Production Technology : Sweet Corn

Land Selection: Since sweet corn needs to be consumed in a short time after harvest it fits well in periutban agriculture. Hence sweet corn can be very profitably grown in areas surrounding large cities and towns which are frost free during the season of growing. It can successfully be grown in well-drained soils with pH of 5.5-7.0. However, it may be grown in all types of soil and is moderately salt-tolerant. Places where sweet corns are to be grown must have facility for 5-6 irrigations. Moisture stress, particularly at the time of anthesis adversely affect the yield and quality of the produce. Since all genes governing sweet corn are recessive, sweet corn is to be grown at a field isolation of 250 m from other corn or by a tassel date of 14 days.

Land Preparation: Sweet corn requires thorough disc ploughing followed by land leveling initially. Thereafter ridges and furrows are to be laid out with an inter row distance of 75 cm. It is recommended to incorporate FYM @5-6 tonnes/ha• at the last ploughing. Seedbed preparation and seed handling is critical for all types of sweet corn, but especially supersweets. Good soil to seed contact, uncrusted soil, and optimal soil moisture help seedlings emerge. Careful seed handling is also important because if the seed coat is cracked, solutes leak from the seed, attracting pathogenic fungi. Rapid uniform seed emergence also pro¬motes uniform maturity.

Time of Sowing: The planting time is June-July in kharif (rainy) and September-October in rabi (winter) season.

Varieties Both white and yellow grain types of sweet corn are grown. Some of the recommended sweet corn varieties released for cultivation are Madhuri, Priya and Almora sweet corn in India. A brief description of these sweet corn varieties is given in Table 1.

Seed Rate:

New seed should be used each year as seed quality (vlgour) gets reduced substantially within a year, especially in the case of supersweets. Recommended plant population for optimum yield is 45000-60000 plants per hectare with row spacing of 75-100 cm and intra row spacing of 15-30 cm. This will require 10-11 Kg of su or se corn seed per ha. Supersweet corn seed contains a large quantity of sugar, which causes the seed to be crinkled and smaller. Therefore, supersweet sweet corn varieties have a higher seed count (500 seed per Kg) than other sweet corn types (325 seed per Kg) and will require less seed (5 to 6 Kg/ha).

Seed Treatment: Seed treatment with Imidachloprid 70WS @5g/Kg seed takes care of insect pests up to 30 DAS and fungicide treatment helps to prevent the attack of damping-off fungi.

Method of Sowing: Two seeds per hill are dibbled manually or mechani¬cally one third from the top on the side of the ridge. Sowing on ridges serves the twin purpose of water conservation and protection from water logging in the initial stages to which sweet corn is highly susceptible. Planting depth should be 3-4 cm for all cultivars except for supersweets, which are planted 2.5 cm deep. Plants are thinned to one plant per hill 10-12 days after emergence. Normal (su) and sugary enhanced (se) sweet corn should not be planted earlier than 7 to 10 days before the average date of the last severe frost. At least 3 rows of each variety should be sown at each planting to assure good pollination. Seeds of sh-2 varieties are less vigorous than other sweet corn types, which can lead to reduced and uneven crop stand. Researchers have suggested that the reduced vigour is related to reduced starch reserves for germination, cracked seed coats and increased sugars, which render the seed more susceptible to diseases.

Several precautions are suggested to ensure the best possible stand. Seed should be handled carefully and preferably with a plate-less planter in case of mechanized sowing so that seeds will not be damaged. All seeds should be treated with insecticide and fungicide (check with seed companies to see if treatment has been applied be¬fore purchasing seed). Planting of supersweet varieties should be delayed until the soil warms to 16°C.

Transplanting: Sweet corn is seldom transplanted, but this can be done in special cases to ensure plant stands and early yields. To grow transplants, seeds are sown 3 weeks before the expected transplanting date in trays within 4 cm cells. The transplants must be grown and transplanted withoutexperiencing any stress, e.g., water, fertilizer or temperature. Stressed plants will tassel early, and yields will be reduced significantly.

Nutrient Management: Fertilizer must be applied according to soil test results (Table 2). A general recommendation would be to apply a total of 100-120 Kg of nitrogen, 50-60 Kg of phosphate (P₂0₅) and 40-60 Kg potash (K₂0) per ha. For early plantings, when cold conditions are often encountered (16°C), use a "starter" fertilizer (e.g., 18-46-0 per ha) for improving seedling vigour, stand establishment and early plant growth. Fertilizer should be applied in 1 or 2 bands approximately 7-8 cm to the side and 5-7 cm below the seed. The market demands sweet corn with long, green flag leaves and dark green husks. To accomplish this, an adequate supply of nitrogen must be maintained in the soil.

Weed Management: A field that does not contain noxious weeds must be selected. Crop rotation is important for limiting the build up of troublesome weeds. Shallow cultivation should be used in concert with chemicals for weed control. The crop should remain weed-free during the early stages of plant growth; otherwise, yields might be substantially reduced. Several herbicides are available for weed control. Efficient weed control is achieved for 30-35 days through a spray of the herbicide Atrazine @ 1 Kg ai/ha 1-2 days after the initial irrigation. If double cropping is practiced, it must be borne in mind that the crop is sensitive to herbicide carryover, particularly atrazine or atrazine-containing products. Therefore, selection of herbicides should be made accordingly.

Water Management: Depending on the soil type,. the number of irrigations varies from 4-5 irrigations in heavy soils and 7 -8 irrigations in light soils. A grower should be prepared to irrigate at least 2.5-4.0 cm a week in order to produce high quality sweet corn. The most critical time period to have adequate moisture is during tasseling and silking.

Intercultivation: Top dressing of Urea is done 25 days after emergence after running a cultivator in between the rows that will help weed control and better root aeration apart from soil water conservation. Subsequently, a ridger is run to prevent plant lodging and minimize volatilization losses of urea. Top dressing with Urea is followed by irrigation.

Intercropping: Sweet corn being a highly remunerative crop can successfully be cultivated in peri urban agriculture. However, instead of cultivating sweet corn as sole crop it may be intercropped with other highly remunerative crops like marigold, tuberose, gladiolus, spices, pea etc. This provides additional income to the farmers from unit area and makes agriculture more sustainable.

Pest and Disease Management: A preventive spray of Endosulfan 35EC @2m1/1 of water is given to 10-14 days old plants to take care of stalk borers Chilo partellus in kharif and Sesamia inferens in rabi. Sweet corn must be free of insect larvae or worms for shipping or distant markets. The corn ear worm, Helicoverpa armigera is by far the most difficult insect to control in sweet corn. Eggs are laid on the young silks where they hatch and the larvae feed on the silks and tips of the ears. It is advised to direct the spray at the ear zone. When tassel shoots appear, spray insecticide, then consecutive applications should be made depending on the situation.

Disease problems tend to be sporadic. Troublesome diseases include leaf blight and post-flowering stalk rots. Use crop rotation, and avoid sequential planting in adjacent fields to minimize disease. A spray with Bavistin @1 g/litre takes care of foliar diseases.

Harvesting of Cobs: Plant growth picks up after the knee high stage rapidly as this is the grand growth stage. About 45 days after emergence, the tassel emerges and 2-3 days later the silks emerge. Top dressing with urea is done at tassel emergence. Sweet corn is an abundant pollen producer and seed set is usually very high. There should be no moisture stress at tasseling-silking stage as moisture stress reduces seed set and in extreme cases barren ears are also seen. As an ear approaches maturity, sugar changes to starch, the hull becomes tougher, and the kernels pass through stages called pre-milk, milk, early dough, and dough. At field temperatures of 16°C, an ear may remain in prime condition for as long as 5 days, while at 30°C it might remain in prime condition for only 1-2 days. Corn will be ready for harvest approximately 18-22 days after completion of pollination (indicated by drying of silk). As the field nears maturity, a few ears should be examined daily to determine the time for the first picking. Corn is ready for harvest when the ear is full size for the variety, has a tight husk, and has somewhat dried silks. The kernels are fully developed and exude a milky liquid when punctured. Delaying the harvest will progressively reduce the sugar content in the kernels. The ear husks are still green at this stage and the kernels remain lustrous at milky stage. Whether harvested by hand or machine, sweet corn should be collected at night or early in the morning, when the environment remains cool. Every effort should be made to keep harvested ears cool and in shaded areas. Ears can be eaten raw, as a dessert or ears can even be steam boiled, salted and eaten, which is more relishing. Under good management, two ears are borne per plant especially in the rabi (winter) season. Staggered planting of sweet corn at intervals of 7-10 days will enable farmers to harvest sweet corn ears at intervals for continuous supply to the market and thereby get continuous and higher returns.

Harvesting of Green Fodder: In addition to high market value for sweet corn one additional advantage is that immediately after harvest of green cobs, the plants remain green and can readily be used as fodder. It is estimated that up to 250-400 q/ha fodder may be harvested from one crop, which provides additional income to the farmers.

Post-harvest Handling: Because sweet corn has a high respiration rate, it produces heat which can cause ears in bulk trailer loads to heat up considerably during delays between picking and precooling. The longer the delay, the greater the heating, conversion of sugar to starch, and subsequent quality loss. Sweet corn must be moved quickly from the field to packing sheds, where it should be rapidly sorted, packed, and cooled. Sweet corn is generally packed in wire bound wooden crates, which can hold from 4 to 6 dozen ears, depending on the size of the crate or ears. However, cabbage bags or wax impregnated cartons can be used and typically hold about 60 ears. The ears are graded according to the raw ear size and packed in polythene bags and sold in the market. Under room temperatures sweet corn will lose 50% or more of its sugars in 24 hours. Sweet corn should be precooled to as close to 0^oC as possible, although it is rarely cooled below 4^oC in commercial practice. Several methods of cooling sweet corn are available:

Hydro-cooling: It consists of precooling by either showering the corn or immersing it in cold water. It is the most popular method of precooling. Immersing the corn in cold water is much more efficient than showering the corn. A dealer or engineer is to be consulted to ensure maximum efficiency and sufficient capacity to handle anticipated daily volume of sweet corn.

Packaging: This is an excellent method of precooling sweet corn which is used for local, direct shipments. In this method, 7-10 Kg of crushed ice is distributed throughout the container (box or crate) during the packaging process. The amount of ice needed in a package depends on the temperatures of the corn at the time of packing and on the expected length of the marketing period. Typically about 500 g of ice for 2 Kg of sweet corn is sufficient. The main disadvantage is that if shipment to market is delayed, the package is heavy and wet.

Cold Storage: To maintain best quality, sweet corn is placed in cold storage immediately after precooling. Storage can be in a refrigerated truck or in a room. Temperature is maintained as close to 0^oC as possible without freezing the corn, and relative humidity of the air in the cold room at 95 percent or higher to keep the corn fresh. The corn is to be taken out as soon as possible since more than a few days in cold storage will decrease quality.

Cooling in Transit: Sweet corn must also be kept cold in transit. The best method consists of blowing finely crushed ice over the top and through the corn crates, as the trucks are loaded. This method of icing keeps temperatures low during transit but provides little or no additional cooling. The refrigerated truck only maintains the temperature of the already cooled corn. It is not meant to be used as a substitute for precooling. It is important to remember that for maximum quality and value, sweet corn must be continuously and properly refrigerated from harvest until it reaches the consumer.

Obtaining Earliness: Several methods can be employed to obtain an early harvest of sweet corn. The most obvious method is to choose a variety which is early to mature. Also, a more vigorous variety will germinate under less-favourable growing conditions. In addition, seeds can be sown 10 to 20 days earlier using clear plastic mulch. Photo-degradable mulch (30-day type is recommended; the plastic breaks down slowly after exposure to sunlight) can be used to eliminate the cost of removing mulch at the end of the growing season. It is recommended to apply herbicide and then cover with clear plastic. The plastic is left over the emerged plants approximately 30 days, then cut and removed. Growing corn on black plastic mulch can also enhance earliness and weed control.

Corn Colour: Sweet corn comes in three colors: yellow, white and bicolour (yellow and white). Cross-pollination of yellow kernel varieties with white kernel varieties will result in production of bicolour corn. Also, if a bicolour is cross pollinated with a yellow variety, kernel colour will be predominantly yellow. Although there are geographical preferences for certain kernel colors, there is no relationship between colour and sweetness of sweet corn.

Yields: If water requirements are met and other cultural practices optimized, sweet corn yields about 66000 ears per ha. High density planting can give higher number of harvested ears.

Pollination: Unlike most vegetables, corn is wind pollinated and must be planted in blocks. Sweet corn of any type downwind from popcorn or field corn develops starchy kernels. Ears of supersweet sh-2 corn pollinated by a normal (su) sweet corn cultivar or a cultivar with the se gene will develop hard, ugly and starchy dent kernels. To avoid this cross¬pollination, different kernel types must be separated by at least 250 m or staggered planted so that they flower at least 14 days apart.

Precautions with Modified Endosperm Sweet Corn to Avoid Xenia:

Isolation of "sweet corn" cultivar plantings of different genetic types is necessary to prevent cross-pollination. Xenia is the immediate effect of foreign pollen on a variety; on sweet corn (su), it will produce a starchy kernel. Isolation can be obtained by planting at a different time, planting cultivars of different maturities, or providing barriers and border rows. All of these methods will reduce the isolation distances necessary. On a practical basis, commercial growers should provide at least 50 m separation, plant upwind of normal field corn, and use four or more border rows.