A key step in high-yield corn production is monitoring fields and troubleshooting yield-limiting factors throughout the growing season. Corn growers who understand how the corn plant responds to various cultural practices and environmental conditions at different stages of development can use management practices more efficiently and, thus, obtain higher yields and profits. Knowledge of growth and development also helps in troubleshooting problems related to abnormal growth caused by pest problems or inappropriate cultural practices. A helpful resource with information for diagnosing problems related to pests and environmental stresses during the growing season is OSU Extension bulletin 827 Corn, Soybean, Wheat, and Forages Field Guide.
Table 4.3 describes the most widely used staging system for corn growth and development - the leaf collar method - largely used by Extension agronomists throughout the United States. Table 4.4 shows a timeline relating corn growth and development to normal heat unit (growing degree day) accumulation during the growing season, following the leaf collar method.
The second method is the horizontal leaf method, which is used by crop insurance adjusters to assess hail and other weather-related plant damage.
Leaf Collar Method
Start with the first oval-shaped leaf as V1. A field is defined as being at a given stage when at least 50% of plants show collars.
Horizontal Leaf Method
Growth staging system used by hail adjusters for hail damage assessment (Table 4.5).
- Identify the uppermost leaf that is 40%–50% exposed and whose tip is below the horizontal.
- Typically, a horizontal leaf growth stage will be one to two leaf stages greater than the collar method
Table 4.5 lists estimated yield loss resulting from varying amounts of leaf area destruction for several stages of development. Although this table was developed to determine yield losses resulting from hail damage, it can also be used as a reference to help assess losses resulting from other defoliation injuries (such as wind, frost, insect feeding, herbicide damage, and foliar nitrogen “burn”). The most common damage from hail is loss of leaf area, although stalk breakage and bruising of the stalk and ear may also be severe. Note that the largest yield losses result from defoliation damage that occurs during the late vegetative stages and the reproductive stages (silking and tasseling). Defoliation at early growth stages does not affect yield the same way as it does at later growth stages because much of the plant’s total leaf area is not yet exposed. Extensive defoliation of plants in the 10- leaf growth stage (or V8, eight-leaf collar stage) does not result in a large yield loss because only 25% of the leaf area is exposed and the plant can easily recover from early damage. On the other hand, severe damage to plants during tasseling results in a large yield loss because, by that time, 100% of the leaf area has been exposed and cannot be replaced
Early killing frost in the fall may damage immature corn and reduce yield. The effect of frost damage on corn depends on the severity of defoliation, stalk damage, and stage of growth (see Chapter 1, Figure 1.4, for the median fall frost dates). Tables 4.6 and 4.7 provide yield loss and moisture estimates resulting from premature plant death (defoliation) during grain fill.
Additionally, wind-induced damage to plants can affect adequate corn growth and development, such as root lodging and greensnap. For more information, see the section on Stalk Quality and Lodging in this chapter.
Hybrid Selection
Selecting a hybrid for planting is a key step in designing a successful corn production system. To stay competitive, growers must introduce new hybrids to their acreage regularly. During the past 40 years, the genetics of corn hybrids have improved steadily, contributing to increases in grain yield potential ranging from 0.7% to 2.6% per year.
Growers should choose hybrids best suited to their farm operations. Corn acreage, soil type, tillage practices, desired harvest moisture, and pest problems determine the need for such traits as drydown rate, disease resistance, early plant vigor, plant height, etc. End uses of corn should also be considered (see the section on Specialty Corn). Will the corn be used for grain or silage? Will it be sold directly to the elevator as shelled grain or used on the farm? Capacity to harvest, dry, and store grain should also be part of the decision-making. The most important factors for hybrid selection in Ohio are relative maturity, yield potential and stability, stalk quality, and disease resistance.
Relative Maturity
Growers should choose hybrids with maturity ranges appropriate for their geographic area or circumstances. Corn for grain should reach physiological maturity or “black layer” (maximum kernel dry weight) one to two weeks before the first killing frost in the fall. Use days to maturity and growing degree day (GDD) ratings along with grain moisture data from performance trials to determine differences in hybrid maturity. Although yields of full-season hybrids often exceeded those of short-season hybrids in the past, early- to mid-maturing hybrids have been developed in recent years with yields comparable to those of full-season types. Late- to full-season hybrids do not always mature or dry down adequately before frost, possibly resulting in lower yields and/or wet grain at harvest. When confronted with delayed planting or replanting decisions, growers may need to switch to early- to medium-maturity hybrids adapted to their area, but they should generally avoid short-season hybrids that are earlier than those normally used due to lower yield potential. For more information on selecting hybrids for late planting, see the section on Date of Planting.
Days to Maturity Rating System
The most common maturity rating system is the days-tomaturity system. This system does not reflect the actual calendar time between planting and maturity. A 106- day hybrid, for example, does not mature 106 days after planting. A days-to-maturity rating is based on relative differences within a group of hybrids for grain moisture at harvest. A one-day maturity difference between two hybrids is typically equal to a 1/2 to 3/4 percentage point difference in grain moisture. For example, a 106-day hybrid would be, on average, 3 to 4.5 points drier than a fuller season 112-day hybrid if they were planted the same day (6 days multiplied by 0.5 or 0.75).
The relationship between days to maturity and kernel moisture is usually dependable when comparing hybrid maturities within a single seed company. However, because there are no industry standards for the days-tomaturity rating system, grain moisture comparisons of similar hybrid maturities from different seed companies may vary considerably. Days-to-maturity ratings are satisfactory for preseason hybrid maturity selection when the length of the growing season is usually not an issue. For delayed planting or replanting hybrid selection needs, growers need more absolute descriptions of a hybrid’s growing season requirements to manage the risk of a killing fall frost to late-planted corn.
Growing Degree Day (GDD) Maturity Rating System
The GDD maturity rating system is based on heat units. It is more accurate in determining hybrid maturity than the days-to-maturity system because the growth of the corn plant is directly related to the accumulation of heat over time rather than the number of calendar days from planting. The GDD system has several advantages over the days-to-maturity system. The GDD system provides information for choosing hybrids that will mature reliably, given a location and planting date, allows the grower to follow the progress of the crop through the growing season, and aids in planning harvest schedules.
The GDD calculation method most commonly used for corn in the United States is the 86/50 cutoff method. GDDs are calculated as the average daily temperature minus 50.
GDD = Tmax + Tmin ÷ 2 - 50
If the maximum daily temperature (Tmax) is greater than 86 F, 86 is used to determine the daily average. Similarly, if the minimum daily temperature (Tmin) is less than 50 F, 50 is used to determine the daily average. The high cutoff temperature (86 F) is used because growth rates of corn do not increase above 86 F. Growth at the low temperature cutoff (50 F) is already near zero, so it does not continue to slow as temperatures drop further. GDDs are calculated daily and summed over time to define thermal time for a given period. The cumulative GDDs associated with different vegetative and reproductive stages are shown in Table 4.4.
Each corn hybrid requires a certain number of accumulated GDDs to reach maturity; Table 4.4 illustrates the example of a hybrid requiring 2,650 GDDs from planting to physiological maturity (Black Layer). Most seed corn dealers have information on specific hybrids. To monitor GDD accumulations during the growing season, the grower should follow the Crop Progress & Condition Report weekly for Ohio, provided by the U.S. Department of Agriculture National Agricultural Statistics Service (NASS), available online at nass.usda.gov/Statistics_by_ State/Ohio/Publications/Crop_Progress_&_Condition.
An open-source tool, Useful to Usable (U2U: mygeohub.org/groups/u2u/purdue_gdd), can also help monitor GDD accumulation. The tool helps to develop different scenarios; it provides county-level estimates based on historical GDDs accumulation, planting dates, relative hybrid maturities, GDDs to black layer, and historical freeze temperature dates (Spring and Fall). Be aware that U2U assumes the same GDD is needed to reach the black layer for the same hybrid, regardless of when it is planted. Under delayed planting situations, research has suggested that GDD requirements for maturity may be reduced, which is often referred to as “growing degree compression.”
As with any system, the GDD system has several shortcomings. GDD ratings of hybrids with similar daysto-maturity ratings do not always agree, especially if the hybrids are from different companies. Some seed companies start counting GDDs from the day of planting, while others begin from the day of emergence. When this occurs, similar maturity hybrids may vary by 100 to 150 GDDs—the average GDDs required for emergence. Some companies use entirely different mathematical methods to calculate GDD. Although most companies use the 86/50 cutoff method described above, others use different methods to calculate GDDs. Also, under certain delayed planting situations and stress conditions, GDD requirements for maturity may be reduced significantly. For more information on this, see the section on Date of Planting.
If interested in learning more about corn growing degree day and maturity ratings in corn, visit the ANR FactsheetAGF-101 (ohioline.osu.edu/factsheet/agf-101).
Yield Potential and Stability
Choose hybrids that have produced consistently high yields across several locations and/or years. The Ohio Corn Performance Tests (OCPT) indicate that hybrids of similar maturity vary in yield potential by as much as 40 bushels per acre or more. Choosing a hybrid because it possesses a particular trait, such as big ears, many kernel rows, deep kernels, prolificacy, or upright leaves, does not ensure high yields; instead, looking for stability in performance across environments.
Most of the hybrids marketed and planted in Ohio contain transgenic traits for herbicide and insect resistance. Planting herbicide-resistant hybrids allows growers to use herbicide formulations also used on soybeans. Hybrids with glyphosate and/or glufosinate ammonium resistance offer weed management options that generally involve fewer applications and the use of more environmentally benign chemicals. Insect-resistant hybrids contain a gene from bacteria that produces the insecticide known as Bacillus thuringiensis. Planting Bt corn hybrids may eliminate the need for soil insecticide treatments (rootworm) and postemergent insecticide applications (corn borer), which are less effective and potentially harmful to nontarget beneficial insects. See the section on Insect Control in this chapter for more on the use of Bt resistance to minimize crop losses.
A major concern of growers is whether the yield potential of hybrids with fewer transgenic traits or no transgenic traits is less than that of stacked trait hybrids with multiple genes for above- and below-ground insect resistance. One explanation for this concern is that some seed companies are no longer introducing nontransgenic versions of certain hybrids or are releasing non-transgenic versions some years after the original hybrid has been introduced. So when a new high-yielding hybrid is introduced, it is often available with only stacked traits. As a consequence, some might believe that to optimize yields with the newest genetics, it is necessary to plant stacked-trait corn hybrids with transgenic traits for above- and below-ground insect resistance. An assessment of corn hybrids in the OCPT without transgenic traits (non-GMO), with transgenic herbicide resistance only, with transgenic traits for above-ground insect resistance only, and with transgenic traits for above- and below-ground insect resistance indicated that non-transgenic hybrids are available that yield competitively with many transgenic corn hybrids in the absence of corn borer and rootworm pressure. Similarly, yields of hybrids with transgenic traits for above-ground insect resistance only were comparable to yields of hybrids with transgenic traits for above- and belowground insect resistance. As to whether different insect and herbicide traits and combinations thereof affect hybrid performance (in the absence of insect pressure), OCPT data suggested that no set of traits performed consistently much better or much worse than other sets of traits and the number of traits was not highly correlated with yield performance among these sets.
Several major seed companies have introduced corn hybrids that specifically target enhanced drought tolerance. To date, these drought-tolerant hybrids from DuPont Pioneer (AQUAmax) and Syngenta (Agrisure Artesian) contain native traits, and those from Bayer (DroughtGard) contain a transgenic trait. In field studies conducted by The Ohio State University from 2012 to 2014, droughttolerant hybrids from DuPont Pioneer and Syngenta were compared to conventional hybrids of similar relative maturity. Results suggested that in moderate- to loweryielding environments in Ohio (below 185 bushels per acre average yield), the drought-tolerant hybrids can produce greater yields than their conventional counterparts under the same management conditions, but the yield may not be greater when conventional hybrids yield more than 185 bushels per acre. Drought-tolerant hybrids may offer a yield advantage in production environments at greater risk to water deficit with moderate- to low-yield potential.
Some investigators have reported that corn hybrids with different genetic backgrounds vary in their response to plant population and nitrogen fertilizer, and many seed corn companies characterize hybrids based on their response to plant population and nitrogen fertilizer. However, most university research indicates that differential response among hybrids for nitrogen and population is often inconsistent, strongly influenced by environmental conditions, and not a practical consideration when making nitrogen and seeding rate recommendations. Nitrogen and plant population response for different hybrids has also been found to vary by site and weather conditions.
It is recommended to review the results of state, company, and county performance trials before choosing hybrids. Because weather conditions are unpredictable, the most reliable way to select superior hybrids is to consider performance during the last year and the previous year over a wide range of locations and climatic conditions. When using university performance trial results, two years of data from several locations is usually adequate; test summaries for three or more years may exclude new hybrids with better performance potential. Moreover, most hybrids are not evaluated in the OCPT beyond two years.
On-farm strip tests are not reliable in hybrid selection because they cannot predict hybrid performance across a range of environmental conditions. However, on-farm hybrid tests can be useful in evaluating various traits, such as lodging, greensnap, drydown, harvestability (ease of shelling, ear retention, etc.), disease resistance, and stay-green characteristics.
Ohio State conducts corn performance tests across Ohio. Results titled Ohio Corn Performance Test, are published each year and are available online at ohiocroptest.cfaes.osu.edu/corntrials. The presentation summarizes hybrid tests conducted each year at about 10 Ohio locations and includes yield information from the previous two years. The bulletin includes data on grain yield, grain moisture, and standability of hybrids along with other important agronomic variables. Annual records for past years of the Ohio Corn Performance Test are available online at u.osu.edu/perf/archive. This website also includes past results for annual Ohio organic corn tests and Ohio silage tests.
Stalk Quality and Lodging
Hybrids with low stalk quality should be avoided for grain production even if they show outstanding yield potential. Hybrid stalk quality, as measured by stalk lodging (stalk breakage below the ear) at harvest, has improved greatly over the past 20 years. Nevertheless, this trait is particularly important in areas where stalk rots are perennial problems, or where field drying is anticipated—i.e., conditions that often lead to lodging. If growers have their drying facilities and are prepared to harvest at relatively high moisture levels (above 25%) or are producing corn for silage, then standability and fast drydown rates are less critical selection criteria.
Traits associated with improved hybrid standability include resistance to stalk rot and leaf blights, genetic stalk strength (a thick stalk rind), short plant height and ear placement, and high stay-green potential. Resistance to European corn borer conferred by the Bt trait can also enhance stalk quality by limiting entry points in plant tissue through which fungal pathogens can invade the plant. However, the Bt trait will do little to minimize stalk rot and lodging in a hybrid characterized by below average stalk quality.
Another stalk-related problem, greensnap or brittle snap, has started to appear in recent years. Corn plants are more prone to snapping during the rapid elongation stage of growth when severe windstorms occur. According to studies in Iowa, Minnesota, and Nebraska, the V5 to V8 stages (corn approximately 10 to 24 inches in height) and the V12 stage through tasseling are the most vulnerable stages. Vulnerability to greensnap damage varies among hybrids. However, all hybrids are at risk from wind injury when they grow rapidly prior to tasseling. The use of growth regulator herbicides, such as 2,4–D or Banvel, has also been associated with stalk brittleness, especially if the application is late or if the application is made during hot, humid conditions. Once tassels begin shedding pollen, greensnap problems generally disappear.
Disease Resistance and Tolerance
Hybrids should be selected for resistance or tolerance to stalk rots, foliar diseases, and ear rots, particularly those that have occurred locally. Seed dealers should provide information on hybrid reactions to specific diseases in Ohio. See the section on Disease Management for more on the use of hybrid resistance and tolerance to minimize crop losses.
Hybrid response to a high population can be limited by stalk lodging, which often increases at higher plant density. Some hybrids that have shown positive yield responses to higher populations cannot be grown at high plant densities because of the increased risk of lodging at harvest. Lodging reduces yields and slows the harvest operation. Therefore, it is essential that hybrids planted at high seeding rates possess superior stalk quality for standability. Hybrids should also have resistance—or the best levels of tolerance available—to fungal leaf diseases (such as gray leaf spot and northern corn leaf blight), which contribute to stalk lodging problems and stalk rots (such as Anthracnose and Gibberella).
Grain Quality
The composition of corn grain is a major factor affecting grain feeding value. Although the grain market does not include this factor in price determination, growers who feed livestock may use this information to reduce feed costs and optimize diets. Hybrid genetics can significantly affect the protein, oil, and starch content of corn grain. For feed, protein content is of primary interest, whereas for processing uses, oil content is of interest. Corn grain is typically 8% protein, 3.6% oil, and 66% starch (on a 15% moisture basis).
Although significant differences among hybrids for oil and protein are evident under certain testing conditions (2001–2009), the Ohio Corn Performance Test has indicated that protein, oil, and starch levels vary considerably from test to test, between years and regions (Table 4.8). To this point, hybrids, management, and climate have changed over the years. Updated research on protein, oil, and starch for newer hybrids would be important in upcoming years. Some normal dent corn hybrids produced primarily for grain exhibit elevated protein and oil levels. Environmental conditions (e.g., temperatures, rainfall) and cultural practices (e.g., nitrogen fertility, plant populations) can influence grain composition, especially grain protein. Additional information on hybrids developed for special grain composition characteristics is in the Specialty Corns section in this chapter.
Another grain quality consideration that has become increasingly important in Ohio is contamination with mycotoxins. Unlike grain protein and oil composition, mycotoxin contamination may reduce the market value and challenge the utilization of the crop both for grain and silage.
Date of Planting
The recommended time for planting corn in northern Ohio is April 15 to May 10, and in southern Ohio, April 10 to May 10. Approximately 100 to 150 GDDs (heat units) are required for corn to emerge. In central Ohio, this number of GDDs usually accumulates by the last week of April or the first week in May. Improved seed vigor and seed treatments allow corn seed to survive up to three weeks before emerging if soil conditions are not excessively wet. An early morning soil temperature of 50 F at the 1/2- to 2-inch depth usually indicates that the soil is warm enough for planting. Corn germinates very slowly at soil temperatures below 50 F. Short-term weather forecasts should be monitored to make the best decision on early planting. After April 25, planting when soil moisture conditions allow is usually safe. The latest practical date to plant corn ranges from about June 15 in northern Ohio to July 1 in southern Ohio. Plantings after these dates usually yield no more than 50% of normal yields.
Planting should begin before the optimum date if soil conditions will allow the preparation of a good seedbed. Growers should have the equipment capability to plant more than half of their corn acres prior to the optimum planting date; this should allow planting all the corn acres prior to the calendar date when corn yields begin to quickly decline. Ohio corn producers usually cannot perform field operations during all days of their optimum planting date range due to spring rains and cool weather conditions that limit soil drying. On average, during the optimal corn planting time in Ohio, only one out of three days is available for effective fieldwork.
Table 4.9 shows the effect of planting dates in the Columbus region. Yields decline approximately 1 to 1.5 bushels per day for planting delayed beyond the first week of May. Grain yield and test weight were increased by early plantings, whereas grain moisture was reduced, thereby allowing earlier harvest and reducing drying costs. Early planting generally produces shorter plants with better standability. Delayed planting increases the risk of frost damage to corn and may subject the crop to greater injury from various late insect and disease pest problems, such as European corn borer and gray leaf spot.
In one out of four years, excessive rainfall in April and May forces farmers in Ohio to plant or replant up to half of their corn acreage as late as early to mid-June. Since 2005, evaluations of corn yield response to early and late planting dates (late April to mid-May versus early- to mid-June planting dates) at Ohio State research farms in northwest (NW), northeast (WO), and southwest Ohio (SC) indicate that planting date effects on yield vary considerably across years and locations (Figure 4.1). The change in yield associated with the late planting dates ranged from -43% to +38%. Averaged across site years, yields decreased by about 11%. For five of the 14 site years, yields of the later plantings were greater than or comparable to the early plantings, which can be related to stressful early-season growing conditions (excessively cold and wet) and unusually favorable late-season growing conditions. The higher yields associated with June plantings occurred at the northern locations.
Studies have also been performed to determine if various management practices need to be adjusted to optimize yield when planting corn in early to mid-June. These studies indicate that Ohio producers generally do not need to modify plant populations for late plantings based solely on hybrid maturity. There are differences in yield response among hybrids for early to mid-May and June planting dates, but these differences were not strongly related to hybrid maturity. Results suggested that in some Ohio environments, plant populations should be reduced regardless of relative maturity to optimize yield. Although planting in early to mid-June usually results in lower yields, optimum nitrogen rates for late-planted corn were not consistently lower than early-planted corn. Significantly reducing nitrogen recommendations for late-planted corn may place producers at risk of yield loss under certain environmental conditions.
Corn should be planted only when soils are dry enough to support traffic without causing soil compaction. The yield reductions resulting from "mudding the seed in" may be much greater than those resulting from a slight planting delay. No-till corn can be planted at the same time as conventional if soil conditions permit. In reality, however, planting may need to be delayed several days to permit extra soil drying. Planting a full-season hybrid first and then alternately planting early-season and midseason hybrids allows the grower to take full advantage of maturity ranges and gives the late-season hybrids the benefit of maximum heat unit accumulation. When compared with short- to midseason hybrids, full-season hybrids generally show greater yield reduction when planting is delayed. Planting early hybrids first, followed by midseason, and finally the full-season hybrids will spread the pollination interval for all the corn acres over a longer period and may be a good strategy for some drought-prone areas with longer growing seasons.
Planting hybrids of different maturities reduces damage from diseases and environmental stress at different growth stages (improving the odds of successful pollination) and spreads out harvest time and workload. Consider spreading hybrid maturity selections between early-, mid-, and full-season hybrids―for example, a 25–50–25 maturity planting, with 25% in early to midseason, 50% in mid- to full-season, and 25% in full-season. Planting a range of hybrid maturities is one of the simplest and most effective ways to diversify and broaden hybrid genetic backgrounds.
When corn planting is delayed past the optimum dates or if a crop needs to be replanted, it may be necessary to switch hybrid maturities. In most delayed planting situations, however, full-season hybrids still perform satisfactorily and reach physiological maturity (black layer formation) when planted as late as the last week of May. Hybrids planted in late May or early June mature at a faster thermal rate (requiring fewer heat units) than the same hybrid planted in late April or early May.
Ohio and Indiana research has indicated that the required GDD units from planting to kernel black layer decreases with delayed planting. For each day that planting was delayed after May 1, the reduction in GDD requirement was about 6.5 GDDs. A hybrid rated at 2,800 GDDs with normal planting dates (such as late April or early May) may require only 2,605 GDDs when planted on May 30. Therefore, a 30-day delay in planting may result in a hybrid maturing in 195 fewer GDDs (30 days multiplied by 6.5 GDDs per day). However, more recently, during the severely delayed 2019 growing season, the GDD compression was neither evident nor observed in experimental data in Michigan and Indiana. The GDD compression was not apparent during the 2020 season in Ohio.
Useful to Usable (U2U) is a phenology prediction tool that can assist as part of the hybrid selection and planting date decisions. This U2U tool can make countylevel estimations across the Midwest, including Ohio. Estimates are based on current and historical GDDs, planting dates, relative hybrid maturities, GDDs to black layer, and freeze temperature values. You can use U2U to test different scenarios and inform your decisions, specific to your location and conditions.
For using the U2U tool, five basic steps are needed:
1. Access the U2U online at mygeohub.org/groups/u2u/purdue_gdd.
2. Select your location, and zoom in or out as needed on the map. Search by ZIP/City/County can be used.
3. Select the start date for GDD. As a proxy, the planting date can be used here.
4. Select your corn hybrid maturity. For example, 108 days or 114 days.
5. Observe the projections. Ensure all boxes are checked in the upper-left corner of the window.
The U2U projection will include the GDD line, average GDD from 1981 to 2010, last freeze dates in the spring, first freeze dates in the fall, expected silking dates, and black layer. The outcomes can be seen using a predetermined location, date, and hybrid's relative maturity. You can repeat the exercise as many times as needed to evaluate other potential scenarios.
Other factors concerning hybrid maturity need to be considered when planting is delayed. For plantings in late May or later, the drydown characteristics of hybrids should be considered. Although a full-season hybrid may still have some yield advantage over shorter-season hybrids planted in late May, it could have significantly higher grain moisture at maturity than earlier maturing hybrids, there will be less calendar time for field drying, and drying costs will be higher. Later planting dates generally increase the possibility of damage from European corn borer and western bean cutworm and warrant the selection of Bt hybrids that control these lepidopteran pests if suitable maturities are available.
Seeding Depth
The appropriate planting depth varies with soil and weather conditions. For normal conditions, plant corn 1.5 to 2 inches deep to ensure adequate moisture uptake and seed-soil contact, provide frost protection, and allow for adequate root development. Shallower planting often results in poor root development and should be avoided in all tillage systems. In April, when the soil is usually moist and the evaporation rate is low, seed should be planted shallower―no deeper than 1.5 inches. As the season progresses and evaporation rates increase, deeper planting may be advisable. When soils are warm and dry, corn may be seeded more deeply―up to 2 inches on non-crusting soils.
When corn is planted 1.5 to 2 inches deep, the nodal roots develop about 1/2 to 3/4 inch below the soil surface. However, at planting depths less than 1 inch, the nodal roots develop at or just below the soil surface. Excessively shallow planting can cause slow, uneven emergence due to soil moisture variation, and rootless corn (“floppy corn syndrome”) when hot, dry weather inhibits nodal root development. Shallow plantings can increase stress and result in less developed roots, smaller stalk diameters, smaller ears, and reduced yields.
Some corn growers plant at depths less than 1.5 inches. The rationale for this shallow planting is that seeds will emerge more rapidly due to warmer soil temperatures closer to the surface. This is an important consideration as corn growers across the Corn Belt are planting earlier so they can complete planting before yield potential begins to decrease after the first week of May. Particularly in soils that crust, speed of emergence is critical to establish plant stands before heavy rainfalls “seal” the soil surface. In recent OSU research evaluating varying planting depths, grain yields were about 14% greater for the 1.5-inch and 3-inch planting depths than the half-inch planting depth in 2011, and 40 percent greater in 2012. The lower yield of the shallow 1/2-inch planting was associated with final stands that were 7,000 to 12,000 plants per acre less than those of the other two planting depths in 2011 and 2012.
More recently (2017 to 2019), field studies were conducted in Ohio to determine planting depth and soil type effects on corn emergence, growth, and grain yield. In a Strawn–Crosby soil type, yields were not different between planting depths while in a Kokomo soil type, yields increased by 8% or 10% when planting at 2-inch and 3-inch depths, respectively (relative to 1 inch). With these results, it was concluded that planting depth adjustments can enhance the performance of corn and that grain yield responses differ depending on soil types.
Row Width
Since the early 1970s, average row spacing in Ohio decreased from about 35 inches to about 30 inches in 2015. This reduction in row spacing coincided with an increase in average plant population from approximately 18,000 plants per acre to nearly 30,000 plants per acre. Due to considerable interest in narrowing row spacing even further, many university and seed company studies have compared corn planted in narrow rows (row spacing 22 inches or less) and conventional 30-inch row spacing.
Although narrow-row systems are often perceived as a proven method for increasing yield and profitability, studies on narrow-row corn production have produced mixed results. Some of the inconsistency may be related to latitude, with narrow rows in the North Central Region of the United States exhibiting the largest yield increases (2%–3% or more) over 30-inch rows. This advantage diminishes moving southward with little or no yield advantages for narrow rows in the central Corn Belt. Results of a Michigan State University study conducted in 1998–99 showed that corn grain yields increased by 2% and 4% when row width was narrowed from 30 inches to 22 inches and 15 inches, respectively. However, in university research in central Corn Belt states (Iowa, Illinois, and Ohio), the yield advantage of narrow rows over 30-inch row spacings has been smaller (usually less than 2%) and less consistent. When they occur, yield increases with narrow rows have been found to occur at both moderate and high plant populations and at high and moderate yield levels. University of Illinois research found no trend for higher or lower-yielding sites to show more response to narrow rows. Hybrids varying in maturity and plant architecture have generally exhibited yield responses to narrow rows similar to those for 30-inch row spacing. Some companies have marketed hybrids for high populations and narrow rows, but university trials have not shown that these hybrids have an advantage over high-yielding hybrids in 30-inch rows.
Some growers are considering twin rows as another row spacing configuration that may offer some of the yield increases associated with narrow-row corn. In the typical twin-row system, two rows are placed 6 to 8 inches apart on 30-inch centers, although other twin-row configurations are used. Twin rows make it possible to create narrow rows without changing the row configuration of other equipment and to avoid costs associated with equipment conversion to a narrow-row system. Staying on 30-inch centers allows growers to use the same corn header and tractor tire spacing used in 30-inch corn production. In recent university studies, results have generally indicated little or no advantage for a twin-row system compared to 30-inch row spacings.
Narrowing row spacing below 30 inches has usually proven advantageous in silage corn production. Studies at Pennsylvania State University indicate a 10% advantage for silage production using 15-inch or 20-inch rows compared to 30-inch rows.
Potential yield gains from narrow rows must be balanced against the investment for new equipment and higher input costs associated with narrowing row spacing. Key changes for narrowing rows include tractor and combine rims and tires, combine heads, and planter modifications. Greater interest in increasing equipment use efficiency by using the same planter or drill for soybean, sugar beet, and corn may warrant the adoption of narrow-row systems for corn. Producers in northern regions who also grow soybeans and sugar beets in 22-inch rows often find it more efficient to use this same row spacing for corn.
Plant Populations and Seeding Rates When corn is produced for grain in Ohio, recommended plant populations at harvest (or final stand) can range from 24,000 to 34,000-plus plants per acre, depending on the hybrid and production environment. Yield response to plant population is influenced by several factors, including environmental conditions, the hybrid, and the end use of the corn crop. To account for the effects of the production environment, plant population adjustments should be made on a field-by-field basis using the average yield potential of a site over a three- to five-year period as a key criterion for determining the appropriate plant population. When determining the realistic yield potential for a site over a five-year period, it may be appropriate to ignore the highest and lowest yields, which may have occurred during years that were unusually favorable or unfavorable for corn performance.
Hybrids differ in their response to plant population, with some exhibiting stalk lodging at the upper end of the plant population range. Seed companies specify a range in final stands for the various corn hybrids they market. Because of differences in genetic backgrounds for various traits, especially stalk quality, these seed company recommendations should be considered when adjusting seeding rates for specific hybrids.
Based on Ohio State studies, a population of 31,000 to 32,000 plants per acre will optimize yields in most Ohio production environments. For fields with low yield potential, final stands of 24,000 to 26,000 plants per acre will probably be sufficient. For fields with very productive soils and exceptionally high yield potential, final stands greater than 34,000 plants per acre may be necessary. Seeding rates can be cut to lower seed costs, but this approach typically costs more than it saves. In the absence of major environmental stresses, most research suggests that planting a hybrid at suboptimal seeding rates is more likely to cause yield loss than planting above-recommended rates (unless lodging becomes more severe at higher population levels). Relative to plant populations, seeding rates have to be higher to account for germination losses. Under normal conditions, hybrids usually have a 95% germination rate.
Plant populations recommended for corn silage are greater than those for grain. According to recent Pennsylvania State research, optimum plant populations for silage are about 2,000 to 4,000 plants per acre greater for silage than for grain. Higher plant populations can increase silage yields but may reduce forage energy content.
If a grower plans to rely extensively on field drying that can delay harvest, there may be little benefit from using high plant populations much above 30,000 plants per acre. An Ohio State study evaluated the effects of plant population (24,000 to 42,000 plants per acre) and harvest dates (early/mid-October, November, and December) on the agronomic performance of four hybrids differing in maturity and stalk quality (Table 4.10). Although the hybrids exhibited similar yield potential when harvested early (early/mid-October), differences in yield became evident with harvest delays, which could be attributed to differences in stalk quality. Yield differences among plant populations were generally small on the first harvest date, but with harvest delays, major yield losses occurred at the higher plant populations, especially 42,000 plants per acre, due to increased stalk lodging. Grain moisture averaged about 24% on the first harvest date, 18% on the second harvest, and 17.5% on the third harvest date. After the first harvest in early/mid-October, stalk lodging increased to as much as 80% for certain hybrids at high plant populations, resulting in yield losses of nearly 50% by mid-December.
Results from trials conducted at Ohio State and other universities indicate that higher seeding rates do not necessarily require higher nitrogen rates. In Ohio State research, two different cropping rotations (corn after soybeans and corn after corn) and two seeding rates (30,000 and 40,000 seeds per acre) were evaluated across a range of nitrogen rates. In five out of eight site-years, the seeding rate had no impact on fertilizer nitrogen response (the optimum nitrogen rate was similar regardless of the seeding rate). When there were differences in optimum nitrogen rates, it was not because the higher seeding rate required more nitrogen. Only one out of the eight site-years (for corn after corn) revealed that the higher seeding rate required more nitrogen.
Final stands are always less than the number of seeds planted per acre. Cold, wet soil conditions, insects, diseases, cultivation, and other adversities will reduce germination and emergence. Generally, you can expect up to 5%–10% fewer plants at harvest than seeds planted. To compensate for these losses, you need to plant more seeds than the desired population at harvest.
To calculate your planting rate, consider the following formula:
Planting Rate = Desired Population per Acre ÷ (Germination x Expected Survival)
Germination is the percent seed germination shown on the seed tag (converted to decimal form). Expected survival is the percent of seedlings and plants that you expect to reach harvest maturity under normal conditions (converted to decimal form). If you are planting very early when the soil will likely remain cool for several days following planting, you may want to increase the seeding rate by 5%. A similar approach should be followed when planting no-till, especially in heavy corn residues.
Example:
Target stand at harvest—30,000 plants per acre
Seed tag indicates 95% seed germination
Assume 97% survival (3% plant mortality)
Planting rate = 30,000 / (0.95 x 0.97) = 32,556 seeds per acre
According to the formula, you should consider a planting rate of approximately 32,600 seeds per acre to achieve the desired final stand of 30,000 plants per acre.
Uneven plant spacing and uneven emergence reduce yield potential. The impact of uneven emergence is usually greater than that of uneven spacing. Seed should be spaced as uniformly as possible within the row to ensure maximum yields and optimal crop performance, regardless of plant population and planting date. Corn plants next to a gap in the row may produce a larger ear or additional ears (if the hybrid has a prolific tendency), compensating for missing plants. These plants, however, cannot make up for plants spaced so closely together in a row that they compete for sunlight, water, and nutrients. Crowding, especially with uneven emergence, can result in barren plants or ears too small to be harvested (nubbins), as well as stalk lodging and ear disease problems. Although uniformity of stand cannot be measured easily, studies have indicated that reduced plant stands will yield better if plants are spaced uniformly than if there are large gaps in the row. As a general guideline, yields are reduced an additional 5% if there are gaps of 4 to 6 feet in the row and an additional 2% for gaps of 1 to 3 feet. Studies at Purdue University suggest that corn growers could improve grain yield from 4 to 12 bushels per acre if within-row spacing were improved to the best possible uniformity (depending on the unevenness of the initial spacing variability).
The most effective way to improve planter accuracy is to keep planting speed within the range specified in the planter’s manual. The following are additional considerations for improving seed placement uniformity:
• Match the seed grade with the planter plate.
• Check planters with finger pickups for wear on the back plate and brush (use a feeler gauge to check tension on the fingers, then tighten them correctly).
• Check for wear on double-disc openers and seed tubes.
• Make sure the sprocket settings on the planter transmission are correct.
• Check for worn chains, stiff chain links, and improper tire pressure.
• Make sure seed drop tubes are clean and clear of any obstructions.
• Clean seed tube sensors if a planter monitor is being used.
• Make sure coulters and disc openers are aligned.
• Match the air pressure to the weight of the seed being planted.
Uneven emergence affects crop performance because competition from larger, early-emerging plants decreases the yield from smaller, later-emerging plants. The primary causes of delayed seedling emergence in corn include shallow planting depths, poor seed-to-soil contact resulting from cloddy soils, the inability of no-till coulters to slice cleanly through surface residues, worn disc openers, and misadjustment on the closing wheels. Other causes include soil moisture and temperature variability within the seed depth zone, soil crusting prior to emergence, the occurrence of certain types of herbicide injury, and variable insect and/or soil-borne disease pressure.
Based on research at the University of Illinois and the University of Wisconsin, if the delay in emergence is less than two weeks, replanting increases yields less than 5%, regardless of the pattern of unevenness. However, if one-half or more of the plants in the stand emerge three weeks late or later, then replanting may increase yields by up to 10%. To decide whether to replant in this situation, growers should compare the expected economic return of the increased yield with both their replanting costs and the risk of emergence problems with the replanted stand.
Use Tables 4.11 and 4.12 to determine the number of kernels dropped or the plant population per acre.
Table 4.12 provides an example of a single 30-inch row. For twin rows, measure from the center of the twin rows to the center of the next set of twin rows to determine the effective row width. Count the plants in both of the twin rows on each side of that center. For example, if twin rows are planted 6 inches apart every 30 inches, the effective row spacing is 30 inches (There are rows 3 inches to each side of that 30-inch center). You need 17 feet, 5 inches of row in 30-inch rows. Measure off 17 feet, 5 inches of row, and count the plants in both of the twin rows that are on each side of the 30-inch center, and multiply the total count that you get from both twin rows by 1000 to get the total number of plants per acre.
Making Replant Decisions
Although it is not unusual that 5%–10% of planted seeds fail to establish healthy plants, additional stand losses resulting from insects, frost, hail, flooding, or poor seedbed conditions may call for a decision on whether to replant a field. The first rule in such a case is not to make a hasty decision. Corn plants can and often do outgrow leaf damage, especially when the growing point is protected beneath or at the soil surface (up until about the six-leaf collar stage). If new leaf growth appears within a few days after the injury, then the plant is likely to survive and produce normal yields.
When deciding whether to replant a field, assemble the following information: original planting date and plant stand, earliest possible replanting date and plant stand, and cost of seed and pest control for replanting. If the plant stand was not counted before damage occurred— providing that conditions for emergence were normal— estimate population by reducing the dropped seed rate by 10%. To estimate stand after injury, count the number of living plants in 1/1,000 of an acre (Table 4.12). Take counts as needed to get a good average―one count for every 2 to 3 acres.
Table 4.13 shows the effects of planting date and plant population on final grain yield. Grain yields for varying dates and populations are expressed as a percentage of the yield obtained at the optimum planting date and population When the necessary information on stands, planting, and replanting dates has been assembled, use Table 4.13 to locate the expected yield of the reduced plant stand by reading across from the original planting date to the plant stand after injury. Then locate the expected replant yield by reading across from the expected replanting date to the stand that would be replanted. The difference between these numbers is the percentage yield increase (or decrease) to be expected from replanting.
Here is an example of how Table 4.13 might be used to arrive at a replant decision. If we assume that a farmer planted on May 9 at a seeding rate sufficient to attain a harvest population of 30,000 plants per acre. The farmer determined on May 28 that his May 9 planted stand was reduced to 15,000 plants per acre as a result of saturated soil conditions and ponding. According to Table 4.13, the expected yield for the existing stand would be 79% of the optimum. If the corn crop were replanted the next day on May 29 and produced a full stand of 30,000 plants per acre, the expected yield would be 81% of the optimum. The difference expected from replanting is 81 minus 79 or 2 percentage points. At a yield level of 150 bushels per acre, this increase would amount to 3 bushels per acre, which would probably not justify replanting costs.
Keep in mind that replanting itself does not guarantee the expected harvest population. Corn replant decisions early in the growing season can be based mainly on plant stand and plant distribution. Later in the season as yields begin to decline rapidly because of delayed planting, calendar date assumes increased importance.