This section presents information on water relations and water management of sugarbeet and provides links to other sources of information.

Sugarbeet (Beta vulgaris) provides about 16 percent of the world's sugar production. Present world production is about 234 million tons of beets from about 5.9 million ha.(FAOSTAT, 2001).

The crop is believed to originate from Asia. Sugarbeet is a biennial crop but for sugar production beets are harvested in the first year. Flowering occurs during the second year. The crop is grown under rainfed conditions but also widely under irrigation in the subtropics where the crop is known for its high tolerance to saline and alkali soils.

The crop needs a relatively long growing period, normally from 140 to 160 up to 200 days. Large amounts of sugar are formed in the leaves. The greater part is used for growth processes during the vegetative period, while in the late growing period when vegetative growth slows down a large part is stored in the roots. However, sugar yield is determined by both root size and sugar concentration. With rapid growth of the storage root the sugar concentration reaches a steady value which is principally determined by climate, water supply and nitrogen level in the soil and is influenced to some extent by variety and plant spacing. Sugar percentage in the root is often greater than 15 percent of the fresh root weight. The crop is harvested toward the end of the first season's growth, when the roots contain maximum amount of sugar.

The crop is grown in different climates. Seed germination is possible at 5°C but the effective minimum is considered to be 7 to 10°C. Higher temperatures during vegetative growth are preferred, but high sugar yields are obtained when night temperatures are between 15 and 20°C and day temperatures between 20 and 25°C during the latter part of the growing period. During this period temperatures greater than 30°C greatly decrease sugar yields. For high sugar yields and low vegetative growth in the latter part of the growing period, progressively cooler nights should be accompanied by an exhaustion of available soil nitrogen and soil water.

When the crop is grown for seed, several weeks at low temperatures, near 4°C, are required to induce flowering, which tends to be accelerated by long days.

The crop can be grown on a wide range of soils with medium to slightly heavy textured, well-drained soils preferred. Restricted deep root growth in the early part of the growing period due to soil compaction may result in formation of forked and sprangled roots with reduced yields. Soil pH smaller than 5.5 is unfavourable to growth. Crust forming at the soil surface at the time of germination can lead to poor crop stand.

Adequate nitrogen is required to ensure early maximum vegetative growth. Nitrogen is often given in split applications, a small amount at planting and the rest after thinning. Nitrogen either in an excessive amount or when applied late during the growing season reduces sugar content. Fertilizer applications may be up to 150kg/ha N, 50 to 70kg/ha P at planting and 100 to 160kg/ha K.

A deep, well-prepared seedbed is advantageous. Seeds are planted 1 to 2 cm deep in single or double rows, with width between single rows 0.5 to 0.7m and double rows about 1m. When the plant has 4 to 8 leaves, thinning, by hand or by machine, is frequently needed to space 3 to 6 beets per metre row. Seed rates vary between 12 and 30kg/ha. Plant densities under commercial production vary from 40 000 up to 100 000 plants/ha.

Except during the early stages, after crop establishment, the crop is tolerant to salinity. Yield decrease is 0% at ECe 7, 10% at 8.7, 25% at 11, 50% at 15 and 100% at ECe 24 mmhos/cm. During early growth ECe should not exceed 3 mmhos/cm.

The graph below depicts the crop stages of sugarbeet, and the table summarises the main crop coefficients used for water management.

¹ Sugarbeet often experience late afternoon wilting in arid climates even at p < 0.55, with usually only minor impact on sugar yield.

² The Kcend (or late) value is for no irrigation during the last month of the growing season. the Kcend (or late) for sugarbeets is higher, up to 1.0, when irrigation or significant raim occurs during the last month.

For maximum production, water requirements of the crop are related to reference crop evapotranspiration (ETo). The crop coefficient (kc) is 0.4-0.5 during the initial stage (25 to 30 days), 0. 75-0.85 during the crop development stage (35 to 60 days), 1.05-1.2 during mid-season stage (50 to 70 days), 0.9-1.0 during the late-season stage (30 to 50 days) and 0.6-0.7 at time of harvest. Total water requirements are in the range of 550 to 750 mm/growing period, but vary with climate and length of the total growing period. Time of sowing affects the rate of crop development, particularly from emergence to when the crop has reached its maximum height, which for an autumn-sown crop may be 140 days, for a spring-sown crop about 90 days and for a late spring/early summer-sown crop about 60 days.

Following figure shows the duration of the different growth periods of the sugarbeet crop with 140 to 200 day growing period (after G.B. Heathcots)



The relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the total growing period of root yield are shown in the figure below.

The relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the total growing period of sugar yield are shown in the figure below.

When grown for sugar, flowering and seed production is avoided. Sugarbeet is particularly sensitive to water deficits at the time of crop emergence and a period of about a month after emergence (0). Frequent, light irrigations are preferred during this period, and irrigation may also be needed to reduce crust formation on the soil and to reduce salinity of the top soil. Early over-watering may retard leaf development and can encourage flowering during the first year (bolting).

Water deficits in the middle part of the growing period (vegetative and yield formation periods, 1 and 3) tend to affect sugar yields more strongly when occurring during later periods. Ample supply in the later part of the growing period (ripening period, 4) has an adverse effect on sugar concentration although it may increase the root size, with the final effect on yield being small. Water deficits together with a nitrogen deficiency toward the end of the growing period lead to a reduction in root growth but an increase in sugar concentration. In general, top growth toward the end of the growing period tends to be negatively correlated with sugar production. Irrigation supply must be discontinued at least 2 to 4 weeks prior to harvest.

Thus, except during emergence and early growth periods, it appears that the crop is less sensitive to moderate water deficits. The effect of reduced water supply over the total growing period is often masked by the overriding influence of temperature, nitrogen availability and reduced water needs during a period just prior to harvest.

When available water resources are limited and when maximum overall production is aimed at, water supply should be directed toward expanding the area under irrigation rather than concentrating the supply over a limited area to meet maximum water requirements (ETm) over the total growing period. This is because there is an increase in the efficiency of water utilization (Ey) for both roots and sugar yield when water supply is reduced so that yields decrease less than proportionally with the reduction of water supply (ky< 1) provided the growing environment during the later part of the growing period is favourable to sugar storage.

In deep soils the crop can develop a deep tap root system but normally 100 percent of the water is extracted from the first 0.7 to 1.2m soil depth (D = 0.7-1.2m). Under conditions when ETm is 5 to 6mm/day, 50 to 60 percent of the total available soil water can be depleted without reducing water uptake (p = 0. 5-0.6), with higher depletion levels just before harvest. When the plant is under water stress the leaves become dark green in colour and when the water stress is severe the leaves fail to recover from midday wilting in the evening.

Frequent, light irrigations during the establishment period (0) are advantageous and sufficient soil water must be available at emergence. An irrigation is frequently applied after thinning. Irrigation intervals can be selected using Table 21. The irrigation is discontinued at least 2 to 4 weeks before harvest to increase sugar concentration in the beets. The soil should, however, not be too dry to hamper lifting of the beets at harvest.

The most common method is furrow irrigation. Border irrigation is sometimes practised but it is not common. Sprinkler irrigation offers advantages particularly during the germination and emergence period when frequent but light applications may be adequate. However, young plants will be damaged when sprinkling with poor quality (saline) water.

A good commercial yield of 160 to 200 day sugarbeet is 40 to 60 ton/ha of fresh beet at 15 percent sugar. Under certain conditions yields of up to 70 to 80 ton/ha are obtained. The water utilization efficiency for harvested yield (Ey) for beets containing 80 to 85 percent moisture is 6 to 9 kg/m3 and for sucrose containing no moisture 0.9 to 1.4 kg/m³.

A few comparative studies have been made between sugarbeet and sugarcane performance. For a given location, climate is the most important factor in dictating which crop is likely to be more suitable from the point of view of production.