Corn, Sweet for Processing

Zea mays (sweet corn for processing)

Last revised February 2, 2010

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").

XENIA:

Xenia effects are pollen-induced changes in kernels of sweetcorn apparent on the harvested ears. Some of these changes may be intended and beneficial. These changes are of most concern when they result in a loss of cob or kernel quality by adversely changing kernel color, kernel weight, embryo weight, soluble solids percent, kernel moisture, or cause other undesirable direct and indirect effects. A number of ways to manage xenia effects are used. The main method is by isolation of the different genotypes. Consult sections on isolation below.

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

Processor specifies varieties. Some of the more common varieties for processing are:

YELLOW KERNELS

Standard sweet (su) 
Early: GH 1703
Main season: Jubilee (also called Golden Jubilee).
Super Sweet (sh2)
Early: Sheba, Krispy King
Main season: Supersweet Jubilee, Challenger, Crisp 'N Sweet 710, Zenith.

Sugary enchanced (su,se): very limited production.

WHITE KERNELS

Very limited production and must be isolated from yellow or bicolor types.

BICIOLOR KERNELS

Very limited production.
NOTE: Kernel quality of all the above varieties may be dramatically altered under certain pollination conditions. See the sections below on "Genetic types" and "Isolation".

 

GENETIC TYPES:

The threee genetic classes mentioned above are categorized into 6 major sugar-mutant types. Other categories exist, but are not of commercial importance. These may be represented by yellow, white, or bi-color varieties.

The following table modified from a paper by J.W. Courter and others, describes these table corn types and classifies them into categories for isolation purposes:

ISOLATION CLASS	DESCRIPTIVE TERMINOLOGY	HOMOZYGOUS RECESSIVE "SUGAR" GENES	VARIETY EXAMPLES
I	Field, dent, or flour corn	none	Field corn cvs
II*a	Sugary or standard sweet corn	su	Jubilee (yel) Double Taste (bi) Silver Queen (wh)
II*b	Sugary augmented with sugary enhancer; or "EH" types	su se heterozygous	Kandy Korn EH (yel) D'Artagnan (bi) Silverado (wh)
		su se homozygous	Miracle (yel) Calico Bell (bi) (no whites)
II*c	Sugary augmented with shrunken-2; or "SWEET gene HYBRID", or  "Synergistics"	su sh-2 heterozygous	Sugar Loaf (yel) (no bicolors) (no whites)
III**a	Shrunken-2, supersweets or "Xtra-Sweet" hybrids	sh-2	Crisp N Sweet 710 (y) Honey and Pearl (bi) How Sweet It Is (wh)
III**b	Shrunken-2 augmented with sugary; or "Improved Supersweet Hybrid"	sh-2 su heterozygous	Sweetie 82 (yel) (no bicolors) (no whites)

* Class II contains varieties homozygous only for su (IIa) as well as those homozygous su cultivars with additional heterozygous or homozygous recessive genes such as se (IIb), or sh2 (IIc), since cross pollination of such cultivars will still produce su kernels (sweetcorn).
** Class III contains varieties homozygous only for sh2 (IIIa) as well as those homozygous sh2 cultivars with additional heterozygous recessive genes such as su (IIIb), since cross pollination of such cultivars will still produce sh2 kernels (supersweet corn).

 

For an excellent discussion of the origin, biology, production, and uses of sweet corn and other forms of maize, see Iowa State University's The Maize Page.

 

ISOLATION

A recommended isolation distance, in feet, is given in the following table for the different classes of corn. Popcorn and ornamental Indian corn should be considered as two additional, separate, isolation classes.

White-kernel varieties must be isolated from all other corn by 500 feet or more.

	ISOLATION DISTANCE RECOMMENDED (ft.)
ISOLATION CLASS	I	IIa	IIb	IIc	IIIa	IIIb
I	0	250	250	250	250	250
ISOLATION CLASS	I	IIa	IIb	IIc	IIIa	IIIb
IIa	250	0	50	50	250	250
IIb	250	50	0	50	250	250
IIc	250	50	50	0	250	250
IIIa	250	250	250	250	0	50
IIIb	250	250	250	250	50	0

Note: An isolation distance of 250 feet is given between isolation classes I, II, and III, wherever outcrossing will cause flavor, texture, and the starch content of the outcrossed kernels to resemble field corn. Popcorn and ornamental Indian corn should be considered as two additional isolation classes to be separated from all isolation classes by 250 feet.
An isolation distance of 50 feet is given whenever outcrossing is not very detrimental but could result in flavor, texture, and the starch content of the outcrossed kernels to be no different than the non-augmented type (sugary in class II and supersweet in class III).

 

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 (discussed above in the "VARIETIES" section) 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. Processing companies, however, may have different requirements for isolation.

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:

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.
Mechanically top standard sweet 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 (see also section on TOPPING below).
In small sequential 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:
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.
To obtain an effective two to three week spread at pollination, the early planting must germinate uniformly or late germinating plants may cause problems.
Whenever possible, mechanically top the early planting just before the later one begins to silk (CAUTION, see section on TOPPING below). 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 (and using them for silage). In Florida, experience has shown that 6-10 rows (sometimes up to 20 rows) may need to be skipped.

SOILS AND SOIL TEMPERATURE

A wide variety of soils are suitable. It is important that the soil be well drained and well supplied with organic matter. The optimum pH range is 5.8 to 7.0.

The optimum soil temperature range for germination is over 60 F. This is especially true for the super sweet and improved super sweet varieties where germination may be drastically reduced under cool soil conditions. 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.

SEED AND SEED TREATMENT

Sweet corn seed numbers approximately 120-180 per ounce. About 10-15 lb are used per acre. Use only seed treated with fungicides and insecticides. Some seed companies are now offering super sweet varieties with specially coated, sized seed, intended to improve stand establishment.

Research indicates that seed shape and size are related to emergence and stand establishment and performance. In normal sweet corn, flat seed perform better than round, and in both classes, high density seeds perform better than those of lower density.

When planting supersweet varieties, large differences in seed vigor occur between different varieties, particularly under cool, wet or compacted soil conditions. The difference in performance is not apparant from germination information on the identification tags.

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 processing, recommended stands are 26,000 to 27,000/A but where large ear size is 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 till most of the plants in the previous planting had 3 leaves before making the next planting.

SPACING

Plant quick-growing, small stature varieties in rows approximately 30 inches apart, and 6-8 inches in the rows. Vigorous tall-growing varieties should be grown in rows 30-36 inches apart, 9-12 inches between plants. Processing varieties should be planted according to the row spacing and rate required by the processor.

FERTILIZER

Good management practices are essential if optimum fertilizer responses are to be realized. These practices include use of recommended varieties, selection of adapted soils, weed control, disease and insect control, good seed bed preparation, proper seeding methods, and timely harvest.

Western Oregon:

For the most current advice, see Nutrient Management for Sustainable Vegetable Cropping Systems in Western Oregon, available as a free download from the OSU Extension Catalog

For additional advice on fertilizer management, use the following publication: Nutrient Management Guide for Sweet Corn (Western Oregon)

Eastern Oregon:

NITROGEN (N)

Sweet corn requires a good supply of available N. An optimum response to N fertilization depends on adequate irrigation. An irrigation when corn is 12-18 inches tall will insure most efficient utilization of banded fertilizer.

Part (40-60 lb/A) of the N should be banded at planting time. The remainder may be applied before planting and/or during the growing season before tasseling, particularly where leaching is likely to be a problem.

If the band application of N exceeds 60 lb/A, there is danger of seedling injury from the concentration of salt.

Fertilizer salt injury can be reduced by using two rather than one fertilizer bands, not banding too close to the seed, and immediate irrigation of dry soil. Salt injury is likely to be greater in sandy soil compared to finer textured soil and in dry compared to moist soil. The urea or diammonium phosphate forms of N may cause seedling injury if banded close to the seed at planting, especially where the soil pH exceeds 7.0.

Amount of N fertilizer required depends on the following factors:

The preceding crop; the N carry-over from the previous crop; the amount and type of residue to be plowed under; and possible leaching losses due to over-irrigation.

The following fertilizer guides are for mineral soils with low organic matter content.

Eastern Oregon N Fertilizer Guide Based on Soil Test

The amount of residual N in the soil varies considerably. A soil test for nitrate-N helps in evaluating the N carry-over from the previous crops in the case of mineral soils with low organic matter content. Soil samples for nitrate-N should be taken following a growing season and before the application of N fertilizer.

Soil samples should be taken from 0 to 2-foot and 2 to 5-foot depths on deep soils. The soil samples should consist of soil cores removed from the entire 0 to 2 and 2 to 5-foot sections of the soil profile. On soils shallower than 5 feet, soil samples should be taken from 0 to 2 feet and from 2 feet to the expected rooting depth.

OSU soil test results for N are reported in ppm. One ppm N in a 1-foot depth of soil equals about 4 lb N/acre. As an example:

Soil depth	Nitrate-N
(ft)	(ppm)	(lb/A)
0 - 2	4	32
2 - 5	3	36
	Total 68 lb/A

The total Nitrate-N soil test values are used to estimate the N fertilizer requirement as follows:

Nitrate-N Soil** Test (lb/A)	After* non-legume crop	After beans, peas or alfalfa
0	300	250
50	250	200
100	200	150
150	150	100
200	100	50
250	50	0
300	0	0

*When straw is incorporated after Sept. 1, increase N fertilizer rate by 30-50 lb/A. 
**Assumes extraction procedures similar to those used by the OSU Central Analytical Laboratory. Specific information on soil test procedures is available from the Dept. of Crop and Soil Science.
Should the soil test value for nitrate-N be less than 2 ppm in the 0 to 2-foot soil depth, apply a minimum of 30 lb N/A regardless of the soil test value for N below 2 feet. This application is to ensure adequate initial growth of plants.

 

PHOSPHORUS (P)

Phosphorus is essential for vigorous early growth of seedlings. All of the P should be banded 2 inches to the side and 2 inches below the seed at planting.

If soil test* for P reads (ppm)	Band this amount** of phosphate (P2O5) (lb/A)
0 - 5	100 - 150
5 - 12***	0 - 100

*Assumes extraction procedures similar to those used by the OSU Central Analytical Laboratory. Specific information on soil test procedures is available from the Dept. of Crop and Soil Science.
**Double the rate of P application when P is plowed down. 
***For early plantings into cool soil when P soil test exceeds 12 ppm, apply 20-30 lb P2O5/A in a 2 x 2-inch band.
POTASSIUM (K)

 

Potassium should be broadcast and plowed down before planting.

If the soil test* for K reads (ppm):	Apply this amount of potash (K2O) (lb/A)
0 - 100	150 - 200
100 - 150	100 - 150
150 - 200	0 - 100

*Assumes extraction procedures similar to those used by the OSU Central Analytical Laboratory. Specific information on soil test procedures is available from the Dept. of Crop and Soil Science.
SULFUR (S)

 

Plants absorb S in the form of sulfate. Fertilizer materials supply S in the form of sulfate and elemental S. Elemental S must convert to sulfate ion in the soil before the S becomes available to plants. The conversion of elemental S to sulfate is usually rapid for fine ground (less than 40 mesh) material in warm moist soil.

Elemental S should be applied the year preceding the crop using finely ground (less than 40 mesh) material. Elemental S is a strong soil acidifier. Sulfur in the sulfate form can be applied at planting time.

If soil test* for SO4-S in the 0-2' soil depth reads (ppm)	Apply this amount of S** in lb/acre:
	Loamy soil	Sandy soil***
0 - 2	20 - 30	30 - 40
2 - 5	0 - 20	20 - 30
5 - 8	None	0 - 20

*Assumes extraction procedures similar to those used by the OSU Central Analytical Laboratory. Specific information on soil test procedures is available from the Dept. of Crop and Soil Science. 
**When the irrigation water contains over 2 ppm of S, additional S fertilizer is probably not required. 
***These rates should be increased by 50% for sandy soils in central Oregon.
Sulfur requirements will vary with soil texture, leaching losses and the soil parent material. Sulfur is frequently contained in fertilizers used to supply other nutrients such as N, P, and K and may be present in irrigation water which can be tested for S content.

 

ZINC (Zn)

Sweet corn has a relatively high requirement for Zn. An application of Zn is suggested when the Zn soil test value is below 0.8 ppm. Where Zn is required, either 10 lb Zn/A should be broadcast and worked into the soil before planting or 3 to 4 lb Zn/A should be banded with the fertilizer at planting time. An application of 10 lb Zn/A should supply Zn needs for 2 or 3 years.

To correct Zn deficiency during the growing season thoroughly wet plants with a solution containing 1 lb Zn in 50 to 100 gal of water.

BORON (B)

Responses of sweet corn to B have not been observed in eastern Oregon. Where the soil test value for B is below 0.4 ppm trial applications of B are suggested.

OTHER NUTRIENTS

Responses of sweet corn to other nutrients such as copper and iron have not been observed in eastern Oregon.

LIME

Responses of sweet corn to lime have not been observed in eastern Oregon; however, where the soil test pH value is less than 5.5 a lime application is suggested. Soil pH should be measured before application of fertilizer.

Where the subsoil is calcareous or has a higher pH deep plowing will reduce surface soil acidity. On sandy soils where soil acidity is most prevalent, one ton of dry 100-score lime raises the pH about 1 unit. In most instances 1 to 1.5 T/A of lime is adequate to correct soil acidity. With silt loam and clay loam soils 2 to 3 T/A of lime respectively will raise soil pH about one unit.

Lime should be mixed into the seedbed at least several weeks before seeding. A lime application is effective over several years. The liming rate is based on 100-score lime.

Salty Soils: The growth of sweet corn will likely be restricted when the soil test value for soluble salts exceeds 4 mmhos/cm.

The eastern Oregon guides are largely based on the results of experiments conducted by Washington State University and observations of growers' fields, and have been quoted and modified from OSU Fertilizer Guide No. FG71.

LIME

Experimental work has shown that sweet corn will produce good yields over a fairly wide range of soil acidity. Lime applications are suggested when the soil pH is below 5.8 or when calcium (Ca) levels are below 5 meq Ca/100g of soil. The rate of lime application can be estimated from the following OSU lime requirement table:

If the SMP Buffer* test for lime reads	Apply this amount of lime (T/A):
Below 5.6	4 - 5
5.6 - 5.8	3 - 4
5.8 - 6.0	2 - 3
6.0 - 6.3	1 - 2
Over 6.3	None

*Assumes extraction procedures similar to those used by the OSU Central Analytical Laboratory. Specific information on soil test procedures is available from the Dept. of Crop and Soil Science. The liming rate is based on 100-score lime.
Lime should be mixed into the soil at least several weeks before planting and preferably the previous fall. A lime application is effective over several years.

 

Some soils may have a fairly high SMP buffer value (over 6.2) and a low pH (below 5.3). This condition can be caused by the application of acidifying fertilizer. In this case the low pH value is temporary and the pH of the soil will increase as the fertilizer completes its reaction with the soil. This temporary "active" acidity from fertilizer is encountered following recent applications of most nitrogen fertilizer materials. Acidifying fertilizers also have a long term acidifying effect on soil which is cumulative and leads to lower SMP buffer readings.

Sandy soils to which fertilizers have not been recently applied sometimes record low pH and high SMP buffer values. In such cases, a light application of lime (1 to 2 T/A) should suffice to neutralize soil acidity.

For acid soils low in magnesium (less than 1.0 meq magnesium/100 g soil), 1 T/A of dolomite lime can be used as a Mg source. Dolomite and ground limestone have about the same ability to neutralize soil acidity.

Fertilizer Guide #3, "Liming Materials for Oregon," which is available from your local OSU Extension Office, provides additional information on lime.

Phosphorus, K, Mg, B, Zn, and lime suggestions are based on soil test values from the Central Analytical Laboratory, OSU, Corvallis, Oregon.

These guides to fertilization are largely based on the results of experiments conducted by Horticulture and Crop and Soil Science Department scientists of the Oregon Agricultural Experiment Station and are quoted from O.S.U. Fertilizer Guide FG11.

NUTRIENT SUFFICIENCY LEVELS FOR SWEET CORN

Data from the midwest indicates the following sufficiency levels for sweetcorn based on growth stage (from the National Corn Handbook, NCH-43):

Nutrient Sufficiency Levels in Sweet Corn

Growth stage	Plant Part sampled	N	P	K	Ca	Mg	S	Fe	B	Zn	Mn	Mo
12 inch	Whole plant	3.5	0.4	3.0	0.3	0.30	0.2	50	7	20	50	0.3
Silk	Ear leaf	2.8	0.3	1.8	0.3	0.25	0.2	60	6	20	25	0.3

Levels below these values are considered low or deficient. Levels above these values are considered high or excessive.

The above values should be used as guides only for diagnostic purposes. The values may differ for different varieties.

IRRIGATION

For western Oregon 12-14 inches of water provides optimum yields and ear size. In eastern Oregon 25-28 inches may be needed depending on seasonal variation, variety and planting date. Approximate summer irrigation needs for the Hermiston area of eastern Oregon have been found to be: 3.5 inches in May, 5.0 in June, 7.5 in July, and 7.0 in August.

Soil type does not affect the amount of total water needed, but does dictate frequency of water application. Lighter soils need more frequent water applications, but less water applied per application.

Sweet corn is often grown with furrow irrigation in eastern Oregon. Water soluble polyacrylamide (PAM) is useful for flocculating soil particles in irrigation furrows and reducing erosion of soil from the furrow.

Automated lateral-moved tower systems and continuous-move big gun systems are used effectively. Increased use of large center pivot systems (those covering over 40 acres), in the Willamette Valley may be contributing to production problems. This is especially so with low pressure systems. Problems are caused by water application rates along the outer portions of the circles which exceed soil infiltration rates. This causes surface ponding and increased risk from soil erosion. Also, prolonged periods of soil wetness resulting from necessary frequent water applications, increases the probability of soil compaction from cultivation or other mechanical operations when the soil is wet. Systems larger than 40 acres should be restricted to soils with appropriate infiltration rates (generally 0.5 inches per hour or more). Such soils are not common in western Oregon.

To facilitate movement of irrigation pipe at the last irrigation/s, expedite harvest, and to reduce lodging, corn may be topped after pollination is completed (see section on TOPPING).

Sweet Corn Water Use
The following crop water use and irrigation management information is from the Western Oregon Irrigation Guide developed by M. Hess, J. Smesrud and John Selker (Dept. of Bioresource Engineering) and N.S. Mansour:

Total Seasonal Evapotranspiration (inches)	18.8
Peak Evapotranspiration Rate (inches/day)	0.22
Maximum Allowable Depletion (percent)	50
Critical Moisture Deficit Period	silking to harvest

Irrigation of sweet corn should be managed to supply adequate soil moisture while at the same time maintaining adequate aeration and soil temperatures. In the period between seeding and emergence, low soil temperatures can delay or prevent germination. Thus, it is recommended that fields be irrigated before seeding and not again until after emergence whenever possible. In the remainder of the season, available soil moisture should not be depleted by more than 50 percent1. Especially critical is the time between silking and harvest. Water deficit during this period will have the greatest negative impact on yields. Excessive irrigation, however, may also negatively impact yields by promoting excessive stalk and leaf mass.
The use of big guns and center pivot systems is common in corn due to difficulties in moving pipe as fields approach maturity. When using big guns, it is critical to pay close attention to the uniformity of water application which decreases with increasing set spacing. With center pivot systems on the other hand, it is excessive application rates near the edges of the circle which tend to be problematic in the fine-texured soils of Western Oregon. High application rates promote surface sealing and runoff of irrigation water, thus shorting the crop of needed soil moisture.

 

The peak water use for sweet corn, which occurs in July, is approximately 0.22 inches per day. On most soils, weekly irrigation during the peak is adequate. With sandy and sandy loam soils, however, irrigation may be required as frequently as every three days.

References:
1. Sanders, D.C. 1993. Vegetable Crop Irrigation, Leaflet No: 33-E (North Carolina State University, Raleigh).

TOPPING

Corn is sometimes topped to facilitate movement of hand carried irrigation pipe through the fields, expedite harvest, and to reduce danger of lodging. Topping is done by special high clearance machines that use rotary cutting blades.

Top corn when pollination is completed (2/3 to 3/4 of silks of top ears begin to turn brown), and leave 2-3 leaves above the top ear, otherwise yields will be unacceptably reduced. Topping at the correct time may still result in up to a 10% yield reduction but this is usually compensated for by better irrigation, expeditious harvest, and reduction in lodging. This topping information has been developed for the variety Jubilee. Other varieties may have yields seriously depressed by topping (Stylepak), or may have silks that do not turn brown after pollination, making this timing indicator unsuitable.

HARVESTING, HANDLING, AND STORAGE

In western Oregon, sweetcorn 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.

Sweetcorn yields can range widely. Processing sweetcorn average yield is approximately 9 tons/acre with good yields about 12-14 tons/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 sweetcorn, regardless of type, requires immediate cooling and refrigerated transport and handling. Corn intended for processing should be protected from overheating and delivered to the processor as soon as possible.

Limited Oregon research data indicate that there is approximately a 0.356 tons per acre increase in yield 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).

Percent cut-off increased approximately 1.04% for normal and se sweet corn and 2.03% for sh2 supersweets per 1% drop in kernel moisture, depending on season and cultivar (ranging from 0.04% to 1.85% for normal and sugary-extender varieties to 1.12% to 3.38% for sh2 superweets).

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)

Sweetcorn for processing is not normally stored. Where temporary holding is intended, hold sweet corn at 32 F and 95 to 98 % relative humidity. 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. 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). 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 can handle bins or palletized crates.

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, 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.

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.