Ron Heiniger, Crop Science Extension Specialist, NC State University
Dominic Reisig, Entomology Extension Specialist, NC State University
Key management practices for organic corn production:
- Choose organically grown (when possible), non-GMO hybrids with high vigor, high standability rates, disease and pest resistances, stress tolerance, high yield, and a maturity date of 112 days or less.
- Plant on time, at the proper depth, in a well prepared seedbed, on narrow rows.
- Rotate crops.
- Achieve proper soil pH and good fertility.
- Choose the correct plant population.
For organic growers seeking to identify appropriate corn hybrids, yield is not the primary consideration. Producers should consider these key hybrid characteristics for organic corn production:
- rapid early growth and vigor
- pest and disease resistance
- stress tolerance
Rapid early growth and vigor
Rapid early growth is essential to minimize the effects of seedling diseases and insects, increase root volume, and reduce weed infestation. Hybrid seed companies list seed vigor ratings. Few companies, however, list ratings covering early growth. In general, early growth is closely related to hybrid maturity. Early to medium maturing hybrids (102- to 114-day relative maturity) tend to exhibit better early growth than do late hybrids (> 115-day relative maturity). The best way to select hybrids with rapid early growth for North Carolina is to contact Cooperative Extension agents, seed company representatives, and other growers who have had experience with different corn hybrids.
Standability is important because it is a measure of how well the crop will stand under difficult environmental conditions. Because pests and diseases can be problems, it is important that an organic hybrid has the ability to avoid lodging under stress. Most hybrid seed suppliers provide ratings for standability or stalk or root strength.
Pest and Disease Resistance
Resistance to common seedling, leaf, and stalk diseases is an important characteristic for hybrids in organic production systems. There are even some hybrids that tolerate insect pests such as European corn borer and southern cornstalk borer. Unfortunately, most hybrids do not have resistance to a wide range of diseases or pests. Growers should select hybrids that combine good early growth characteristics with a good resistance package to diseases that are major problems in their area. In North Carolina the major diseases of corn are grey leaf spot and northern and southern leaf blights.
Unfortunately, variety testing of corn under organic conditions is sparse. Past NC trials can be found at the Organic Grains portal. Given the rapid turnover of hybrids, test results can rapidly fall out of date. Growers should conduct their own hybrid comparisons by selecting four to six promising hybrids and evaluating them on their farms with their management practices. The best procedure for grower testing of hybrids is the strip test where each hybrid tested is grown adjacent to a common “tester” hybrid. The strip test, with tester hybrids, permits any yield data collected to be adjusted for soil variability. If not using a tester, growers should place the hybrids they are considering beside the hybrid that has performed best for them in the past. Growers conducting their own hybrid evaluations must remember to select uniform test fields with minimal soil variability and restrict comparisons to hybrids of the same maturity.
Planting date is crucial to the success of an organic production system. Planting too early results in slow growth and increases weed competition, the incidence of seedling diseases, and the likelihood of damage from seedling insects. On the other hand, planting too late results in a greater risk of drought stress, increased insect damage from second and third generations of European corn borers, and reduced yield due to decreasing hours of daylight. The recommendations here attempt to balance these considerations. In the NC tidewater and coastal plain, plant organic corn between April 15 and May 15. In the NC piedmont, plant organic corn between April 20 and May 20. In all NC locations, plant following at least two days when average temperatures are above 65ºF. Depending on the soil type, time soil preparation and planting date so that soils are moderately dry at planting to minimize the risk of seedling diseases.
Seedbed preparation should begin with a major tillage operation performed at least a month before planting. If cover crops are used, they may need to be killed and/or incorporated into soils earlier than one month before planting to allow for residue decomposition and to avoid seed corn maggots. Heavy applications of compost or manure should also be incorporated earlier. Follow up with at least two light tillage operations to create a smooth, weed-free seedbed. The final tillage operation should be performed on the day of planting to ensure that all germinated weeds have been destroyed when the seed is placed in the ground. Seeding depth is very important in an organic production system. Seeds planted too deeply will be slow to emerge, and seedlings will have immediate weed competition and a greater likelihood of damage caused by seedling diseases. A seeding depth of just 1 inch is sufficient on most soils and allows for rapid emergences.
Plant population is another important factor in organic corn production, especially when corn is grown on sandy soils. Plant populations should be related to the moisture-holding capacities of each individual field. In organic systems, corn plant populations per acre should be 10 percent higher than in conventional systems. The higher plant population will increase light interception and reduce weed competition and the effects of pest damage. On soils with good-to-excellent water-holding capacity, the goal is a stand of 30,000 to 33,000 plants per acre; on soils with average water-holding capacity, 25,000 to 28,000 plants per acre; and on soils with poor waterholding capacity, no more than 22,000 plants per acre.
Narrow rows permit more uniform plant distribution and result in rapid closing of the canopy. In choosing a row width, balance the potential advantages that come from narrower rows against the additional machinery cost and management that a narrow row system demands. Because cultivation is the primary weed control measure in organic production, make rows wide enough to permit the use of a tractormounted cultivator. Row spaces as narrow as 20 inches have been successful under organic conditions with alterations in cultivators and guided steering systems (see the farm profile in Chapter 7).
Corn generally requires from 120 to 160 lb of nitrogen per acre, 30 to 50 lb of phosphorus per acre, 80 to 100 lb of potassium per acre, and smaller amounts of sulfur and micronutrients to obtain optimum yield. Organic corn growers should design their systems so that the amount of nutrients added to the system offsets the amount removed in the grain or forage. The local offices of the USDA Natural Resources Conservation Service, Cooperative Extension, or the Soil and Water Conservation District can provide guidelines for a nutrient management plan. Chapter 6 also has more information on organic soil management.
Grassy weeds and warm-season broadleaf weeds, such as cocklebur and morning-glory, will be among the most difficult to control. Although tillage prior to planting can help reduce early-season weeds, many of the summer annuals will continue to germinate and grow. It is very important to start with a clean seedbed and to till the soil just before planting so the crop begins with a head start on new weed seedlings. This will make it much easier to use cultivation to control grass and broadleaf weeds that are smaller than the corn. It is also important to take advantage of the corn canopy’s ability to shade the soil. Shade reduces the number of weeds germinating and slows their growth. Use of increased plant populations, narrower rows, row directions perpendicular to the path of the sun, and tall-growing hybrids all increase canopy density and lead to quick canopy closure. Remember that weed competition during the first four to six weeks after planting will cause the most damage in terms of yield reductions. Weeds that emerge after canopy closure will have little effect on yield, although they can make harvest more difficult. Chapter 7 has more information on managing weeds in organic production.
Cultural practices are very important for establishing a vigorous, full corn stand. Stand establishment can greatly influence pest populations as well as crop competitiveness and tolerance to pest feeding. In fields where pests are historically abundant, do not plant organic corn if suitable, effective, and economical pest management options are not available.
Crop rotation is one of the most powerful tools for insect management and is also often the lowest-cost method of control. Rotations of at least two years and use of a nongrass crop will reduce the levels of many pests through starvation, interference with insect reproduction, or both. Rotation also gives the option of isolating corn crops from one year to the next. This may or may not be effective for wireworm. Depending on the species, a single generation of wireworm can take one to five years to complete. As a result, a multiyear rotation out of corn may be needed to avoid this pest. Rotation in large units with a minimum of 800 to 1,000 feet between current and previous corn is the most effective way to manage
moderately mobile pests such as billbugs.
Cover crops may reduce the abundance of some pests, although little research has been done in corn. Alternatively, the density of certain pests, such as cutworms and probably wireworms, can be increased by cover crop use.
Insect pests that feed on seed and small seedlings are typically found in the soil or at the soil surface. Populations of wireworms, cutworms, grubs, seed corn beetles, and other pests can be reduced with winter or early spring disking and the accompanying bird feeding and exposure. The combined action of these factors can give meaningful protection to planted seed and small seedlings. No-till organic corn using cover crop mulches is being tested around the state, but this system is not as well understood as no-till soybeans (see Chapter 6).
Rapid germination and seedling grow-off
Rapid germination and seedling grow-off reduces the time corn seed and seedlings spend in the most vulnerable stage between germination and the six-leaf stage and helps the crop gain a size advantage over weeds. Losses to seedling insects and other pests can be reduced by promoting early germination through row-bedding, seeding at the recommended depth, hybrid selection for performance under cool conditions, and adequate soil fertility.
In corn, timely maturity of the crop almost always reduces insect damage. Certain pest insects and pathogens (for example, late-season corn borers and fall army worms) reach high densities in late July and August and may severely infest late-maturing corn. Timely planting and avoidance of late-maturing hybrids (over 120 days) will reduce the level of pests attracted to the crop in late season and prevent yield loss. When planted early, hybrids that mature in 112 days or less will usually avoid late-season caterpillar attack.
Rapid germination, early vigor, strong ear shanks, tight husks, resistance to stalk rots and other pests, strong stalks, and uniform performance over a wide population range are factors influenced by genetics that may reduce losses to insects.
Billbugs can be serious pests of corn seedlings. No insecticide approved for organic use has activity against billbugs. Combining cultural tactics—rotation and isolation from previous corn crops—along with rapid seedling emergence and grow-off should help prevent concentrations of adult billbugs and promote rapid accumulation of tolerance. Three additional billbug management tactics are (1) avoiding areas with abundant nutsedge, which is an alternative host for billbug; (2) avoiding no-till production for organic corn because no-till soils warm more slowly and delay germination and grow-off; and (3) planting at the earliest possible date to allow seedling growth prior to billbug adult emergence.
Wireworm and black cutworm
In organic systems, the major tactics for reducing populations of these insects will be disk cultivation and avoidance of no-till situations. Cultural methods that promote rapid seedling growth and seeding at adequately high populations to allow some seedling loss can also be important.
European corn borer (ECB) and southern corn-stalk borer
Borers likely occur at some level in all North Carolina cornfields. Their populations fluctuate greatly between years and sometimes within a single growing season. The organic farmer can influence the abundance of these borers through rotation, site selection (away from first-generation ECB nursery areas in potato and wheat fields), early planting, and use of short-season corn hybrids. Taking these actions to manage both space and time will help avoid high populations and promote tolerance for those borers that are present. Organically approved spinosad insecticides are labeled for ECB on corn, but they are expensive and are not likely to be effective when sprayed on tall corn. For ECB scouting procedures and thresholds, consult your county Extension center or the Extension corn portal.
Western corn rootworm
Western corn rootworm is a pest only in nonrotated corn. It can be successfully managed in an organic system by rotating corn with other crops.
Six key diseases—Grey leaf spot, Northern and Southern corn blights, seed rots and seedling blights, stalk rots, and charcoal rot, which are usually controlled in conventional systems either by fungicides or management practices—can have significant impacts on organically grown corn. Growers should be aware of these diseases and select hybrids and management practices that reduce the risk they pose. Grey leaf spot and Northern and Southern corn blights are best managed in organic systems by choosing resistant hybrids. While there are many other diseases that can attack corn, they rarely cause economic loss. Pictures of these field corn diseases can be found in Common and Southern Rusts.
Seed rots and seedling blights
Seed rots and seedling blights caused by species of Fusarium, Stenocarpella, Pythium, and other fungiare often associated with the term “damping-off.” Plants die at emergence or within a few days of emergence. These diseases are more prevalent in poorly drained, excessively compacted, or cold, wet soils. Planting old or poor quality seed with mechanical injury will increase seed rot and seedling blight, as will planting seed too deep in wet, heavy soils. Seed vigor
ratings are often used to select hybrids with genetic resistance to seed rots and seedling blight.
Stalk rots (caused principally by the fungi Stenocarpella zeae and species of Fusarium as well as Colletotrichum graminicola) are present each year and may cause considerable damage, particularly if abundant rainfall occurs during the latter part of the growing season. Stalks previously injured by cold, leaf diseases, or insects are especially susceptible to attack by these fungi. Diseased stalks ripen prematurely and are subject to excessive stalk breaking. Stalk rots not only add to the cost of harvesting but also bring the ears in contact with the ground, increasing their chance of rotting. Adequate fertility (particularly adequate potassium) is the key to controlling stalk rot.
Charcoal rot (caused by the fungus Macrophominaphaseolina) becomes most evident with the onset of hot dry weather. It may cause stalk rot, stunting, and death of the corn plant. This disease is often considered to be stress-related. Typically, when this disease occurs in North Carolina, soil fertility and pH are at very low levels. Although the fungus survives in the soil, rotation is not generally helpful because most crops are susceptible to this disease. Supplying
adequate nutrition and water is the principal means of control. Hybrid resistance in corn has not been documented.
Early harvesting usually avoids crop damage from pests or hurricanes and prevents field losses resulting from ear drop and fungal pathogens. Probably the most important reason for timely harvest is the potential for yield reductions resulting from ear loss and ear rots due to stalk lodging, ear drops, and reductions in kernel weight. Fungal diseases that infect the corn kernel also cause more problems as harvest is delayed. Mycotoxins, such as aflatoxin and fumonisin, which are produced by fungal pathogens, also increase as harvest is delayed and may result in corn that is unsuitable for human or livestock consumption. Ideally, corn harvest should begin as soon as the grain reaches moisture levels of 25 percent or less. Under favorable conditions, corn should be ready to harvest in 10 days or less following the black layer formation at the base of the kernels.
Crops that are propagated by using the pollen from selected (usually inbred) male plants to fertilize selected (usually inbred) female plants are called hybrids. Neither the female or male plants may exhibit robust growth and/or vigor. Yet, when these plants are crossed, they may exhibit vigor, resistance to disease or drought, or other desirable characteristics. Probably the most common hybrid crop plant today is corn. Hybrid corn seed is produced by allowing male plants to have tassels to produce pollen. Tassels are removed from the female plants and pollination takes place by the male plants. The resulting seeds contain a mixture of genes from both parents at each point on the chromosome. Because the two parents are inbreds that carry the same genes at each point on their chromosomes (homozygous) the resulting hybrid exhibits the same characteristics (such as height, color, ear placement) across the seed lot. This gives the crop its uniformity and makes it easier to handle and harvest.
This is the term used to describe crops that are not hybrids and that produce seed by the crop pollinating itself. For simplicity these crops are often referred to as OP crops. Some of the most common OP crops are soybeans and wheat. In the corn industry, most of the crop seeds produced 75 to 100 years ago were OP types. Today there is a thriving niche seed business in “old” or “antique” varieties. Nearly all old or antique varieties are OP materials. Because a crop is from open-pollinated seed stock does not mean the crop has not undergone serious plant breeding efforts through isolation, inbreeding, crossbreeding, and other means of genetic manipulation. Unlike open-pollinated soybeans and wheat, which are largely self-pollinating, open-pollinated corn exhibits a large amount of cross-pollination (pollen from one plant fertilizing another plant). This means that most open-pollinated corn varieties are not homozygous with similar genes at each point on the chromosome. Instead open-pollinated corn exhibits mixtures of genes (heterozygous) at each chromosome similar to a hybrid. However, unlike a hybrid where each parent is identical and imparts identical mixtures of genes to the offspring, open-pollinated corn parents differ in their genetic makeup with the result that the offspring exhibits a wide range of characteristics—such as wide variations in height, maturity, ear placement, and kernel color. This variation is often undesirable because it makes the crop more difficult to manage and harvest.
The Problem with Saving Hybrid Seed
As stated earlier, hybrid seed has a mixture of genes from each parent at each of its chromosomes (alleles). The only saving grace is that because the parents were homozygous, each seed has the same mixture. When hybrid seed is saved, however, it is no longer possible to control which plant contributes the male genes and which contributes the female genes. Therefore, saved hybrid seeds are genetically different from each other. When these seeds are planted again, these different genes start to express characteristics (such as height and kernel color) that differ from each other. A fourth of the seeds will produce plants that look more like the female parent, a fourth will look more like the male parent, and the remaining half will have a combination of characteristics from the two parents. This segregation of genes means that seed saved from hybrids will start to produce plants that have the same problematic variation as open pollinated varieties—such as a range of maturity, height, and color—which makes the crop difficult to harvest and manage.
George Place, Crop Science, NC State University
Major Goodman, Crop Science, NC State University
Genetic contamination of organic corn with genetically modified (GM) genes is a growing concern for organic producers. Although corn pollen does not travel far in comparison to many other grass species, if temperature, humidity, and wind are favorable, corn pollen can travel thousands of feet. Research has indicated that cross-pollination between cornfields could be limited to 1 percent or less on a whole-field basis by a separation distance of 660 ft, and limited to 0.5 percent or less on a whole-field basis by a separation distance of 984 ft. However, cross-pollination could not be limited to 0.1 percent consistently even with isolation distances of 1,640 ft.
Organic certifying agencies and organic grain buyers will need to know how organic corn farmers avoided genetic contamination from neighboring GM corn crops. Some buyers (and all who ship products to Europe) will utilize a serological test that can detect a GM protein in the corn and will reject loads that are above a certain percentage. Contamination tolerance levels must be 0.9 percent or less for corn to qualify as organic in the European Union. The United States does not have a set threshold of contamination tolerance for organic certification, but many buyers are establishing their own product thresholds.
To reduce genetic contamination, farmers must plan ahead to spatially or temporally distance organic corn from conventionally-grown corn. Organic corn should be planted at least 660 ft from any neighboring GM corn (or conventionally grown corn), if possible. This may mean planning a rotation around what your neighbors are doing. If distance separation is not possible, another strategy is to plant later or earlier than your neighbors so that your corn is pollinating at a different time. A typical corn plant will shed pollen for five to six days. A whole field will usually complete pollen shed in 10 to 14 days. If a neighbor producing GM corn is planting a 110-day corn hybrid, the organic corn producer could plant a later maturity hybrid (say 118-day corn) 10 days after his neighbor has planted. This would create a maturity separation of 18 days, leaving plenty of time for the neighbor’s GM corn to complete pollination. However, the organic producer must be careful not to confuse this temporal separation. If the neighbor is planting a later maturity variety (118-day), then the organic producer wanting temporal separation would also need to chose a later maturity variety (118-day or later) and plant at least two weeks later (pollination times could match if the organic producer chose an earlier maturing hybrid and planted later). Temporal separation strategies also must take into account that rarely could the organic corn producer plant earlier than the GM corn because organic corn seed is untreated and susceptible to early-season diseases. To significantly reduce any genetic contamination that may have occurred despite these measures, many farmers harvest the outside rows of their organic corn separately and sell it on the conventional market. The number of buffer rows needed depends on how susceptible the field was to cross-pollination contamination.
Blue River Hybrids has been marketing Pura Maize hybrids that will use what is known as the Ga1-s isolating mechanism. This is a naturally occurring gene in corn that stops pollen originating from a plant that does not have the Ga1-s gene from being able to pollinate a plant that does have the Ga1-s gene. This crossing barrier is utilized extensively in commercial popcorn hybrids because popcorn hybrids are grown in the Midwest, as is dent or field corn. If a popcorn ear is pollinated by dent corn pollen, then the resulting popcorn kernel does not behave as a true popcorn—which would irritate a lot of movie goers. Thus, popcorn hybrids that have the Ga1-s gene do not accept pollen from the surrounding GM field corn hybrids that do not have this Ga1-s gene. Dr. Major Goodman, NC State corn breeder, has also incorporated new crossing barrier genes into several breeding lines that will be released to the public in 2013. We hope that hybrids with these genes will become commercially available soon.
Publication date: Feb. 10, 2014
Other Publications in North Carolina Organic Grain Production Guide
- Chapter 1: Introduction
- Chapter 2: Organic Crop Production Systems
- Chapter 3: Crop Production Management - Corn
- Chapter 4: Crop Production Management - Organic Wheat and Small Grains
- Chapter 5: Crop Production Management - Organic Soybeans
- Chapter 6: Soil Management
- Chapter 7: Weed Management
- Chapter 8: Rolled Cover Crop Mulches for Organic Corn and Soybean Production
- Organic Certification for Field Crops: A Guide
- Chapter 10: Marketing Organic Grain Crops and Budgets
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