Organic Field Crop Production and Marketing in North Carolina

Chapter 3: Crop Production Management - Corn

Ron Heiniger, Crop Science Extension Specialist, NC State University

John Van Duyn, Entomology Extension Specialist, NC State University

Production Management

Key management practices for organic corn production:

 Hybrid Selection

For organic growers seeking to identify appropriate corn hybrids, yield is not the primary consideration. The key hybrid characteristics for organic corn production are

Table 3-1 provides a list of organic and conventional untreated hybrids that have been evaluated and rated for these key characteristics.

Rapid early growth and vigor

Rapid early growth is essential in minimizing the effects of seedling diseases and insects, increasing root volume, and competing with weeds. In general, early growth is closely related to maturity date. Early- to medium-maturing hybrids (102 to 114 days) tend to exhibit better early growth than do late hybrids (longer than 115 days). The best way to select hybrids with rapid early growth for North Carolina is to contact Extension agents, seed company representatives, and other organic growers who have had experience with different corn hybrids.

Standability

Standability is an important measure of how well the crop will stand under difficult environmental conditions. Because pests affect stalk strength, an organic hybrid needs to be able to resist lodging under stress.

Pest and disease resistance

Resistance to common seedling, leaf, and stalk diseases is an important characteristic for hybrids in organic production systems. Some hybrids even tolerate insect pests such as European corn borer and southern cornstalk borer. Growers should select hybrids that combine good early growth characteristics with good resistance to diseases that are likely to be problems in their fields.

Stress tolerance

Stress tolerance is the ability to produce acceptable yield under drought or other environmental stresses. Hybrid seed suppliers often refer to this characteristic as “drought tolerance.” This characteristic is important since limited available nitrogen, which is often a problem in the early years of an organic system, can lead to nutrient and drought stress. Hybrids that can tolerate this stress will produce higher yields and compete more successfully with weeds.

Yield

The only reliable indicators of yield potential in organic systems come from tests conducted using organic practices. Most of the hybrid comparisons done in organic systems use hybrids best suited to the upper Midwest, and there is only a limited amount of organic-yield test information available in North Carolina. In these circumstances, growers may get the best information from local hybrid comparisons when drought or other types of stress were factors. For instance, official variety tests conducted in North Carolina during 2002 reflect results from drought conditions and might be indicators of hybrid performance in organic systems. Those hybrids with good compensating mechanisms may do well in situations of low nitrogen availability or high pest pressures. Growers should conduct their own hybrid comparisons by selecting four to six promising hybrids and evaluating them under their own management practices. The best procedure is the strip test, where each test hybrid is grown adjacent to a standard hybrid (see Figure 3-1). This pattern allows researchers to adjust the yield data for soil variability. If a standard hybrid is not used, test hybrids can be alternated with a hybrid that has the best past performance. Growers conducting their own hybrid evaluations should remember to select uniform test fields to minimize soil variability and to restrict comparisons to hybrids of the same maturity class.

Figure 3-1. Sample planting pattern for a strip test

Table 3-1. Evaluations of organic and untreated corn hybrids for relative maturity, seed vigor, early growth rating, standability, disease ratings, and stress tolerance*

* Ratings are based on a scale of 1 to 10. A perfect rating of 10 represents a plant with complete resistance or tolerance to disease or stress.

Planting Date

Planting date is a crucial factor in the success of an organic production system. Planting too early results in slow growth and increases the amount of 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 from a decrease in intercepted sunlight due to decreasing hours of daylight. The recommendations here attempt to balance these considerations. In the tidewater and coastal plain, plant organic corn between April 15 and May 15. In the piedmont, plant organic corn between April 20 and May 20. In all 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 and Planting Depth

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. Perform the final tillage operation on the day of planting to ensure that all germinated weeds have been destroyed when the seed is placed in the ground. Place the seed exactly 1 inch deep, and compress the soil to provide maximum seed-soil contact for rapid germination and growth. Seeding depth is a very important factor 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.

Plant Population

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 populations 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 water-holding capacity, no more than 22,000 plants per acre.

Row Spacing

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 row widths wide enough to permit the use of a tractor-mounted cultivator. Where weeds are not a major problem, use row spacing of 20 to 24 inches. Where weed control will require multiple passes of a cultivator, use row spacing of 30 to 36 inches.

Soil Fertility

Corn generally requires from 120 to 160 pounds of nitrogen per acre, 30 to 50 pounds of phosphorus per acre, 80 to 100 pounds 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, the Cooperative Extension Service, or the Soil and Water Conservation District can provide guidelines for a nutrient management plan. Chapter 6 in this manual also has more information on organic soil management.

Weed Management

Grassy weeds and warm-season broadleaf weeds, such as cocklebur and morningglory, will be among the most difficult to control. While 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 prior to planting so that 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 ability of the corn canopy 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 in this manual has more information on managing weeds in organic production.

Insect Pest Management

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 at high levels, do not plant organic corn if suitable, effective, and economical pest management options are not available.

Crop rotation

Crop rotation is the most powerful tool 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 reduces 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. 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.

Control of insects with tillage

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. In organic corn production, no-till methods should be avoided or used with caution, especially in areas where southern corn billbug, wireworms, and cutworms are common.

Rapid germination and seedling grow-off

Rapid germination and seedling grow-off reduces the time corn seed and seedlings spend in their 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.

Crop maturity

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 levels 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.

Hybrid selection

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. Some hybrids have European corn borer-resistance traits that will reduce susceptibility to this important pest.

Major Corn Insect Pests and Management

Corn billbugs

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 disc 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 cornstalk borer

Borers occur 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 white potato and wheat fields), early planting, use of short-season corn hybrids, and selection of hybrids with ECB tolerance. Taking these actions to manage both growing space and planting 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 office or the following Web site: http://www.ces.ncsu.edu/plymouth/pubs/ent/index1.html.

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.

Key Diseases and Management

Three key diseases—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. While there are many other diseases that can attack corn, they rarely cause economic loss. Pictures of these field corn diseases can be found at this Web site: http://www.btny.purdue.edu/Extension/Pathology/CropDiseases/Corn/.

Seed rots and seedling blights

Seed rots and seedling blights caused by species of Fusarium, Stenocarpella, Pythium, and other fungi are 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

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

Charcoal rot (caused by the fungus Macrophomina phaseolina) 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 since 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.

Harvesting

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.