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The potential of guayule for commercial rubber production in the southwest

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Guayule shrub
Fig. 1. Guayule plants. Credit: Jack Dykinga. This photo is in the public domain. 

Agricultural innovators have known for decades that guayule (pronounced ‘gwah-YOO-lay’) an unassuming shrub native to the Chihuahuan Desert, contains “an economically valuable amount of rubber“[1]. In addition to its rubber, guayule's by-products, such as resin and bagasse (the fibrous material left over after latex extraction), hold potential for use in biofuels. From the farmer' perspective, if grown as a commercial crop, guayule could provide a reliable source of income, while using far less water than many commodity crops grown on southwestern irrigated croplands. From a national security standpoint, guayule offers a dependable domestic alternative, potentially reducing U.S. reliance on rubber imports. 

The cultivation and research support of guayule has expanded and contracted over the past 120 years, with a notable uptick in interest and investment over the last 10 years. Currently, guayule is not being cultivated by many farmers; however, scientists and engineers are making significant progress in the genetics, cultivation, harvesting, and processing of the crop. To fully understand guayule's potential, it's important to consider the global context of rubber production and recognize efforts to make guayule a commercially viable crop.

Where does rubber come from?

Natural rubber is essential to U.S. manufacturing and the economy, so much so that the U.S. government classifies it as a strategic material critical to national security. Despite its importance, the U.S. relies entirely on imports to meet its rubber needs. As the second largest market for natural rubber, the U.S. imported almost $1.5 billion worth in 2023 [2]. The U.S. is also the largest market for rubber tires, importing over $19.5 billion worth in 2023 [3]. In fact, in the U.S. and worldwide, tire manufacturing accounts for over 70% of natural rubber consumption, so it's easy to overlook the over 40,000+ other uses [4]. Rubber combines strength, durability, elasticity, and temperature tolerance and, depending on the application, there are few synthetic alternatives to natural rubber. A world without rubber is difficult to imagine! From the tires that keep our cars running to everyday items like shoes and medical supplies, rubber is everywhere. Without rubber bushings, belts and mechanical seals, industries would struggle to keep manufacturing equipment running, affecting the production of all kinds of factory-made products. 

Rubber is indispensable, yet most people are unaware of where it comes from or the precarious nature of the rubber supply chain. While over 20,000 plant species produce latex, only a small number contain the specific type of latex with the right kind of polymers [Cis-1,4 polyisoprene] to make natural rubber.  A single species dominates commercial rubber production: the Pará rubber tree (Hevea brasiliensis), also known "in the trade" as Hevea. Asia leads in global rubber output, producing 12.9 million metric tons in 2022, followed by Africa with 1.8 million metric tons and Latin America with 0.5 million metric tons [5]. Three countries supply most of the rubber for the U.S. market: the Republic of Indonesia (47%), Thailand (27%), and Côte d'Ivoire (11%) [2]. If we consider the distance between rubber growing countries and rubber importing countries, the U.S. tops the list, with an average supply distance of 4,700 miles [2].

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Harvesting rubber from para rubber trees
Fig 1. A rubber plantation and a worker. Photo by Isuru Ranasinha on Unsplash. 

Speaking of that precarious supply chain, even though demand comes from some of the world's biggest manufacturing companies, it is smallholders farming on less than five acres of land who produce over 85% of the world's rubber. These smallholders face various challenges, including the long wait for rubber trees to mature for harvest (about five years) and the labor-intensive process of harvesting the latex (Fig. 2). Hevea cultivation also suffers from a lack of genetic diversity, meaning that disease can devastate production across wide areas [6]. Most recently, Rubber Leaf Fall disease caused by the fungus Pestalotiopsis microspora has affected production in Southeast Asia with reported yield losses of 15- 50% [7]. It is no surprise, then, that smallholders, with little control over the price they receive for their product and facing rising disease pressures, are increasingly switching from rubber to oil palm, drawn by the promise of better returns for their land and labor investment [6,8]. 

On the supply side, rubber resources are under threat, while globally, demand for rubber continues to increase (apart from a period of contraction in 2020). In 2022, the International Rubber Study Group projected rubber demand would grow on average by 2.4% between 2023 and 2031 [9]. Acknowledging the uncertain future where rubber supplies may not meet demand, USDA-ARS researchers are investigating alternative latex-producing plant species suitable for commercial cultivation. And this is where guayule, that little known relative of the sunflower family, a unremarkable-looking shrub native to the Chihuahuan Desert, enters the scene. 

USDA Guayule Research 

The USDA has conducted extensive research over many years on guayule as a promising alternative domestic source for natural rubber. And research has come a long way since experiments in the early 20th century and the Emergency Rubber Project during World War II. The last 30 years have seen advances in biotechnology and genetic engineering that have improved guayule's rubber yield and quality. Current USDA research focuses on making guayule a viable, sustainable source of domestic rubber that supports southwestern farmers, uses less water, and offers environmental benefits.

Improving Yields and Production Efficiency: Agricultural Research Service (ARS) scientists have been working on improving the rubber and resin content of guayule (10). They have evaluated a large guayule germplasm collection to understand its genetic and phenotypic diversity, including assessing traits such as plant biomass, rubber and resin content, and plant responses to different growing conditions (11). Ultimately, the goal is to breed superior guayule genotypes for improved rubber and resin production, that will provide the best yields for commercial production.

Developing Sustainable Farming Practices: Ongoing research in Arizona is investigating irrigation management to determine optimal levels of water use for maximizing rubber yield and whether deficit irrigation practices affect guayule biomass, and rubber and resin content. (12,13). Recent research in California tested guayule as an alternative crop for soils with high levels of salt, boron, or selenium. Results have shown that it tolerates saline irrigation water and could be useful for selenium bioremediation (14). 

Rubber Extraction Techniques: ARS scientists are refining methods for efficient rubber extraction from guayule, aiming to lower processing costs and enhance the quality of the rubber produced, making it competitive with Hevea rubber

Materials research: Guayule-based latex is hypoallergenic because it lacks the proteins that trigger Type I latex allergies, unlike latex from Hevea rubber trees. It was USDA-ARS scientists in the 1990s who were the first to test guayule as a source of hypoallergenic latex (15).

Exploring Multiple Uses of Guayule: Guayule has a high energy content, providing a comparable energy output to charcoal. ARS researchers are studying guayule bagasse, the plant material that remains after latex extraction, as a source of biofuel, including ethanol, bio-oil, and syn-gas. 

Economic Viability and Market Development: The USDA is collaborating with private sector partners (such as Bridgestone) to assess the economic feasibility of guayule as a commercial rubber source, with particular focus on its use in tires and other rubber products.

 

References

  1. Lloyd, F.E. (1911) Guayule. A Rubber Plant of the Chihuahuan Desert. Carnegie Institution of Washington, Washington D.C. Available: 

  2. Statistics available from trademap.org. Natural rubber code: 4001

  3. Statistics available from trademap.org. Pneumatic tires code: 4011

  4. Mooibroek, H., & Cornish, K. (2000). Alternative sources of natural rubber. Applied microbiology and biotechnology, 53, 355-365. 

  5. Food and Agriculture Organization of the United Nations. FAOSTAT https://www.fao.org/faostat

  6. Cornish, K. (2017). Alternative natural rubber crops: why should we care?. Technology & Innovation, 18(4), 244-255.

  7. Azizan, F. A., Astuti, I. S., Young, A., & Aziz, A. A. (2023). Rubber leaf fall phenomenon linked to increased temperature. Agriculture, Ecosystems & Environment, 352, 108531.

  8. Jayathilake, H. M., Jamaludin, J., De Alban, J. D. T., Webb, E. L., & Carrasco, L. R. (2023). The conversion of rubber to oil palm and other landcover types in Southeast Asia. Applied Geography, 150, 102838.

  9. Rubber World. (2022). IRSG World Rubber Industry Outlook (WRIO) is now available.  

  10. Abdel-Haleem, H., Foster, M., Ray, D., & Coffelt, T. (2018). Phenotypic variations, heritability and correlations in dry biomass, rubber and resin production among guayule improved germplasm lines. Industrial Crops and Products, 112, 691-697.

  11. Luo, Z., & Abdel-Haleem, H. (2019). Phenotypic diversity of USDA guayule germplasm collection grown under different irrigation conditions. Industrial Crops and Products, 142, 111867.

  12. Hunsaker, D. J., Elshikha, D. M., & Bronson, K. F. (2019). High guayule rubber production with subsurface drip irrigation in the US desert Southwest. Agricultural Water Management, 220, 1-12.

  13. Wang, G. S., Elshikha, D. E. M., Katterman, M. E., Sullivan, T., Dittmar, S., Von Mark, V. C., ... & Dierig, D. A. (2022). Irrigation effects on seasonal growth and rubber production of direct-seeded guayule. Industrial Crops and Products, 177, 114442.

  14. Banuelos, G. S., Placido, D. F., Zhu, H., Centofanti, T., Zambrano, M. C., Heinitz, C., ... & McMahan, C. M. (2022). Guayule as an alternative crop for natural rubber production grown in B-and Se-laden soil in Central California. Industrial Crops and Products, 189, 115799.

  15. Siler, D. J., & Cornish, K. (1994). Hypoallergenicity of guayule rubber particle proteins compared to Hevea latex proteins. Industrial Crops and Products, 2(4), 307-313.