Hemp facts and hemp fiction Hayo M.G. van der Werf

International Hemp Association, Postbus 75007, 1070 AA Amsterdam, the Netherlands

Jack Herer's book, The Emperor Wears No Clothes, edited by Chris Conrad, Lynn Osburn and Judy Osburn, provides fascinating reading. It was one of the major causes of the enormous increase in interest in fibre hemp in recent years. Although most of the information contained in the book is valid, some of its claims are clearly incorrect.

Like Jack Herer and his editorial team, I am convinced that hemp has real potential as a 'new' crop and that it deserves more attention from farmers and agronomists. We are currently witnessing the renaissance of fibre hemp, but I think this renaissance is not well served by making unrealistic claims regarding the potential of hemp.

Some quotes from The Emperor Wears No Clothes :

"Hemp [...] uses the sun more efficiently than virtually any other plant on our planet" (page 2).

"Depending on which U.S. agricultural report is correct, an acre of full grown hemp plants can sustainably provide from four to 50 or even 100 times the cellulose found in cornstalks, kenaf or sugar cane - the planet's next highest annual cellulose plants" (page 44).

"Hemp hurds are 77 % cellulose" (page 44).

Herer and his editorial team claim that hemp produces higher yields than other crops and that hemp hurds, which make up 60 to 80 % of the stem dry weight, contain 77 % cellulose. These claims are repeated in other books: Energy Farming in America (by Osburn), Hemp, Lifeline to the Future (by Conrad). Both claims are simply wrong.

For a start, cellulose content of hemp hurds has been found to vary between 32 and 38 % (Bedetti and Ciaralli 1976, van der Werf 1994). Possibly, Herer confuses the hurds, which form the woody core of the hemp stem, with the bark, which forms the outer layer of the hemp stem. The bark contains the long bast fibres which are used in textile manufacturing. The cellulose content of the bark is much higher than that of the core. It has been found to lie between 53 and 74 % (Bedetti and Ciaralli 1976, van der Werf 1994).

One of the objectives of the research I conducted over the last four years was to investigate the yield of hemp relative to that of other crops.

Essentially, all agricultural crops do the same thing: they intercept light with their canopy and, in the leaves, sugars are formed as a result of photosynthesis. Most of the sugars move out of the leaves to other parts of the plant and are transformed into other organic substances, e.g., cellulose and lignin in the stem, and starch, protein and oil in the seed.

When growing conditions are optimal, that is, in the absence of limitations due to lack of water or nutrients, or due to pests, diseases, or other stresses the yield of any crop will depend on:

a) the total amount of light (L) intercepted by the crop canopy during the growing season;

b) the radiation use efficiency (RUE): the amount of dry plant material produced per unit of light intercepted by the canopy;

c) the harvest index (HI): the fraction of the plant parts of economic value in the total plant dry matter produced.

This can be summarized in a simple formula:

Yield = L x RUE x HI

Grown side by side under favourable growing conditions, different crops may have different yields. Relative to most other crops grown in the Netherlands, fibre hemp intercepts quite a lot of light (L is high) because the crop can be sown early and it grows well at low temperatures, so the canopy is established rapidly. The radiation use efficiency of fibre hemp is relatively low, probably as the result of the structure of the canopy and the relatively high lignin content in the stem. As with other annual fibre crops, such as flax or kenaf, the harvest index of fibre hemp is relatively high.

In the most favourable growing conditions, we obtained yields of up to 15,000 kg of stem dry matter per hectare (6,070 kg per acre). Under simular conditions, other crops such as maize, sugar beet or potato produced similar dry matter yields. All results indicate that as far its yield is concerned, fibre hemp is in no way exceptional.

These results have been detailed in my thesis, Crop Physiology of Fibre Hemp (Cannabis sativa L.). From these results I cannot help but conclude that the claim that hemp produces higher yields than other crops, simply is not true. This, of course, does not necessarily mean that hemp is not an interesting crop for farmers. I think that hemp is an excellent candidate to fill the niche for an annual fibre crop in temperate climates. Hemp seems to be an attractive crop for a sustainable agricultural system, as it has moderate fertilizer requirements, needs little or no pesticides and suppresses weeds and some major soil-borne pathogens.

Cannabis is certainly a valuable crop plant, and its future succes will depend in part on the responsible dissemination of accurate information concerning its benefits.