Stevia (Stevia rebaudiana) is an herbaceous perennial, endemic to a semitropical region in the highlands of Paraguay, where it adapted to small niche environments between wetter marshlands and drier vegetated areas (Shock, 1982a, 1982b). The acidic, sandy soil is relatively infertile with an underlying shallow water table that provides a continuous source of water without surface soil saturation.
Stevia is among many species of economic importance from the Asteraceae or Compositae family, including sunflower, lettuce, and herbal teas. Stevia usually grows as an herbaceous perennial but can be cultivated in northern latitudes or other areas, such as China, as an annual. Stevia is an obligate short-day plant that has a critical daylength of less than 13 h for flower initiation, although daylength sensitivities are subject to varietal differences (Metivier and Viana, 1979; Valio and Rocha, 1977; Zaidan et al., 1980).
Paraguay’s native Guarani peoples have used stevia leaves for centuries as a sweetening agent for normally bitter medicinal teas (Madan et al., 2010; Ramesh et al., 2006). The sweet flavor, reportedly up to 300 times sweeter than sugar, comes from noncaloric steviol glycoside compounds found in the leaf (Brandle et al., 1998). The two most prominent steviol glycosides are stevioside and rebaudioside A.
Sugar consumption in the United States is projected to decline 10%, from $2.18 billion in 2013 to $1.95 billion by 2018 (Sprinkle, 2014). In 2014, 48% of the sweetener market consisted of sugar and 17% consisted of all noncaloric sugar substitutes. Within the noncaloric sugar substitutes, steviol glycosides comprised 29% of the market compared with 45% for sucralose. Over the previous 2 years, 33% of the U.S. adults increased their consumption of steviol glycosides, 55% maintained their consumption, and 12% reduced their consumption. Manufacturers of diet soft drinks are using steviol glycosides (principally rebaudioside A) as sweeteners, which reduces sugar content and associated costs, while marketing a potentially healthier alternative to soda sweetened with sugar or high-fructose corn syrup.
Currently, the U.S. Federal Drug Administration (FDA) regulations limit stevia use in food products to processed combinations of stevioside, rebaudioside A, rebaudioside D, rebaudioside M, enzyme-modified steviol glycosides, and other steviol glycosides, and they must be at least 95% pure (U.S. Food and Drug Administration, 2015). The FDA allows stevia leaf use as a dietary supplement but not as a dietary ingredient, a sweetener, or flavoring agent. Industrial extraction of steviol glycosides from stevia leaves is currently only performed overseas. The U.S. market for domestically produced leaf may be limited by shipping costs until leaf processing facilities are built.
Commercial cultivation of stevia was reported in Paraguay and Japan during the early 1960s (Madan et al., 2010; Yadav et al., 2011). Today, stevia has been introduced for crop production in many countries including China, India, Brazil, Mexico, Canada, parts of Europe, and Africa, and to a lesser extent, the United States.
Efficient production of stevia will require efficient cultivars and sound cultural practices. Planting densities, fertility requirements, and pest management practices have been reviewed by several authors (Brandle et al., 1998; Madan et al., 2010; Ramesh et al., 2006), yet there are many unsolved problems in stevia production. Shock (1982a, 1982b) studied stevia yield for possible cultivation in California. The field study at the University of California, Davis, CA, produced dry leaf yield up to 9.2 Mg·ha−1 in small plots in a single growing season with a density of 191,400 plants/ha, but most yields were lower. The stevia was grown with frequent, shallow irrigations.
Over the past decade, several stevia trials have examined stevia dry leaf yields, and in some instances, steviol glycoside content on single cultivars from several regions of the world. In Paraguay, where stevia occurs naturally, the Cooperativa Colonias Unidas, Obligado, Itapúa, Paraguay, compared microsprinkler and drip irrigation with or without patchouli mulch in areas of high annual rainfall (2019 mm), low soil fertility, and high erosion (Prieto et al., 2010). Stevia dry leaf yields were compared at various plant densities over multiple sequential harvests. After the initial planting, any dead plantlets were replaced with live stevia plants to maintain densities. Dry leaf yield over 3 years improved from 4.12 Mg·ha−1 per year with no irrigation to 4.6 Mg·ha−1 per year with microsprinkler irrigation, and 5.22 Mg·ha−1 per year with drip irrigation at a planting density of 111,000 plants/ha. With mulch and replanting, using microsprinkler irrigation, harvest yields increased to 5.08 Mg·ha−1 per year. Before these advancements by Prieto et al. (2010), Ramesh et al. (2006) reported typical stevia leaf yields in Paraguay in the range of 1.5–2.5 Mg·ha−1 per year.
Fronza and Folegatti (2003) studied stevia in San Piero, Italy (43° N, 11° E; 5 m above sea level), where the climate is Mediterranean. Stevia leaf yield was 4.37 Mg·ha−1, stevioside content was 6.49%, and rebaudioside A and total steviol glycosides were not reported. In southwestern Belavatagi, India (15°34′ N, 75°21′ E, 578 m above sea level), Aladakatti et al. (2012) studied flood-irrigated stevia with various irrigation regimes and planting densities. Seedlings were planted at densities of 74,000, 83,000, 111,000, and 167,000 plants/ha. Harvest was initiated after a 90-d interval from planting date and subsequent harvests were performed on 70-d intervals totaling five harvests per year for 2 years. Yields as high as 10.54 Mg·ha−1 per year were obtained.
In Bhubaneswar, Odisha, India, Behera et al. (2013) grew stevia with various drip irrigation and fertigation regimes at 55,600 plants/ha. Stevia dry leaf yield of 2.74 Mg·ha−1 was obtained. Lavini et al. (2008) studied stevia water consumption, leaf yield, and steviol glycoside content in the Mediterranean climate of southern Italy with 50,000 plants/ha and two harvests per season. Yearly dry leaf yield reached 4.6 Mg·ha−1.
In the United States, the S&W Seed Company, Fresno, CA, had an interest in stevia leaf production in California and planted stevia from seed purchased from China and a patented clonal cultivar PC1 (Alvarez, 2012) in 2010, 2011, and 2012 at Chowchilla, CA. Stevia leaf yields were below 1 Mg·ha−1 in 2011 and 2012, but the leaf steviol glycoside analyses were favorable.
Stevia leaf and steviol glycoside productivity need to be better understood in the western United States. Limited information is available to growers who are interested in stevia production. Stevia seedling vigor is very low, so crop establishment currently depends on transplants from established seedlings or vegetatively propagated rooted cuttings. Major stevia production costs include labor for plant propagation and transplanting, weed control, irrigation, leaf harvest, and drying. Those investing in stevia production have little available information as to leaf yield and quality that could be expected. The stevia cultivar trials reported here were conducted at four western locations to obtain an understanding of the effects of location, harvest strategy, and cultivar on stevia dry leaf yield, steviol glycoside content, and steviol glycoside yield.
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