P. R. Carter1, D. R. Hicks2, J. D. Doll1, E. E. Schulte2 R. Schuler2, and B. Holmes2

1Departments of Agronomy, Soil Science and Ag. Engineering, College of Agricultural and Life Sciences and Cooperative Extension Service, University of Wisconsin-Madison, WI 53706.
Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN 55108.
Nov., 1989.

I. History:

Popcorn is a special kind of flint corn that was selected by Indians in early western civilizations. Although U.S. commercial popcorn production has always been centered in the Corn Belt, the major growing areas within this region have shifted over the years. Up to the mid 1940s, Iowa was the number one producer, with both production and processing plants concentrated in the western part of the state. Then, as popcorn hybrids began replacing open-pollinated varieties, production shifted eastward, first to Illinois and then to Indiana. Another major shift occurred in the mid 1970s when production jumped back to the west, so that Nebraska is now the leading popcorn producer.

Total U.S. popcorn acreage over the 5-year period 1977-81 averaged 189,000 acres per year, with 55% of that total in Nebraska and Indiana, and another 25% in Iowa, Ohio and Illinois.

II. Uses:

Popcorn is sold either as a plain or flavor-added popped product, or as an unpopped product in moisture-proof containers ranging from plastic bags and sealed jars to ready-to-use containers both for conventional and microwave popping. Popcorn flavor is enhanced to individual tastes with the addition of salt and butter. There is no end to the uses of popcorn. One recipe book lists 200 different recipes.

Nutritionally, it is one of the best all-around snack foods, providing 67% as much protein, 110% as much iron and as much calcium as an equal amount of beef. An average 1.5-ounce serving of popcorn supplies the same energy as two eggs; and a cup of unbuttered popcorn contains less calories than half a medium grapefruit. In addition, hull is excellent roughage, comparing favorably with bran flakes or whole wheat toast.

III. Growth Habits:

Popcorn moves through commercial channels primarily in three kernel types: white, small yellow and large yellow. White popcorn characteristically has a rice-shaped kernel, while yellow popcorn kernels are pearl-shaped. The three kernel types meet different needs within the industry, and growers and/or

processors must take that into consideration when selecting hybrids. Kernel color of specialty popcorn may also be blue, red, black, or brown.

The popped appearance may resemble either a butterfly or a mushroom. The butterfly type is preferred for eating; while the mushroom type is used in confectionery products.

In performance trials, a hybrid is usually identified as one of the above types, although some medium-kernel yellows are now available that meet either the small- or large-kernel requirements after grading. No industry standards have been developed for kernel size determinations; but a commonly used measure is based on number of kernels in 10 grams and defines kernel size as follows:52-67 = large, 68-75 = medium and 76-105 = small.

Popcorn hybrids generally have smaller plants and about 2/3 the yield per plant than dent corn. Stalk strength is relatively poor compared to dent corn hybrids.

IV. Environment Requirements:

A. Climate:

Popcorn growth requirements are similar to those for dent corn, with less adaptability to environmental extremes. Popcorn seed germinates more slowly than dent corn, and seedling growth is also slower. Although popcorn varieties are available with maturities as early as 90 days, extensive development of top performing hybrids is limited to those with maturities of 110-days or later.

B. Soil:

Although there is little experimental data regarding effects of soil type on popcorn production, grower experience has shown that any soil type suitable for dent corn should produce a good popcorn crop. However, experience also indicates that popcorn is likely to perform better on medium- to coarse-textured soils (coupled with adequate rainfall or supplemental irrigation) than on fine-textured, poorly drained soils for at least two reasons:

Popcorn seed germinates more slowly than dent corn, and the seedlings grow more slowly; thus, medium- to coarse-textured soils, which warm slightly faster than fine-textured soils, should improve germination, emergence and seedling establishment.

The popcorn root system is less extensive than that of dent corn; thus, high-clay-content and/or poorly drained soils foster weak, shallow rooting that reduces yields and increases lodging.

V. Cultural Practices:

A. Seedbed Preparation:

Careful seedbed preparation is important because seed size is small. A clod-free seedbed with good tilth will ensure coverage of the seed placed just deep enough (1-2 inches) to be in contact with moist soil.

B. Seeding Date:

Timely planting of popcorn is very important because of its slow germination and seedling growth and because it must reach harvest maturity (see Harvesting section) for maximum popping expansion. Planting should not occur until early-May when soils warm to temperatures conducive to rapid germination and emergence. Planting delays after mid-May need to be balanced with popcorn maturities and the length of growing season.

C. Method and Rate of Seeding:

For plate planters, special popcorn plates are required, and recommendations of the equipment manufacturer should be followed. Both seed size (number of kernels per pound) and grade should appear on the popcorn seed bags.

Little information is available concerning the effects of plant density on the performance of popcorn hybrids. Commonly, seeding rates for popcorn are higher than for dent corn because of its smaller plant size and lower yield per plant. However, if plant populations are too high, the relatively poor stalk strength of popcorn hybrids can result in severe lodging. Generally, a 5 to 25% increase over the recommended dent corn plant densities should be considered for most soils and cultural practices.

D. Fertility and Lime Requirements:

A pH of 6.0 is adequate for popcorn. Test the soil and apply the amount of lime recommended to achieve this pH. Thorough incorporation and mixing are important.

Nutrient requirements for popcorn are estimated to be 85% of those for field corn. Soil P and K should test in the medium to high range (30 to 75 lbs of P and 160 to 240 lbs K per acre). Apply any corrective phosphate or potash recommended on the soil test report.

Maintenance fertilizer equivalent to crop removal should be applied to maintain soil test P and K levels. Nitrogen requirements and maintenance recommendations for phosphate and potash are given in Table 1.

Table 1: Annual nitrogen, phosphate and potash recommendations for popcorn.
  Soil organic matter, T/a Soil Test P Soil Test K
  0-20 21-35 36-75 76-100 >100 H or below VH EH H or below VH EH
bu/A ------------ lbs N/a ------------ --- lbs P2O5/a --- --- lbs K2O/a ---
80-100 100 80 60 30 20 30 15 0 20 10 0
101-120 120 100 80 40 30 35 20 0 25 15 0
121-140 140 120 100 60 40 40 20 0 30 15 0
141-160 160 140 120 80 60 45 25 0 35 20 0
Credits for a preceding legume crop and use of manure should be subtracted from these recommendations.

Dent corn fertilizer programs adapted to popcorn production should take into consideration popcorn's relatively poor standing ability. Very high rates of N can compound lodging problems, especially if soil test K levels are low. Also, because popcorn seedlings grow more slowly than dent corn, the application of starter fertilizer is probably of greater importance.

E. Variety Selection:

Popcorn growers shifted from open-pollinated varieties to hybrids for some of the same reasons that dent corn growers did –improved yield and better standing ability. Also, hybrids have better popping expansion and more uniform kernel type and maturity.

A major consideration in hybrid selection is maturity. Maximum popping potential of a hybrid can be achieved only if it reaches full maturity. Any factor that prematurely terminates plant development (e.g., drought stress, disease, frost, etc.) reduces popping potential, and may result in a crop not marketable as popping corn. Therefore, growers should select hybrids that will usually mature before frost in their area. If planting must be delayed much past normal dates, consider an earlier-maturing hybrid or an alternative crop.

Some popcorn hybrids are dent-sterile and cannot be pollinated by ordinary types of dent or sweet corns. In popcorn seed fields (where the harvested crop is to be used as seed for the following year's crop), dent sterility is important because it prevents cross pollination with dent corn. Corn harvested from plants grown from outcrossed seed has very poor popping ability. Thus, if the ears from these outcrossed plants are not sorted out before shelling, it is virtually impossible to remove all kernels from these ears in the cleaning and grading operation. This adversely affects the quality and appearance of the total crop. For this reason, dent-sterile hybrids are especially desirable where popcorn is to be field-shelled.

In popcorn production fields where the harvested crop is to be used for popping, isolation from other types of corn is unnecessary, even if the popcorn is not dent-sterile. Pollen from other corn does not have any effect on the popping ability of popcorn hybrids.

The "right" popcorn hybrids must meet the needs of both grower and consumer. The grower wants high yields, strong stalks and good disease resistance; the consumer wants popped corn that's tender, good tasting and free from hulls. Current commercial hybrids involve some compromise among these requirements.

A large portion of commercial popcorn acreage is contracted with growers by popcorn processors. In most instances, the contracts specify that the processor will determine the hybrids to be used. Growers producing uncontracted popcorn may, of course, grow any hybrid they choose. Sources of information about hybrids include state Extension Services, popcorn seed companies or the current "Hybrid Popcorn Performance Trials" from the Agricultural experiment Station, Purdue University, West Lafayette, IN 47907.

F. Weed Control:

The same weeds that commonly infest dent corn fields are also found in popcorn fields. Yield losses will occur when popcorn has to compete with weeds for the nutrients, light and moisture essential to maximum growth and development. Also, certain weed species may serve as alternate hosts for disease and insect pests of popcorn.

Cultural mechanical, and chemical methods for weed control in popcorn are similar to those used for dent corn. For chemical control methods, consult your county Extension office or popcorn company agronomist. Follow recommendations, and apply as directed on the herbicide label. Neither Banvel nor 2,4-D are registered for use in popcorn.

G. Diseases and Their Control:

As with weeds and insects, popcorn is subject to the diseases common to dent corn. Fortunately, not all of these diseases are of economic importance and may pass from year-to-year without notice. A few, however, are widespread and can substantially reduce yield and quality if conditions are optimum for infection.

Stalk and root rot diseases are often the most destructive in popcorn. Symptoms are usually first noted when the crop nears physiological maturity. The disease complex is generally caused by several fungal and/or bacterial pathogens rather than by a single causal agent. Yield losses are the result of infected plants having poorly filled ears or lodged plants and dropped ears that escape harvest.

Satisfactory control of popcorn diseases involves a combination of sound cultural practices, which include crop rotation, proper fertilization, proper management of crop residue, use of disease-resistant hybrids and appropriate applications of chemical treatments. Consult state recommendations for specific practices tailored to fit local situations.

H. Insects and Their Control:

Most, if not all, insects that attack dent corn also can attack popcorn. Follow state recommendations and the specific information on the product labels regarding application, safety, and restrictions.

I. Harvesting:

Much popcorn acreage today is harvested by combine, despite the fact that there is likely to be more kernel damage, and thus a reduction in popping volume. Processors who want maximum popping volume may contract with farmers to harvest their popcorn on the ear. While this usually increases production costs, it also results in a higher quality popcorn with more potential popping volume. Combined popcorn can give satisfactory popping volumes if it is harvested at the correct moisture content by a properly adjusted combine.

Shelled Grain-harvesting. Popcorn shelled with a combine in the field should have a field moisture of between 14 and 18%, with the optimum being 16-17%. Above 18% moisture, shelling losses are high and there is much physical damage to the kernels. Below 14%, the kernels are too susceptible to impact damage from combining and associated handling operation; and as already mentioned, kernel damage lowers popping volume.

Combine settings are different for popcorn than for dent corn, and adjustments must be made when switching to popcorn. The cylinder speed, concave clearance and other adjustments should be set to provide a balance between shelling efficiency, machine losses and degree of physical damage to the popcorn. Slower cylinder speeds and wider concave clearances reduce kernel damage. A combine operated at or near its rated capacity will produce less kernel damage than when operated at relatively low levels of throughput. Further "fine tuning" will also likely be needed to account for specific harvest conditions, harvest moisture and popcorn hybrid.

Ear-harvesting. With a heated forced-air system to dry the ears promptly to a safe storage moisture, popcorn can be harvested at 25% moisture. Such a system must be so designed that the drying process does not affect the potential popping volume of the popcorn. With an unheated forced-air system or naturally ventilated storage, ear-harvested popcorn should field-dry to at least 20 (preferably 18) % moisture and then be harvested promptly to minimize field losses.

Like combines, ear pickers can also damage popcorn kernels if not operated properly. Special rubber snapping rolls are available for ear harvesting of popcorn; they cause less damage than the steel ones used for dent corn picking.

Hand-harvesting. The home gardener or small, non-commercial producer can hand-harvest popcorn anytime after it reaches physiological maturity (approximately 35% moisture). Well-ventilated storage must be available to allow the ears to dry without molding. They can be dried by spreading them on covered concrete floors for several weeks.

Factors other than moisture content can influence when to harvest a given popcorn crop. These include incidence of disease, insect and bird damage and freezing temperatures. Popping volume is not affected by a light frost once moisture content drops below 30%; but it could be significantly reduced by a hard freeze when kernel moisture is above 20%. On this basis, it may sometimes be better to harvest and artificially dry popcorn than risk a hard freeze. Below 20% moisture, freezing apparently has little or no effect on popping volume.

J. Conditioning:

To be high-quality, popcorn must be free of microbial contamination and insect and rodent damage. Aside from that, the most important factor influencing the economic value of popcorn is popping volume - i.e., the volume of popped corn produced from a given weight of unpopped kernels. Processors may reject popcorn that does not meet specified minimum popping volume.

Popping volume is affected somewhat by harvesting and handling practices, and by the moisture history of the popcorn prior to popping; but the primary factor is the moisture content of the kernels when popped. Studies have shown that maximum popping volume is produced at moistures ranging from 13.0 to 14.5%, with 13.5% being optimum. Data also indicate that popcorn must be Initially dried to at least 13.5% moisture before it attains maximum popping volume. After that, moisture can increase to 15% without significantly decreasing popping volume. Overdried popcorn (11% or below) can be rewetted to 13.5% moisture, but it will not recover the maximum popping volume it had on initial drydown to 13.5%.

Conditioning shelled grain popcorn. Popcorn with less than 18% moisture and combine-harvested can usually be conditioned (i.e., dried down to the proper moisture) without spoilage by using an in-bin forced-air drying system. Such a system should supply approximately 2 cubic feet of drying air per minute per bushel of stored grain. This will not overdry the popcorn, unless the season is unusually dry - i.e. relative humidity (RH) often below 60%.

In areas where RH above 80% is expected for extended periods, a minimum amount of supplemental heat (producing no more than a 7-9oF temperature rise) can be provided, if it can be controlled by a reliable humidistat set at 60% RH and activated only when the outside RH exceeds 80%.

In areas where outside RH is consistently below 60%, it may be advantageous to condition popcorn in unheated air portable-batch- or continuous-flow-type dryers before storage. If operated and monitored properly, this type of system prevents the overdrying problems that can occur with an in-storage drying unit.

Conditioning ear-harvested popcorn. Popcorn mechanically harvested on the ear at 20-25% moisture content must be dried promptly to prevent mold growth in storage. Storage structures should incorporate high-volume forced-air ventilation systems to insure uniform drying in all areas of the crib. The high volume of uniform air flow is the key to preventing mold growth; but provision for supplemental heat during periods of higher humidity, as was recommended for shelled popcorn, is also desirable.

Ear popcorn conditioned by heated forced air needs to be monitored frequently to prevent overdrying or too rapid drying, which would reduce potential popping volume. Moisture content can be checked using a commercially available electronic moisture tester either calibrated for popcorn or supplied with a conversion chart for popcorn.

Ear popcorn harvested at 20% or less moisture can usually be stored in naturally ventilated cribs. Natural ventilation should dry the popcorn to a moisture content near that desired for popping without any overdrying problems. To insure that air will move freely through the stored popcorn, cribs should be no more than 3-4 feet wide, and the popcorn itself clean and free of husks and other residue. Once ear popcorn has dried to below 16%, it can be shelled and conditioned to the correct popping moisture. This may be done with a forced-air system similar to that previously described.

Re-conditioning overdried popcorn. If popcorn has been overdried, it can be rehydrated to the desired moisture content but, as mentioned above, it will not fully recover its initial popping volume. Usually the best way is to move 70-85% RH air through it over a long period (i.e., from a week to several months, depending on air flow rate and amount of rehydration needed). Another way is to blend the over-dried popcorn with high-moisture popcorn to produce a desired "average" moisture.

Blending, however, does not always provide satisfactory results. One requirement in blending is an accurate knowledge of both the moisture contents of the lots being blended and the blending flow rates to insure that the desired average moisture content of blended popcorn is attained. Even then, the moisture levels of the blended grains will not be exactly the same at equilibrium. The higher-moisture grain will always maintain a slightly higher moisture content than the lower-moisture grain in the blend. However, a difference of less than 1% will not have a measurable effect on popping volume.

Heated-air drying is generally not recommended for popcorn because of problems with overdrying, non-uniform drying and occurrence of stress cracks from too rapid drying. Its use, however, may be necessary to prevent spoilage or speed the drying process. If so and to minimize the amount of stress cracking, drying air temperatures should be in the 90-100oF range, with 120oF being the maximum.

Higher quality popcorn can be expected if drying is done in several stages, with a 12-24 hour tempering period between stages to allow for moisture redistribution within the kernels. An alternative to this is a two-stage combination drying system where a heated-air dryer is used to reduce moisture content to approximately 17-18%, then a natural-air or low-temperature drying system finishes the conditioning process.

K. Storage:

For best storage, the moisture content of popcorn must be low enough to prevent significant fungal and microbial activity, but not so low as to adversely affect its popping volume. Popcorn at 14.5% moisture can be safely stored over winter and into early spring. For longer term storage, it should be dried to 13.5-12.5%.

Aeration systems similar to those systems used for dent corn should be provided for shelled popcorn storage bins to prevent moisture migration and help maintain grain quality during storage. Ear popcorn cribs must allow good natural ventilation and be designed to prevent rain and snow from getting into the popcorn.

Before storing a new crop, all bins and cribs should be cleaned and treated for insects; the grain can also be treated as it is put into storage. Use only those insecticides approved for this type of application, and apply according to manufacturer's recommendations. Popcorn that has not been treated with any insecticide must be watched carefully for any sign of insect activity, particularly if it is to be stored after the weather warms up in the spring. An aeration system will tend to reduce insect activity in the winter by keeping the grain near the average outside temperature. Storage in refrigerated warehouses will prevent damage by stored grain insects. All cribs and bins for popcorn storage should also be rodent proofed. To do this, keep the surrounding area free of weeds and trash, which can harbor rats and mice, and apply an approved rodenticide as necessary to prevent problems from developing in the storage facility.

VI. Yield Potential and Performance Results:

In the major commercial production regions, popcorn yields (measured as pounds of shelled corn per acre) for the years 1977-1981 averaged nearly 2900 pounds which, at 65 pounds per bushel, is equivalent to about 44 bushels per acre. Nebraska usually reports the highest yields because of its high proportion of irrigated acres. On a weight basis, popcorn hybrids can be expected to yield a little less than half as much as dent corn hybrids. No popcorn yield trials have been conducted in Wisconsin or Minnesota in recent years.

VII. Economics of Production and Markets:

As with any specialty crop, marketing and economics are extremely important considerations in profitable popcorn production. The grower considering a large acreage must be familiar with marketing outlets for the crop and the economics involved.

Generally, three markets are available for good-quality popcorn: processor-contracted acreage, open-market sales, and local sales. However, since not all harvested popcorn may be marketable as popping corn, thought should be given to alternative uses or outlets.

Processor-contracted acreage. Most popcorn is grown under contract to processors. This acreage is adjusted annually to the processors' estimated needs as determined by market analysis. Normally, these estimates are very close to actual demands, which tends to stabilize the popcorn market.

Most contracts specify that a grower plant a given number of acres with a certain hybrid for a fixed price per 100 pounds of delivered popcorn. As an alternative to fixed pricing, some processors write contracts using the commodity price of popcorn on the Board of Trade on a given day, thus allowing the grower some flexibility in the system. By keeping abreast of popcorn supply and demand as well as the price of dent corn and soybeans, processors are able, under normal growing conditions, to contract popcorn at prices that provide a reasonable profit for the successful grower. Hence, growing contracted popcorn is generally competitive with dent corn.

Open-market sales. A popcorn grower who plans to sell on the open market assumes the risk of fluctuations in price. A year or so of high popcorn prices relative to dent corn not only tempts regular popcorn growers to increase their acreage, but also attracts new growers, the result often being over-production, low prices and financial loss. Thus, it is unwise, especially for the novice, to plant a large acreage of popcorn immediately following a year of high prices.

An open-market grower must be aware of current acreage and crop conditions, as well as probable market demand and carryover. The grower should be prepared, financially and storage facility-wise, to hold the crop for an extended length of time. Good-quality popcorn stored under good conditions will keep indefinitely, allowing the grower to wait for a price that ensures a profit.

Local sales. This market alternative requires a longer-term commitment to popcorn production. Depending on the level of involvement, it can entail becoming a popcorn processor on a small scale. Success in the local sales market depends on the ability of the grower, as both producer and merchant, to grow and process a high-quality product and utilize proper packaging. Growers lacking in any of these areas will likely not fully satisfy their direct sales customers, and sales will drop rapidly, especially in light of the number of competitive popcorns readily available in stores.

Alternative outlets. If the crop is not marketable as popping corn, it can be ground and fed to livestock or poultry. Any outlet for unmarketable popcorn that provides some monetary return on the crop will lessen the extent of the financial loss.

Home Garden Production. All that is required to grow popcorn for home use is adequate space and a little gardening know-how. Most seed catalogs list popcorn varieties for home gardeners. To find one that grows best under your conditions, try several over a couple of years; and keep testing new ones as they come on the market. Maturity is important in variety selection because popcorn that does not reach full maturity before frost will have very poor quality.

Plant popcorn according to package directions. It is better to plant several short rows side by side than one long row. Also, do not plant sweet corn and popcorn in the same garden; if they happen to shed pollen at the same time, the sweet corn quality might be reduced. Popcorn requires adequate nitrogen and should be fertilized accordingly.

Harvest popcorn only after the kernels are hard and the husks completely dry. After picking, remove the husks and store the ears in bags that allow air movement so ears can dry. Each week, shell a few kernels and try popping them. When they pop well, shell the remaining ears and store in moisture-proof containers. Because popcorn can become infested with several types of insects, refrigeration is the best long-term storage.

Determining if moisture content is optimum for the best popping volume is a difficult problem. If the popcorn is "chewy" after popping, it is probably still too wet; so allow the kernels to dry some more, popping a sample every couple of days until the flakes are no longer chewy.

Popcorn that pops poorly with many unpopped kernels is probably too dry and needs moisture. Start by adding one tablespoon of water to a quart of popcorn, mix well a couple of times that day, then after 2-3 days try popping another sample. Continue this procedure until the popcorn pops well.

VIII. Information Sources:

Popcorn Production and Marketing. 1985. K. E. Ziegler, R. B. Ashman, G. M. White, and D. S. Wysong. National Corn Handbook, NCH-5, Purdue University, West Lafayette, Indiana.

Popcorn. 1987. L. W. Rooney and S. O. Serna-Saldivar, in Corn: Chemistry and Technology, Edited by S. A. Watson and P. E. Ramsted, American Association of Cereal Chemists. p. 420-421.

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