undesirable, as plants which die in the course of the growing season will not be harvested. As a result of self-thinning, the maximum density of living plants is smaller when the yield level is higher. In other words, one hectare will accomodate fewer 100 year old oak trees than 6 week old radishes.
In a hemp crop yielding 10 t/ha of above ground dry matter, the maximum density of living plants is about 460 per m2. At a dry matter yield of 15 t/ha, the maximum density of living plants is about 120 per m2. In most hemp growing areas of Europe, hemp yields do not exceed 8-10 t/ha of dry matter. As a result of a later flowering dates and a better water supply, much higher yields have been obtained in The Netherlands. At these higher yields, the optimum plant density will be lower.
In 1991 and 1992, the cultivar Kompolti Hibrid TC was grown at 10, 30, 90 and 270 plants per m2 at Randwijk (Van der Werf et al. 1995). The maximum stem yield (15.1 t/ha) and the maximum bark content in the stem (35.7%) were obtained at 90 plants per m2 at harvest in September. At 270 plants per m2, severe self-thinning caused a lower stem yield (12.9 t/ha). Therefore, a density of about 90 plants per m2 seems optimal for late-flowering high-yielding fibre hemp crops. When plant densities of several hundreds of plants are used in a situation which allows dry matter yields of well above 10 t/ha, self-thinning may cause significant losses of dry matter.
In our experiments, we used the French cultivar Fedrina 74, three Hungarian cultivars (Kompolti Hibrid TC, Kompolti Sargaszar, Kompolti), a high-fibre selection from Kompolti ('Kompolti Hyper Elite') and the Japanese cultivar Kozuhara zairai. Fedrina 74 is monoecious, it flowers in the beginning of August. All Hungarian cultivars are dioecious; they flower in the second half of August. Kozuhara zairai is dioecious and flowers in the second half of September.
Flowering date has a major effect on yield potential. After flowering, the growth rate of the crop decreases and an increasing proportion of the assimilates are used by the inflorescence, so that stem yield hardly increases (Meijer et al. 1995). In 1990 and 1991, ambient daylength was compared with a 24-hour daylength in field experiments for Fedrina 74 and Kompolti Hibrid TC at Randwijk. The 24-hour daylength did not totally prevent flowering, but did greatly reduce the allocation of dry matter to floral parts. Furthermore, it enhanced the efficiency of post-flowering light use, and boosted stem dry matter yield by 2.7 t/ha in both cultivars (Van der Werf et al. 1994b).
In 1991 and 1992 the cultivar Kompolti Hibrid TC was compared to Kozuhara zairai at a density of 90 plants per m2 at Randwijk (Van der Werf et al. 1995). Kompolti Hibrid TC reached its maximum stem yield (15.1 t/ha) in September, when the stem yield of Kozuhara zairai was 15.4 t/ha. The stem yield of Kozuhara zairai, however, continued to increase and reached 16.6 t/ha one month later. From these experiments, it is clear that stem yield of hemp can be limited by early flowering date. Ideally, a hemp crop grown for stem production should not flower before harvest. Therefore, flowering date is a initial criterion in cultivar choice.
In the current hemp growing regions, disease problems in hemp crops are rare. In our experiments, hemp crops suffered severe damage from fungal diseases in rainy years, unless fungicides were frequently applied. Most damage was caused by the fungus Botrytis cinerea. Sclerolinia sclerotiorum was also found, but much less frequent. Both fungi cause stem lesions. The cultivar Kompolti Hibrid TC seemed somewhat more susceptible to B. cinerea than the other cultivars investigated.
In 1992 and 1993, the effect of nitrogen fertilization (0, 40, 80, 120, 160 and 200 kg per ha of nitrogen) was investigated for the cultivars Kompolti Hibrid TC and Kompolti at Lelystad (Van der Werf & Van Geel 1994). Plant density at emergence was 149 per m2 in 1992 and 114 per m2 in 1993. In these experiments, no fungicides were applied. In 1992, diseased plants were rare, but in 1993 many plants carried stem lesions caused caused by B. cinerea and some carried S. sclerotiorum lesions. The level of nitrogen fertilization did not seem to affect the level of disease infestation. In 1992, plant density was 96 per m2 at harvest in September and above ground dry matter yield of living plants was 12.7 t/ha. Another 1.1 t/ha of dead plant material was also present. In 1993 plant density was 96 per m2 at harvest in September and yield of living plants was 8.3 t/ha, and 3.9 t/ha of dead material was present. These data indicate the extent to which fungal diseases can reduce hemp yield.
In 1991, 1992 and 1993, the effect of the fungicide application was investigated for the cultivar Kompolti Hibrid TC at Valthermond (Van der Werf & Van Geel 1994). Plant density at emergence was about 100 per m2. Treatments were: control (no spraying), one application of vinclozolin (0.5 kg per ha at 50% ground cover by the crop), two applications of vinclozolin (0.5 kg per ha at 50% ground cover and again 10 days later). In order to avoid border effects, plots were large (225 m2). Spraying of fungicides did not affect hemp yields for 1991 and 1992. The 1993 growing season was rainy and large number of diseased plants were found in all treatments. In 1993, above ground dry matter yield was 11.2 t/ha for the control, 12.3 t/ha where vinclozolin had been applied once and 11.7 when the fungicide had been used twice. The amount of dry matter of dead plants was 0.8 t/ha for the control and 0.4 t/ha for both other treatments.
In rainy years, the occurrence of Botrytis cinerea can cause large reductions in hemp yield in The Netherlands. Preventive use of fungicides is not a solution to this problem as it has a modest effect on yield only in wet years. Sources of genetic resistance to B. cinerea, e.g. some drug Cannabis cultivars of equatorial origin, (D. P. Watson, personal communication 1995) do exist, so it should be possible to breed fibre cultivars less susceptible to B. cinerea.
Hemp in a crop rotation
Any 'new' crop that Dutch farmers would appreciate should help reduce disease and weed problems in the current crop rotations. Kok et al. (1994) investigated the effect of fibre hemp on three major soil pathogens: the fungus Verticillium dahliae and the root-knot nematodes Meloidogyne chitwoodi and Meloidogyne hapla. All three pathogens were suppressed by hemp, and the authors concluded that the introduction of hemp in a crop rotation might improve soil health.
Except in the cases where plant densities were very low (10 or 30 plants per m2), our experiments demonstrated that hemp crops effectively suppressed weeds and that no herbicides were needed. This confirms literature reporting such effects (Heuser 1927, Tarasov 1975, Lotz et al. 1991).
Agronomically, hemp corresponds rather well to the profile of an attractive 'new' crop for arable farmers in The Netherlands. Hemp stems potentially have a large non-food market and the crop can give good dry matter yields, though not higher than those of other annual crops. In order to obtain high yields, plant density should be set in accordance with the expected dry matter yield and a sufficiently late-flowering cultivar should be used. The fungus Botrylis cinerea can cause important damage in rainy years, although this problem may be solved by breeding resistant cultivars. Hemp requires little or no biocide and suppresses weeds and some major soil-borne diseases.
To what extent hemp will fulfill its promising agronomic characteristics will largely depend on economic factors: the level of production costs and subsidies and the demand for and prices of hemp products.
The authors thank D. W. Pate and A. J. Haverkort for helpful comments on the manuscript.