Stephen Kaffka, University of California, Davis
F. Jackson Hills, University of California Davis
I. Classification, Origin, Adaptation, and Production
Sucrose is synthesized in most plants as a temporary storage product for photosynthetically reduced carbon, and is the most common form of carbon translocated in plants. Most plants convert photosynthate to starch for long-term storage. However, sucrose accumulates to an exceptional degree in two species, sugarbeets (Beta vulgaris L.) and sugarcane (Saccharum officianarum L.), which together form the basis for greater than 90% of the world's sugar trade. The sucrose derived from these two species represents approximately 11% of the world's food supply, and 0.2% of all the carbon fixed via photosynthesis by the world's crops each year. The most recent estimate for world sugar (sucrose) production is 114.3 million tons, of which one third is derived from sugar beets, and two thirds from sugar cane.
A genus of the family Chenopodiaceae, sugarbeet is one of a diverse and useful group of cultivars from the same species that includes swiss chard, fodderbeets and red beets. The crop has an interesting history associated with war, sacrifice, and undoubtedly, romance. The first modern sugarbeets originated as selections made in the middle of the 18th century from fodderbeets grown in then German Silesia, but food and medicinal uses of the genus are much older. A precursor is known to have been used as food as early as dynastic times in ancient Egypt. In 1747 a German chemist, Andreas Marggraf, demonstrated that the crystals formed after a crude extraction from pulverized beet roots were identical in all properties with sugarcane crystals, and attempts to derive sugar from beets originate from his work. His student, Karl Achard, developed processing methods for sugar extraction from the beet, and made the first selections of higher sugar type beets. The blockade of shipments of cane sugar to Europe by the British during the Napoleonic wars stimulated a more intensive search for sweeter beets, a plant breeding program and the construction of many crude factories in France and elsewhere to produce sugar from the sugarbeet. After Waterloo and the lifting of the British blockade, the incipient sugarbeet industry in France declined but the modern sugarbeet had been created and the efficacy of sugar extraction from beet had been demonstrated. The first successful commercial factory in the USA was constructed by E. H. Dyer at Alvarado, California in 1879. Soon after sugarbeet culture and factories expanded in many states. By 1917 there were 91 factories operating in 18 states. By 1989 there were 36 operating factories in 13 states processing sugarbeet grown on about 550,000 ha. The major sugarbeet producing states in the U.S. are California, Colorado, Idaho, North Dakota, Minnesota, Michigan and Texas.
Sugarbeet is grown predominantly in regions with temperate, Mediterranean
or arid climates. Sugarcane production is
confined to tropical and subtropical zones. For the most part,
beet-producing regions lie north and south of the 30th parallels.
Table 1 contains data for sugarbeet production in
the major beet-producing regions of the world. All of these regions
except the EC also must import sugar from sugar cane producing countries
to meet a portion of their sugar demand. Sugar consumption has been
growing at roughly the same rate as world population or 2% per year.
There are substantial differences in per capita sugar consumption among
nations. In part these differences are cultural, but per capita consumption
also is correlated with wealth and is highest in Europe and lowest in China
and Africa.
II. Industry Organization
Beet sugar production worldwide often is vertically integrated. Companies that process sugar from the beet root have considerable influence over all aspects of production from the area planted through the sale of the final product. The crop is of little value without a processor to extract the sugar, and once a sugar factory is constructed, a company must have a reliable supply of beets. So there is usually a closer and more cooperative relationship among growers and companies than is found with other agronomic commodities. In the United States, the crop is grown most often under contract between the individual grower and a processing company. The contract specifies the area that can be grown, various details concerning the delivery of beet roots and the method on which payment to the grower will be based. In the United States, the contract is of a participating type, guaranteeing the grower a share of the return the processor realizes from the sale of sugar. Contracts often contain yield and quality incentives. Sugarbeet growers, through the formation of local associations, influence the terms of the contract, inspect company operations such as the method of sampling delivered sugarbeets, and methods of sucrose analysis and tare determinations. In the United States, sugarbeet processors and growers associations band together to influence national policy and legislation concerning the growing of sugarbeet. Ironically, trade in sugar has often been the subject of bitter dispute. In both Europe and the United States, sugarbeet variety improvement and seed production are carried out primarily by private companies. The USDA developed most of the varieties grown in the first half of the 20th century in the United States and current variety development often uses genetic lines derived from them.
III. Growth and Development of the Sugarbeet Crop
The sugarbeet is a remarkable plant. A rapidly growing crop is capable of high rates of sucrose accumulation. California has regions which are ideally suited to commercial sugar beet production. The world's commercial record has come from a field in the Salinas Valley and equalled 19.1 t ha-1 of sucrose from a crop grown over a 240 day period (115 t ha-1 of roots at 16.5% sucrose. Averaged over the total period of growth, the crop accumulated 185 kg total DM ha-1 day-1 and 107 kg sucrose ha-1 day-1. Sucrose accumulation is not uniform throughout the growing season, (initially it is relatively slow), peak sucrose accumulation rates are considerably higher than the average reported and likely approximate rates of 200 kg DM ha-1 day-1. More typical crops reach half the biomass and sugar yields of record crops. An average hectare of sugarbeet in California produces 56 Mg of roots containing 15% sucrose. Upon processing, the beets yield 7.4 Mg of recoverable sugar, 3.4 Mg of dry root pulp and 98 kilograms of monosodium glutamate, an amino acid salt used to enhance the flavor of foods. The sugarbeet pulp left after sucrose extraction is used widely in the dairy and beef cattle industries as a feed supplement due to its highly digestible fiber and energy content. Another 6.7 Mg dry matter of beet tops may be left in the field to fertilize subsequent crops or used as feed for cattle or sheep. When the beets are harvested, approximately 150 kg N, 20 kg P, and 150 kg K are removed with them. These amounts of nutrients are roughly similar to those removed by a 10 Mg crop of corn grain. Because beets are efficient at accumulating photosynthate in a useful form, they are also efficient convertors of agricultural inputs such as water and nitrogen (Table 2). One of the reasons sugarbeet requires relatively low use of fertilizer nitrogen is its efficiency in recovering residual soil nitrogen.
Figure 1 illustrates the growth and development of a sugarbeet plant in a controlled climate. Under such appropriate conditions, the plant develops quickly from seed with the seedling emerging from the soil as soon as five days after planting. The taproot grows rapidly and may reach 30 cm or more by the time the first true leaf is developed. During the first 30 days, growth is confined primarily to top and fibrous roots. After about 30 days both top and storage-root growth proceed rapidly with tops reaching near maximum fresh weight in 60 to 90 days. Subsequently, with favorable climate, top growth remains fairly constant but storage roots continue to grow rapidly for 20 to 24 weeks. Beyond that period, crown (stem) growth becomes an increasingly larger percentage of the commercial storage root. As the storage root increases in size, there is a constant translocation of sucrose from the leaves to the root where it is stored primarily in concentric rings of vascular tissue derived from secondary cambium early in the root's development and in root parenchyma cells that enlarge during growth. On a fresh weight basis, the sucrose content of the root remains relatively constant until suitable external factors cause the concentration to increase. A rapid increase in root sucrose content is correlated with cool night temperatures in the fall of the year coupled with a nitrogen deficiency. Both of these conditions slow vegetative growth, particularly top growth, and shift photosynthetically produced sucrose from use for growth into storage. Figure 2 shows sugarbeet plants that have been grown for three years in a constant favorable climate. Sugarbeet is a bienniel and when the growing plant (A., Fig. 3) undergoes prolonged exposure to cold temperatures (approximately 90 days at 5 to 7? C) followed by warmer temperatures and longer day lengths, seed stalk production ("bolting") takes place (B., Fig. 3). In the United States, sugarbeet seed is produced most efficiently in the Willamette Valley of Oregon, where winter temperatures are low but the roots do not freeze.
IV. Management Practices and Production Problems
In most sugarbeet growing regions, the earlier the planting and longer the growing season, the higher the yield, provided that temperatures at planting are conducive to rapid growth and plants are not retarded by diseases or other problems. Most sugarbeet varieties currently grown are monogerm hybrids that out-yield older, open-pollinated types by from 10 to 20%. The use of monogerm seed, discovered by V. F. Savitsky in the late 1940's, eliminates seed balls with multiple embryos and crowding of seedlings when plants emerge. In turn, this improves the operation of mechanical thinners or eases hand thinning, and makes it easier to plant directly to a stand. Before planting, seed is processed and graded to permit precision planting, and is treated to protect germinating seedlings from soil fungi and insects.
Seeds are planted in rows from 50 to 76 cm apart. Within a row plants should be spaced at least 13 cm apart. Closer spacings will reduce sugar yield as will spacings beyond 30 cm. Where conditions are conducive to good field emergence (50% or better) seeds can be planted 10 to 15 cm apart with the expectation that the resulting stand will not need to be thinned. Many fields, however, are planted at closer seed spacings and, with good emergence, require the use of a mechanical thinner or long handled hoes to space the plants from 13 to 30 cm in the row. Good stands contain approximately 65,000 plants per hectare.
Generally, nitrogen fertilization is required for profitable sugarbeet production. However, sugar yield is sensitive to the timing of nitrogen availability, requiring ample amounts early for maximum vegetative growth but also a period of nitrogen deficiency prior to harvest for proper sugar accumulation in the storage roots. Figure 4 shows a typical response of a sugarbeet crop to fertilizer nitrogen. Highest sugar yields are a function of root yield and sucrose concentration, and usually are achieved with a fertilizer rate that nearly maximizes root yield. However, this rate can be considerably less than the rate required for maximum total biomass production (roots plus tops) and usually is not the rate giving the highest root sucrose concentration. Data in Fig. --4 indicate that application of 112 kilograms fertilizer N/ha resulted in maximum sugar yield and maximized profit to the grower. In this instance, plant analyses indicated that the crop was deficient in nitrogen for about eight weeks prior to harvest. Analysis of petioles (leaf stalks) and leaf blades is a reliable means of assessing the nutrient status of sugarbeet and serves as a guide to the application of fertilizers.
Sugarbeet is a C3 plant with broad, dark green, succulent leaves. In arid areas of the temperate zone it must be irrigated. Careful and timely irrigations are essential to a good sugarbeet yield. Either furrow or sprinkler irrigation is possible. Sprinkler irrigation, though more costly, has the advantages of improving seedling emergence and using less water in the early stages of plant growth. Irrigation water requirements range from as little as 450 mm of water per hectare per season in a cool climate where the soil is filled with plentiful winter rain, to as much as 1400 mm per hectare in a hot, dry climate with limited winter rain.
The control of pests and diseases is important for profitable crop production. Sugarbeet should not be planted in fields heavily infested with weeds. Moderate weed infestation is controlled by crop rotation and a combination of chemical and mechanical methods. Sugarbeet is susceptible to the pre-emergence and post-emergence seedling rots known collectively as the damping-off diseases. Other important diseases which must be controlled in areas where they occur are: curly top, a virus disease transmitted by the sugarbeet leafhopper; sugarbeet yellows, a virus complex transmitted primarily though not exclusively by the green peach aphid; powdery mildew (Erisphe polygoni DC) and Cercospora leafspot (Cercospora beticola Sacc.) diseases caused by leaf fungi; rhizomania, caused by a virus (beet necrotic yellow vein) transmitted by a soil borne fungus (Polymyxya betae Keskin) and moved from field to field by human activity; and the sugarbeet cyst nematode (Heterodera schachtii Schmidt) and root-knot nematodes (Meloidogyne sp.). Strategies for the control of these diseases involve development of resistant varieties, attention to time of planting, isolation of new plantings from old sugarbeet fields that can serve as sources of virus inoculum, the selective use of fungicides, soil fumigation and careful attention to crop rotation. Where diseases are spread by migratory aphids or leafhoppers, control cannot be achieved by individual farmers. Cooperation at a regional level is required.
Table 1. Estimated average crop growth rates and sucrose accumulation
rates for above average yields in different growing districts of California.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|