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Oregon State University
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Sweet Corn for Fresh Market

Zea mays

Last revised September 7, 2004.

Isolation o Seeding o Harvest, Handling, Storage


Note: This file contains only information specific to production of sweet corn for fresh market. For more information on sweet corn genetic types and isolation classes, cultural practices, fertilizer needs, and pest control, see the file Sweet Corn for Processing.


TYPES

A number of genes affect sweetness in corn. These are recessive mutants of the starchy gene found in field corn (Su) and their modifiers, and other genes. Normal sweet corn has the recessive mutant of field corn (su). Modifiers and other genes include the sugary-extender gene (se) and the supersweet or shrunken gene (sh2). These make up three major genetic classes of importance in commercial production :

1. Normal sugary (susu) corn is the standard corn grown for processing and much of the fresh market. Sugar content at normal maturity is 5-10 percent. The seed germinates well at 55-60 F.

2. Sugary enhanced (sese) corn results in increased sugar levels, in the range of 12-20 percent. Heterozygous sugary enhanced (Sese) runs 7-15 percent. Kernels are very tender with good "corn" flavor. Seed germinates well at temperatures of 55-60 F.

3. Super sweet or extra sweet (sh2) corn produces kernels with two to three times the complex sugars of the standard corn varieties (20-30 percent). Texture is crispy rather than creamy as with the standard and enhanced varieties. Fresh market shelf life is extended because of the slower conversion of sugars to starch after harvest. Seed kernels are smaller, lighter in weight and shrunken in appearance (giving the gene the name "shrunken").


VARIETIES (approximately 70 days for early and 100 days for main season varieties in the Willamette Valley; warmer areas, 7-10 fewer days).

Yellow Kernels

Standard sweet (su), 5-10 percent sugar content at maturity
Very early: Northern Vee. For trial: Earlivee
Early: Sundance. For trial: Flavorvee
Main season: Jubilee (also called Golden Jubilee), Commander, Stylepak.

Super Sweet (sh2), 20-30 percent sugar at harvest
Early For trial: Butterfruit, Summer Sweet 6700
Main season: Supersweet Jubilee, Challenger, Crisp'N Sweet 710. For trial: Excel, Fanciful, Shaker, Showcase, Pinnacle, Sweet Treat, Summer Sweet Series (6710, 6720, etc. where first two numbers designate relative days to maturity), Zenith.

"Improved Super Sweets" (sh2,su), 20-30 percent sugar at harvest: For trial: Sweetie 82.

Sugary enchanced (su,se), 12-20 percent sugar at harvest
Very early: Sugar Buns. For trial: Daybreak
Early: Precocious, Bodacious. For trial: July Gold, Miracle, Spring Treat.
Main season: Kandy King, Kandy Korn. For trial: Amaze, Chase, Conquest, Fantasia, GH1887, GH2684, Incredible.

White Kernels (must be isolated from yellow or bicolor types)

Standard sweet (su): For trial: Silver Queen, Sterling.

Supersweet (sh2): Even Sweeter, How Sweet It Is. For trial: Aspen, Frontier, Vale.

Sugary enhanced (su,se): For trial: Alpine, Divinity, Silverado, Snowbelle, Sugar Snow, SummerSweet series (first two numbers designate relative days to maturity.), White Lightning.

Bicolor Kernels

Standard sweet (su): For trial: Dandy, Double Taste, Harmony, Honey and Cream, Summer Sweet series (first two numbers designate relative days to maturity), Sweet Sue.

Supersweet (sh2): Honey and Pearl. For trial: Appaloosa, Phenomenal, Quest.

Sugary enhanced (su,se): Calico Belle. For trial: Double Delight, Double Gem, Double Treat.

NOTE: Kernel quality of all the above varieties may be dramatically altered under certain pollination conditions. See Sweet Corn for Processing sections on "Genetic types" and "Isolation Classes" and the section on Isolation, immediately below.


ISOLATION

Isolation is necessary from two points of view, color and kernel quality (sugars and texture). Since colored kernels in white varieties are very obvious, a 500 foot or more isolation distance is recommended between white and colored varieties. A two week difference in silking may also be used, but is less reliable. For isolation regarding kernel quality considerations the following is recommended:

Supersweet corn varieties and other new types of corn requiring isolation from standard sweet types should be isolated based on their Isolation Class categorization. The use of 2-4 border rows helps minimize contamination in all situations described below. Isolation may be accomplished in three ways, by distance, time of pollination, and blocking. Isolation by distance is the preferred method.

Isolation by distance

Observations at Oregon State University over several seasons indicate that if no isolation is used between standard sweet (Isolation Class II) and supersweet types (Isolation Class III), outcrossing of kernels in adjacent rows and extending for 6 to 10 rows into each type, is high enough to render the ears from these rows unsalable. This outcrossing can result in over 50% of the kernels on ears in adjacent rows being starchy. Outcrossing drops off rapidly beyond 10 rows, until at about 100 feet, only up to 1% of kernels (up to 4 kernels per ear) may be starchy. This level of outcrossing is probably not discernible by fresh market buyers or consumers.

Where large plantings are made for fresh market production, a distance of 250 feet is recommended between Isolation Classes I, II, and III. Where isolation of fields is convenient, maximum isolation would not need to exceed 600 feet, which is a conservative assumption based on distances used for seed production, where isolation is even more important. Whenever practical:

1. Locate supersweet varieties (Isolation Class III) upwind of varieties in all other isolation classes since outcrossed kernels may be more apparent in the supersweet ears.

2. Mechanically top standard sweet corn plantings, of the variety Jubilee, two leaves above the top ear after the silks have turned brown, and before nearby supersweet plantings begin to silk. Topping an earlier nearby supersweet planting, or a standard sweet variety other than Jubilee would also be helpful, but timing and the effect of topping on yields of supersweet corn, and other standard sweet varieties, have not been researched adequately. Unacceptable reductions in yield have been observed in limited research on topping of other varieties.

3. In small sequential fresh market plantings, plant all varieties of one Isolation Class (I, II, OR III) together in a block located 250 feet or more from a block containing sequential plantings of varieties of any other Isolation Class. For best quality results, varieties of different subclasses (IIa, IIb, OR IIc) should be isolated 50 feet from other subclasses within the same Isolation Class.

Isolation by time of pollination

If the 2-3 week pollination time difference is to be used as a means of isolation between Isolation Classes, and plantings of different Isolation Classes are adjacent, several things need to be considered:

1. The later planting must not be planted based on calendar day difference, but rather on growth stage or heat units. Specifics on this need to be obtained from the individual seed company regarding their variety. The maturity difference between the two types of corn has to also be figured into the planting date difference. Assuming the standard sweet (Isolation Class II) and supersweet (Isolation Class III) varieties have the same maturity (days from seeding to pollination), delay planting the other Isolation Class of corn until the first planting has 8 or more leaves, or 300 or more heat units (base 50 F) have elapsed.

2. To obtain an effective two to three week spread at pollination, the early planting must germinate uniformly or else late germinating plants may cause problems.

3. Whenever possible, mechanically top the early planting just before the later one begins to silk. Fresh market growers may choose to hand-top the late flowering plants or suckers in 10 or 20 rows adjacent to the later planting. Be especially careful of late flowering suckers in these rows.

Isolation by blocking

Fresh market growers who have a use for, or a market for ensilage, may also choose to "block" plantings that have not been isolated by distance or pollination time. This practice consists of walking progressively further from the boundary of the two plantings, examining a sample of ears in each row visually until one finds the row where the outcrossing incidence is acceptable, abandoning the intervening rows (or using them for silage). In Florida, experience has shown that 6-10 rows (sometimes up to 20 rows) may need to be skipped.


SOIL TEMPERATURE

The optimum soil temperature range for germination is over 60 F. This is especially true for the super sweet, improved super sweet and popcorn varieties where germination may be drastically reduced under cool soil conditions. In marginal situations, fresh market growers should consider prewarming soil with clear plastic mulch strips. For small plantings, large sheets of clear plastic may also be used and removed just prior to each planting, and moved to areas to be planted later. Sweet corn takes about 20 days to emerge from 50 F soils, but only about 5 days to emerge at 70 F. Soil temperature is one factor in scheduling plantings.


SEEDING

For early fresh-market standard varieties, seeding may start as soon as soil temperature reaches 60 F. In western Oregon, this is generally about the end of April, and planting extends through June.

In eastern Oregon, depending on location, planting may start about 2 weeks earlier and may extend into mid July. Care should be taken that sweet corn is planted after the danger of spring frost has passed.

Use 10-15 lb/acre of seed, depending on the variety and seed size. Seeding at a depth of l-2 inches is generally satisfactory. Shallow planting (1/2 inch) and maintenance of high soil moisture is recommended where head smut may be a problem, and for supersweet types.

For fresh market, where large ear size and good husk color are important, stands should be between 20,000 and 25,000 per acre.

A rough planting schedule that would provide about 10-14 days between mid season peak harvests between plantings would be to wait until most of the plants in the previous planting had 3 leaves before making the next planting.


HARVESTING, HANDLING, AND STORAGE

In western Oregon, sweet corn harvest ranges from about early August to mid October. The prime harvest season is from about August 25 to the end of September.

In eastern Oregon harvest ranges from about mid July to the end of October with the prime harvest season being from about the first of August to the end of September.

Sweet corn yields can range widely. Early, small-eared corn yields can be approximately 80 cwt/acre, while main-season varieties yield approximately 130 cwt/acre, or about 300 crates/acre.

For optimum quality and returns, harvest of standard sugary (su) and sugary-extender (se) varieties begins when kernels reach 70-75% moisture. Supersweet (sh2) varieties have a much higher sugar content than su or se varieties and maintain their sugar content longer after harvest. They are usually harvested at 77-78% moisture.

Kernel moisture drops approximately 0.5% per day in normal sweet and sugary extender corn varieties with considerable variation depending on season and variety. Kernel moisture of supersweet (sh2) varieties changes at a slower rate. All sweet corn, regardless of type, requires immediate cooling and refrigerated transport and handling.

Limited Oregon research data indicate that there is approximately a 0.356 tons per acre increase for each decrease in 1% kernel moisture, with considerable variation depending on season and cultivar (ranging from 0.173 to 0.792 T/A over 5 seasons and 9 varieties). Supersweet (sh2) varieties averaged 0.700 T/A increase per 1% kernel moisture drop (ranging from 0.225 to 1.011 T/A over 2 seasons and 7 varieties).

Self-propelled and tractor-pulled harvesters are available from several manufacturers. These come in single-row or multiple-row units of up to 8 rows. For fresh market corn harvest some of the harvesters have to be slightly modified so that they do not damage the butt portion of the ear. These modifications are generally easily made, and usually offered as options from the manufacturer.

STORAGE (Quoted or modified from USDA Ag. Handbook 66 and other sources)

Hold sweet corn at 32 F and 95 to 98 % relative humidity. Sweet corn is seldom stored, although occasionally it may be desirable to store an excess supply temporarily. However, storage for more than a few days results in serious deterioration and loss of tenderness and sweetness. The sugar content, which so largely determines quality in corn and which decreases rapidly at ordinary temperatures, decreases less rapidly if the corn is kept at about 32 F. The loss of sugar is about four times as rapid at 50 F as at 32 F. At 85 F, 60 % of the sugars may be converted to starch in a single day as compared with only 6 % at 32 F. However, corn loses sweetness or desirable flavor fairly rapidly, even when iced and held at 32 F. Long shanks and flag leaves should be trimmed before marketing, as they induce denting of the kernels by drawing moisture from them. Denting is an indication of loss of quality. A loss of 2 % moisture from sweet corn may result in objectionable kernel denting.

Rapid removal of field heat from sweet corn, when at 86 F or higher, is especially critical to retard deterioration. Maximum quality retention can be obtained by precooling corn to near 32 F with an hour after harvest and holding ears at 32 F during marketing. In practice cooling to this extent is rarely achieved. However, cooling is the first step in a good temperature management program. Sweet corn has a high respiration rate, which results in a high rate of heat evolution.

Sweet corn can be precooled adequately by vacuum cooling, but it must be wetted first ( and top iced after vacuum cooling). Crated corn can be vacuum cooled from about 85 F to 40 F in a half hour. Hydrocooling by spraying, showering, or immersion in water at 32 to 38 F is effective, although it takes longer than vacuum cooling for the same temperature reduction if the corn is packed before it is cooled.

Crated corn would take over an hour in a hydrocooler to cool to 40 F, and few if any, operators leave it that long. It is important to check cob temperatures during hydrocooling to determine if temperatures are being lowered to at least 50 F. Hydrocooling nomographs for bulk and crated sweet corn are available. Many hydrocoolers now handle palletized crates, with crates four or five layers high. These coolers, with overhead spray nozzles, can be effective if they use a large volume of water and allow an hour or more of operation. After hydrocooling, to icing is desirable during transport or holding to hasten continued cooling, remove the heat of respiration, and keep the husks fresh. When precooling facilities are not available, corn can be cooled with package ice and top ice.

Sweet corn should not be handled in bulk unless copiously iced, because it tends to heat throughout the pile. Corn should not be expected to keep in marketable condition even in cold storage at 32 F for more than 5 to 8 days. The storage life at 40 F is about 3 to 5 days and at 50 F about 2 days.

Some corn is prepackaged in moisture-retentive film, with the husks re- moved after precooling. The film should be perforated to prevent development of off-odors or off-flavors. This product is very perishable and must be marketed with continuous refrigeration.

Use of controlled atmospheres to extend storage offers little promise. Research has shown that injurious atmospheres contain less than 2 % oxygen or more than 20 % carbon dioxide. In an atmosphere with 2 % oxygen, the sucrose content of sweet corn remained higher than in other atmospheres tested.

Some of the new, high-sugar sweet corn cultivars should improve consumer satisfaction. As compared with standard cultivars, which contain 3 to 5 % sugar at harvest, the new cultivars contain 7 to 10 % sugar and also lose their sweetness more slowly during marketing. Thus, consumers purchasing the sweeter cultivars after several days' storage should get corn with 5 to 6 % sugar as compared with standard cultivars containing only 2 to 3 % sugar after similar post-harvest handling.


PACKAGING

Wirebound crates, 42-50 pounds, are commonly used as containers for corn. Corn is packed 4 or 5 dozen per crate.


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