In Hevea, the present commercial source of rubber, production is continuous and non-destructive to the plantations. Trees are tapped for many years for the collection of latex. In guayule, on the other hand, rubber is stored in individual cells and its recovery requires the destruction of whole plants. Tapping and collection of latex from Hevea is labor intensive, making the plant unsuitable for industrialized countries. All phases of production of rubber from guayule, however, including planting, harvesting and processing of the shrubs, are amenable to mechanization, thus making guayule an ideal rubber crop for mechanized agriculture.
Renewed interest in developing guayule as a domestic rubber crop is based on the fact that the United States is totally dependent on foreign sources and spends close to one billion dollars annually to import approximately 750,000 metric tons of Hevea rubber, principally form Malaysia. Natural rubber, an industrial commodity of strategic importance, constitutes 28% of the total domestic rubber market and is indispensable for automobile, bus, truck, and airplane tires.
Concerted basic and applied research on genetics and breeding, biochemistry, physiology, agronomy, shrub processing, and co-products are being conducted in California, Arizona, Texas, New Mexico, Illinois, and Mississippi toward successful commercialization of guayule. In January 1988, through the joint efforts of the U.S. Department of Agriculture, the Department of Defense, and several private companies, a prototype guayule processing facility began operation near Sacaton, Arizona. This facility will produce about 1 00 tons of rubber for application tests by the Department of Defense, and may also be used for shrub processing research in the future.
Guayule must become more productive in order to become commercial. For this reason, germplasm enhancement and development of high rubber-producing cultivars through plant breeding is considered to be the highest priority (Thompson and Ray 1988). Another production limitation is the prohibitive cost of stand establishment by transplanting rather than direct sowing of seed. One of the primary objectives of the guayule genetics and plant breeding project at the University of California is to develop high rubber-yielding cultivars that are also capable of regrowth following their harvest at ground level. Three complimentary approaches: selection and hybridization of apomictically reproducing polyploid plants (2n = 54 and 72 chromosomes), recurrent selection of sexually reproducing diploid plants (2n = 36), and interspecific hybridization of guayule to its related species, are being used to develop high rubber-yielding cultivars (Estilai and Youngner 1984). This paper reports on the yield performance of two new selections and examines the regrowth potential of different guayule materials. The new cultivars, known as 'Cal-6' and 'Cal-7', are apomictic progenies of a tetraploid and a triploid plant, respectively. They were selected and released on the basis of their increased biomass and rubber yield (Estilai 1986). The old cultivars are apomictic polyploid selections developed in the 50's and are commonly referred to as "USDA varieties" (Hammond and Polhamus 1965).
At Shafter, harvests were made in March 1985 and March 1987, when plants were 21- and 45-months old. At each harvest, six plants from the two inside rows of each plot were cut at ground level. At Riverside, harvests were made in April 1987 and April 1988, when plants were two- and three-years old. At each harvest, six plants from the two inside rows were dug from a depth of 40 cm. In both locations, after removal of leaves and flower heads, plants were weighed, chipped and sampled for rubber and resin content determination.
At Shafter, rubber and resin contents were determined by the double extraction procedure using acetone and cyclohexane to extract resin and rubber form finely ground shrub samples, respectively (Black et al. 1983). At Riverside, the near-infrared reflectance spectroscopy (NIR) method was used to determine rubber and resin content (Black et al. 1985). The two methods produced similar results.
For the regrowth study, 21-, 23-, 33-, and 45-month-old plants from different selections were harvested at ground level in February and March when plants were semi-dormant, or in May when they were actively growing. Three months after each harvest, plants that had survived and had regrown were counted. The data were subjected to analysis of variance procedures.
At Shafter, California, 'Cal-6, with a mean annual rubber yield of 908 and 796 kg/hectare at the age of 21 and 45 months, respectively, produced approximately 128% more rubber than the check cultivar 'N565'.
At Riverside, California, 2- and 3-year old 'Cal-6' yielded 652 and 914 kg/hectare-year. At the age of two years, 'Cal-6' yielded 34% more rubber than check cultivar 'N396', and after three years, it produced 62% more rubber than 'N396'-
All entries produced substantial amounts of resin, an important co-product, that may provide additional revenue for economic commercialization of the crop. Beyond two years, the rubber content decreased by age. Resin content on the other hand, stayed the same or showed a small increase. Since plants continue to produce biomass each year, the total rubber and resin yields continue to increase.
Regrowth studies showed that the time of harvest is a critical factor in regrowth of guayule plants cut at soil surface. Approximately 5% of the plants cut in May, when they were actively growing, survived. The survival rate improved substantially for the dormant plants harvested in February-March. Regrowth was also found to be genotype dependent. Entry C250 showed more than 98% survival rate over three consecutive years. On the other hand, only 13% of the plants from entry C255 survived over the three-year study
Improved selections such as 'Cal-6' and 'Cal-7' have the potential of producing 600 to 900 kg/ha-year of rubber, depending on the age of the plant and location. These selections, although substantially more productive than the old USDA cultivars, are still not productive enough to bring about economical commercialization, given the current price of natural rubber. The regrowth potential, observed in some of the guayule genotypes, makes guayule a more attractive crop. Regrowth will make multiple harvest of guayule a possibility and may eliminate the need for re-establishing the fields with transplanted seedlings after each harvest. Guayule plants are normally harvested after 3 to 5 years of growth. Cultivars that could be harvested in 2-year cycles will reduce the time that guayule growers must wait for a financial return from the crop. The number of cycles that guayule may be harvested successfully must be investigated.
The combined effects of four major factors, rubber yield, price of rubber, productions costs (growing, harvesting, transporting, and processing plants), and the level of revenues generated from by-products (primarily resins and bagasse), will eventually determine if guayule can succeed as a commercial rubber crop in the United States. With the present price of rubber ($1.10-$1.30 per kg) approximately 1,500 kg of rubber per hectare per year is needed to make guayule a profitable crop. This indicates that cultivars twice as productive as those currently available must be developed in the next 10 to 15 years. On the other hand, if the price of natural rubber were to double, the best available cultivars appear sufficiently productive to make guayule a commercial crop immediately.
An examination of the increase in the rubber yield of Hevea between 1946 and the present demonstrates that the development of new guayule cultivars with an annual rubber yield of 1,500 kg per hectare and higher is definitely within the realm of possibility In 1946, the yield of natural rubber from Hevea in Indonesia and Malaysia was under 300 kg/ha-year. At present, the average yield is about 3,000 kg/ha-year which shows that over a period of 40 years, Hevea rubber yield has been increased about one order of magnitude.
In 10 to 15 years, given an adequate level of support for research and development, guayule can become a domestic source of renewable hydrocarbons. To achieve this goal, it is essential to exploit the natural variability of the sexual diploids, the development of cultivars that regrow successfully after harvests in 2-year intervals (thus reducing the cost of stand establishment), the establishment of new markets for guayule by-products, the reduction in processing costs, and the use of desirable traits of related species.
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