Of all prospective alternative rubber sources, Guayule (Parthenium argentatum Gray), a shrub growing in semi-arid regions in Mexico and the Southern USA, has received the most attention. It appears to be a viable alternative to the rubber tree (Hevea brasiliensis) because it produces high-quality rubber with essentially the same molecular weight as the material obtained from the rubber tree. In fact, it is the only non-tropical plant that has been used as a commercial alternative source of natural rubber.
However, due to exhaustion of the native stands and increased competition by Hevea brasiliensis rubber in the early 20th century, guayule processing was abandoned in the 1930s.
In response to rubber shortages during WWII and the oil crises in 1979 and 1982 two cycles of focused efforts allowed the collection and generation of germplasm, and progress in breeding, agronomy, processing, and potential products (see pictures of the efforts in Salinas, 1942). As a result of this work, guayule is already a partially domesticated crop. However, because of the long pauses between intensive research and development efforts, much of the germplasm was lost.
Because hypoallergenic rubber and latex products are in high demand, guayule has an extra advantage of Hevea rubber. Guayule rubber is special in that it has a much lower protein content than the H. brasiliensis-based product. In addition, fewer protein species are associated with purified rubber particles from guayule. Even more important is that these do not cross-react with immunoglobulin (Ig)E (Type I latex allergy) and IgG antibodies to H. brasiliensis latex proteins. This makes allergic reactions to guayule rubber by consumers sensitized to H. brasiliensis rubber unlikely. Therefore, guayule is again being developed as a source of hypoallergenic latex by Yulex Corporation, and a commercial plant for guayule processing now operates in Arizona (view on YouTube).Guayule does have several disadvantages, which – in the past – have presented major barriers to its commercialization. Besides being domesticated only partially, it does not tolerate the low winter temperatures in much of Europe and the
USA. It is introduced currently as a biannual or multiannual crop, also because most of the rubber is produced during winter months. Stand establishment and optimal harvesting year and method have not been optimized fully. Cultivation systems in which the crop is harvested several times by cutting of the branches and enabling the plant to regrow gives higher yields than simple harvesting of the whole plant after 2–5 years. This system is now used in the commercial cultivation of guayule.
In contrast to Hevea rubber production, which is labor intensive, guayule cultivation, harvesting and latex production can be mechanized fully. However, processing of the guayule shrub is technically complicated and involves significant capital and operating costs. This is because the rubber is produced as µm-size particles in the bark parenchymal cells and not as free-flowing latex as with the rubber tree. To harvest the rubber, the plant material has to be disrupted thoroughly to release the rubber particles from the individual cells. In subsequent steps, the rubber can be separated either by pressing out the latex, followed by centrifugation and creaming steps to purify the latex, or by solvent extraction.
Major issues during processing guayule for bulk rubber using solvent extraction are mainly technical. For example, the difficult separation of the viscous extractant from the finely dispersed solids, accumulation of terpenes and fine particles in the recycled solvents, separation of a low molecular weight rubber fraction and loss of solvents must be solved.
Agronomic characteristics and economic perspectives of available guayule cultivars have been assessed in a number of as yet uncoordinated field trials in Morocco, Greece, Argentina, South Africa, Israel, Australia and the US in the 1980s and 90s. Presently, guayule is only grown in Arizona. However, the guayule shrub appears to be well suited to the semi-arid areas of Southern Europe and, in the context of the reformed and market-focussed Common Agricultural Policy, would offer an alternative production choice in areas such as those currently dominated by cotton production.
Guayule has the additional benefit of being a low input crop with the potential to reduce environmental impact and contribute to sustainable development. Although current varieties could be grown immediately, the species is relatively unimproved and there is potential to improve rubber yield and quality, water use and other agronomic issues.