Crop Management

R.W. Heiniger, J.F. Spears, D.T. Bowman, E.J. Dunphy, D.T. Bowman, Crop Science Department

Hybrid Selection

Hybrid selection is a critical component of any profitable corn production system. Skillful hybrid selection requires that growers:

• Understand the field environment.
• Know how a corn plant grows and develops.
• Collect and properly evaluate information describing the characteristics of hybrids available in their area.

Understanding Corn Hybrids

Corn hybrids fall into three categories, single crosses, three-way crosses and double-crosses. Single crosses, derived from the crossing of unrelated inbred lines, are characterized by uniformity of plant height, ear height and yield. This uniformity improves machine harvesting and is most advantageous when growing conditions are favorable. The three-way cross is slightly less uniform than the single cross because it is created by hybridizing a single cross seed parent with an unrelated pollen parent inbred. The double-cross is developed from four unrelated inbreds. Accordingly, it is less uniform in plant height, ear height, silking date, etc. than the single or three-way cross. In stress situations, this lack of uniformity is an advantage and it is the reason why double- crosses yield more consistently than single- crosses in a wide range of environments. Performance of the best single and three-way crosses will generally exceed that of the best double-crosses.

Genetically-Engineered Hybrids

Genetic engineering is a powerful, new hybrid development tool that is complementing traditional plant breeding. Genetic engineering is currently being used to introduce traits that confer insect, herbicide, or disease resistance to existing hybrids. Therefore, the basic yield potential of the hybrid is unchanged. Low yielding varieties before genetic engineering will be low yielding after genetic engineering. Corn growers should not select a hybrid based solely on the fact that it is genetically engineered. Instead, selection of a genetically-engineered hybrid should depend on whether the resistant traits that hybrid has are needed in the corn cropping system.

Hybrid Characteristics

For most growers selecting corn hybrids, yield is the primary consideration. However, successful corn producers choose hybrids on the basis of agronomic characteristics that complement their specific farm environment. For example, medium maturity, standability, seedling vigor and leaf disease tolerance are critical on farms comprised of organic soils. Early to medium maturity, standability, grain quality and drought tolerance are desirable hybrid characteristics for coastal plain peanut and tobacco growers. Livestock producers value good grain quality, while many piedmont and mountain bottomland silage growers demand gray leaf spot and viral disease tolerance. Most corn seed companies now routinely distribute information that allows growers to carefully scrutinize the relative agronomic attributes of hybrids they sell.

Hybrid Maturity

Corn producers should plant several hybrids differing in maturity. A 300-acre corn producer, for example, should plant a minimum of three hybrids. Planting hybrids that differ in maturity increases the odds that corn will be combined at optimum grain moisture levels throughout the harvest season and minimizes risk caused by the adverse effects of short-term water deficits and high temperatures. In North Carolina, the highest corn yields are produced by medium-season hybrids with a relative maturity of 110 to 118 days. Corn farmers with highly productive soils with good water-holding capacities should choose hybrids in the 112 to 120 day maturity range, while corn farmers with droughty soils should choose hybrids in the 108 to 115 day maturity range.

Maturity Rating Systems. Since it is important that growers know the maturity of hybrids they plant, several systems have been used to describe the "relative maturity" (RM) of corn. The "Minnesota System" is widely used by most seed companies to describe corn RM. Table 2-1 shows the RM of several corn hybrids commonly grown in North Carolina. Unfortunately, the actual days to maturity of corn hybrids will change based on when the hybrid is planted and the seasonal weather.

Growing Degree Days. The most precise way to determine if hybrids actually differ in days to tasseling and silking or days to physiological maturity is to examine growing degree day (GDD) data supplied in the sales brochures of seed companies (Table 2-1). Growing degree days are calculated every 24 hours using the formula, GDD = [(Tmax + Tmin) / 2] - 50 where Tmax equals maximum temperature during the day and Tmin is the minimum temperature encountered during the day. Fifty degrees is substituted for the minimum temperature when temperature falls below 50 degrees. Eighty-six degrees is substituted for the maximum if maximum temperatures exceed 86 degrees. Those numbers are substituted in the calculation because corn grows very little below 50 degrees and growth slows markedly above 86 degrees. Tables 2-2 and 2-3 show the number of GDDs commonly recorded in central North Carolina. However, the actual number of GDDs experienced for any given period will vary from year to year due to changes in temperature. If a grower wants to select hybrids that minimize the crop's susceptibility to drought, hybrids should be chosen that, on average, will accumulate enough GDDs to start silking before June 20. Growers should also choose hybrids that vary by at least 100 GDDs in days to mid-silk. Growers can spread harvest by selecting a group of "companion " hybrids with a range of 300 to 400 GDDs to physiological maturity.

Corn Hybrids for Silage.

High quality corn silage consists of a grain component that supplies 80% of the energy and 90% of the protein on a per-pound basis, and a stover component that supplies fiber and important amino acids such a carotene. The ideal corn hybrid for silage combines maximum grain production with a highly digestible, high-tonnage stover. A good hybrid for corn silage has the following characteristics: medium to tall stature, excellent grain yield, superior brittle stalk resistance, and excellent standability. Growers who use corn for silage should select hybrids with medium to late maturity. These hybrids tend to grow longer and have more tonnage. While tropical hybrids which have the capability for prolific growth may appear to be ideally suited for silage, keep in mind that much of the forage quality (energy and protein) come from the grain component. Most seed companies select materials that combine good grain yield with maximum tonnage. The North Carolina Official Variety Testing program measures silage yield performance and these results are available from your county Cooperative Extension office.

Table 2-1. Relative Maturity, GDD requirements, and other plant characteristics for several popular hybrids grown in North Carolina.

The Hybrid Selection Process.

Corn growers are confronted with the difficult task of selecting three to four hybrids from approximately 100 varieties offered by more than 20 companies. Significant effort is required to sort through the confusing array of numbers and yield. To simplify hybrid selection, the process may be divided into eight steps that generate intelligent hybrid selections and increased corn profits.

Step 1) Analyze the role of corn on the farm. Pinpoint special corn hybrid traits needed in an operation. Are fields plagued with viral diseases or gray leaf spot? Do billbug problems and no-till demand that hybrids grow off quickly? Does the presence of swine or poultry make long-term storage and grain quality a priority? Priorities should be written down!

Step 2) Ask for product information describing the agronomic traits (yield, standability, maturity, disease ratings, grain quality, seedling vigor, etc.) of hybrids grown in local tests (see Table 2-1 above). Attend meetings where seed suppliers describe the strengths and weaknesses of their products. Notes should be taken at those meetings. Special attention should be paid to the hybrid characteristics that were listed as priorities in Step 1.
Step 3) Obtain hybrid data from independent sources. Call your Cooperative Extension agent and ask for N.C. State University's statewide corn variety testing results. Those data are found at the Department of Crop Science Home Page at http://www.cropsci.nscu.edu under the commodity heading. Growers should also ask for the results of any county tests involving hybrid comparisons, as well as, a summary of state and local yield contests (http://plymouth.ces.nc.us/cropsci/cyc) and make a list of those hybrids that consistently rank among the best performers.

Step 4) Discuss corn hybrid performance with seed dealers, grain buyers and fellow farmers. Everyone has strong opinions about corn hybrids. As they encounter tales of super hybrid performance and failure, growers should not give great credence to individual comments. However, when several experienced corn people observe that, "Every stalk was standing!" or "It grew off extremely fast!", astute growers can gain much insight into hybrid performance.

Step 5) Make a list of hybrids that appear repeatedly at the top of yield comparisons and yield contest summaries. Growers should strive to identify hybrids that perform well under diverse management regimes on numerous farms. Good hybrids will not always win yield contests and comparisons but they will appear among the leading entries. Hybrid performance may vary with seasonal conditions and soil types. Growers should be wary of newly introduced hybrids that may perform atypically in their first year of availability.

Step 6) Eliminate from the list those hybrids that do not meet the special requirements of your operation. Remember the special farm needs written down in Step 1! Corn producers should use notes taken at company-sponsored meetings, any hybrid literature obtained (Step 2) and information gathered locally (Step 4) to cull hybrids from the list that are not suited to their style of corn farming.

Step 7) Choose from the hybrids remaining on your "short list". Corn producers should review their information and notes to ensure that they have selected hybrid maturities that adequately spread their harvest (Table 2-1). If there are two promising hybrids of equal yield potential, their performance can be compared with a formula traditionally used by plant breeders. The formula,
Yield + [% standing plants - (5 x % grain moisture)]
can identify hybrids with the most profit potential because it emphasizes standability and low grain moisture as well as yield.

Step 8) Conduct your own hybrid comparisons. Each year, corn producers should select 4 to 6 promising hybrids and evaluate them on their farm 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 uniformtest fields with minimal soil variability and restrict comparisons to hybrids of the same maturity. On-farm hybrid evaluations are simple for growers who use combine-mounted GPS receivers and yield monitors to create GIS yield maps. Corn producers with yield monitors willfind it easy to compare several hybrids at multiple farm sites.

Table 2-2. Average monthly temperatures and daily GDD for Raleigh, NC.

Month	Daily Maximum	Daily Minimum	Daily Average Temperature	Daily GDD*
March	63	38	56	6
April	74	48	62	12
May	81	58	70	20
June	86	64	75	25
July	88	67	76	26
August	88	67	76	26
September	83	60	72	22

* Base temperatures of 50 and 86^oF

Table 2-3. Average Accumulative, Weekly and Daily Growing Degree Days for Raleigh, NC. (Long-Term Averages).

Week Ending	Accumulative GDD	Weekly GDD	Daily GDD
March
8	39	39	6
15	82	43	6
22	118	36	5
29	174	56	8
April
5	240	66	9
12	311	71	10
19	389	78	11
26	497	108	15
May
3	604	107	15
10	718	114	16
17	839	121	17
24	976	137	20
31	1113	137	20
June
7	1255	142	20
14	1420	165	24
21	1579	159	23
28	1757	178	25
July
5	1936	179	25
12	2118	177	25
19	2298	185	26
26	2489	191	27
August
2	2678	189	27
9	2863	185	26
16	3046	183	26
23	3227	181	26
30	3399	172	25
September
6	3572	173	25
13	3725	153	22
20	3868	143	20
27	3997	129	18
October
4	4114	117	17
11	4217	103	15
18	4314	97	14
25	4395	81	12

High Oil Corn Production

High oil corn is a special type of corn that has higher percent oil content than regular #2 yellow corn. Typically #2 yellow corn has from 3.5 to 4.0% oil. Ideally, high oil corn should contain 7.0 to 8.0% oil. In samples tested in 1996-1998, the oil content of high oil corn hybrids ranged from 4.6 to 8.1% compared to normal dent corn that had oil contents ranging from 2.7 to 4.0%. In addition to the higher oil content, high oil corn kernels usually have a slightly higher percent protein and, even more importantly, higher amounts of amino acids such as lysine, threonine, and tryptophan that are important in the diets of poultry and swine.

The primary advantage of high oil corn is to the livestock producer, particularly poultry, dairy, and swine producers. For livestock producers who are not using added fat, high oil corn increases daily weight gains by as much as 10%. As a result, feeders do not have to pay for expensive fat or amino acid additives. They save money and maintain or increase the productivity of their operation. For corn farmers the benefits comes from having satisfied customers who are willing to share some of the increased profits with them in the form of premiums paid for high oil corn. Currently, livestock feeders are paying form $0.15 to $0.30 more for high oil corn.

The disadvantages of high oil corn come from the potential for lower corn yields, increased kernel damage, and the potential for increases in insect damage. Because almost 10% of the seed in the bag is the unproductive male pollinator, yield decreases of up to 10% are possible. Furthermore, in a year when drought stress occurs during pollination, the smaller amount of pollen available and the mismatch in the timing of silk emergence and pollen shed could reduce yields even more. Companies producing high oil hybrids recommend increasing seeding rates to obtain higher plant populations that make up for the loss of productive plants. This may not be feasible on droughty soils. The Dupont pollinators are designed to compensate for drought conditions by shedding pollen over a longer period (2 weeks vs 7-10 days), and by producing more pollen. Table 2-4 shows the yield and oil content of a number of high oil hyrids and compares them with three conventional hybrids.

The increase in embryo vs the hard endosperm results in a softer kernel that is more prone to damage. Trials have shown a significant increase in kernel damage in the midwest. Softer kernels and high oil also makes this corn attractive to insect feeding. Currently, there are no high oil Bt hybrids, but this is being considered.

If corn yields from a high oil hybrid and a conventional hybrid remained the same there would be no need for premiums. The increase in demand for high oil corn and the increased income available to purchase high oil corn would result in market price increases for these hybrids. Therefore, the open market system would provide the incentive for growers to select high oil hybrids. Unfortunately, the reduced yield potential from high oil hybrids make premiums necessary. For instance, consider the following scenario: a producer plants a high oil corn hybrid and a conventional hybrid side by side. At harvest, the conventional hybrid yields 100 bu per acre verses the high oil hybrid that yields only 90 bu per acre (10% yield reduction due to the unproductive male pollinator). At the current market price of $2.50 per bushel the conventional field returns $250 per acre. The high oil corn returns $225 per acre. In order to make up for the decrease in income from the high oil hybrid the producer would have to be paid a premium of $0.28 per bushel. The reduction in yield potential that exists in the current high oil corns are the reason premiums are necessary in the marketplace. A corn producer who is interested in the high oil corn market must determine the potential yield reductions before agreeing to a premium contract. Otherwise, he/she will not be able to calculate the return from high oil corn.

Table 2-4. Oil Content, test weight, and yield of three
conventional hybrids compared with several high oil corn hybrids.
Data represent the average of two sites in Beaufort County in 1998.

* These are conventional corn hybrids not high oil topcross hybrids.

Despite the potential for yield reductions, the future for high oil corn in North Carolina is very bright. Because livestock feeders make up a large portion of our corn market, there will be an increasing demand for value added corn products. Livestock feeders don't want to have two separate facilities, one for high oil corn and one for #2 yellow corn. Therefore, because high oil corn has such important feeding benefits, they will be moving to make high oil corn the only product they buy. North Carolina corn producers are in a unique position because our corn is usually harvested in August at a time when corn stocks are low. Livestock feeders interested in maintaining their supply of high oil corn should be willing to pay better premiums for this early crop in order to make sure that they do not have to convert back to feeding normal #2 yellow dent corn.

This is only the beginning of the trend toward value added products. High lysine corn, low phytic acid corn (results in better phosphorus utilization by livestock), low linolenic oil soybeans and other value added crops will be coming in the near future. Growers will be moving toward contract marketing with livestock producers to provide these new value added crops.

Corn Seed

Recent improvements in planter technology have produced planting units that use vacuum or air pressure to hold the seed to a plate or drum until it reaches the release point. Usually, these types of planting units can use different seed sizes and shapes depending on the size of the hole in the plate or drum. However, experience has shown that round kernels tend to work best because they flow better in the units and fit the plate or drum better. Finger pickup units can also plant a wide range of kernel shapes and sizes. Again, farmer experience has shown that small to medium kernels with a round shape work best. The key to accurate planting is to fit kernel shape and size to the planter plate, drum, or finger pickup unit that you are using. Corn producers should carefully select kernel size and shape based on their equipment.

Kernel Size Effects on Yield and Emergence

It may be argued that large kernels possess greater carbohydrate reserves that enable them to germinate more consistently and uniformly than small kernels in cold, wet or compacted soils. In contrast, smaller seed sizes require less moisture for germination; they may emerge more reliably in dry planting seasons. However, research has not found any relationship between kernel size or shape and emergence or yield. This observation suggests that growers with placeless or vacuum planters should take advantage of lower prices asked for less popular seed sizes and shapes. It also suggests that, in years of limited seed availability, growers should purchase corn hybrids on the basis of their agronomic performance, not their kernel size.

Germination and Stand Establishment

To determine the number of live plants expected from a given seeding rate, growers should use the following equation:

Expected emergence = Seeding Rate X (%Pure Seed/100) X (%Germination/100)

Under field conditions it is not uncommon to find that as many as 15% of the seeds planted do not produce a live plant.

Planting Dates for Corn in N.C.

Maximum corn yields are obtained from a plant that grows for the longest period of time in the absence of heat or moisture stress. This means that the selection of planting date at a given location is based upon the desire to obtain the longest growing period while at the same time avoiding periods of drought or high temperatures. While this sounds simple in principle, it is difficult to accomplish in practice. Across North Carolina the period of least rainfall and maximum evapotranspiration occurs from July 10 to August 1 (Figure 2-1). This period also experiences some of the highest temperatures particularly night temperatures. In addition to these factors, effective rooting depths in most soils in North Carolina are restricted to the upper 8 to 24 inches of the soil profile by acidic subsoils, compacted layers, and other root restrictions. The combination of low rainfall, high temperature, high water demand, and shallow rooting depths almost guarantees that the corn crop will experience moisture or temperature stress during the summer months.

In any given year, it is impossible to accurately predict when moisture or temperature stress will occur. Long-term data suggests that dry periods with high temperatures occur most often during the period from June 20 to July 15. Therefore, the best strategy that a corn grower can follow in selecting a planting date is one that seeks to avoid pollination during this risky period. Depending on the maturity of the hybrid, field data indicate that corn should be planted either early in the growing season or late enough so that only the early growth occurs during July. Unfortunately, planting corn later in the growing season increases insect and disease pressure and late harvest misses the opportunity of higher corn prices that occur early in the fall. This means that the early planted corn has the best chance of producing high yields and higher profits. With this in mind, recommended planting dates are based on getting corn planted as soon as possible in the spring.

Corn should be planted when soil temperatures reach 55^oF at a 2 inch depth and the weather forecast shows a good chance of warm temperatures over the next few days. Figure 2-2 shows the dates when soil temperatures generally reach 55^oF. In the tidewater region on organic soils this usually occurs before March 20. In the coastal plain 55 ^oF soil temperatures occur from March 20 to March 25, in the piedmont from March 25 to April 5, and in the mountains from April 5 to April 20. Since soil temperatures are effected by the amount of soil residue and moisture, planting dates for no-till systems will be later than those used for conventional tillage. When using no-till practices, planting dates can be delayed by 3-5 days.

Fig 2-1. 30-year averages for rainfall and potential evaporation at Smithfield, NC.>

Planting date studies conducted at NCSU have demonstrated that corn yields decrease with late planting. In the coastal plain and piedmont areas, corn yields decrease, on average, one bushel per acre for every day that planting is postponed after April 15. In the tidewater and mountain regions of the state, corn yields start decreasing after May 1. The later dates in the mountains and tidewater regions are due to the capacity of the soils to hold greater amount of water that extends the period during which corn growth occurs without stress. The accepted cutoff date for corn planting in North Carolina is May 10. After this date, it is generally more profitable to plant another crop. The risk of low corn yields increases because pollination will most likely occur during a period of moisture stress.

One way to reduce the risks associated with planting corn late is to switch from full-season hybrids to medium- or early-season hybrids. Best results are found when growers are advised to switch from full-season to medium-season hybrids around April 28, and from medium-season to early-season hybrids around May 7.

Fig 2-2. Average date when soil temperature at 2 inches exceeds 55 degrees F for most of the day.">

There are some cases where growers should consider planting later in the growing season. Such as when tropical corn hybrids are grown for silage. Tropical hybrids experience rapid growth early during their growth cycle and have more prolific root systems than conventional corn hybrids. Therefore, to avoid lodging tropical hybrids must be planted late. The recommended planting dates for tropical hybrids are from June 1 to June 20. Another situation that is showing some promise in the mid-Atlantic region is the use of early maturing hybrids as a double crop following wheat. By using a Bt hybrid, the grower can avoid insect damage and by planting late the corn will be in the vegetative stage during July. Research in Virginia has found that corn yields range from 80 to 140 bushels per acre in this system. We recommend that growers use double cropped corn only on their better land.

Selecting Plant Population

Plant population is a critical factor in corn production, especially when corn is grown on sandy soils in dry seasons. Plant populations should be selected according to the soil moisture-holding capacities of individual fields. Corn plant populations per acre should increase with increasing soil moisture holding capacity (Tables 2-5 and 2-6). On soils with good to excellent soil moisture holding capacity, growers should seek to obtain a maximum of 27,000 plants/acre. Seldom is there a need to seed dryland corn at final stands exceeding 28,000 plants/acre. On soils with average water holding capacity, they should plant to obtain a final stand between 22,000 and 24,000 plants/acre, and on soils with poor water holding capacity final stands should not exceed 19,000 plants/acre.

Plant Population and Crop Management

A grower should also select a plant population that complements his package of production practices. Seeding rates should be matched to tillage systems, capability of the hybrid to tolerate increasing plant populations, the standability of specific hybrids and intended use of the crop (for example, silage versus grain). Corn growers should be aware of the recommended plant populations for the hybrids they grow (See section on hybrid selection on page 11).

Plant Population, Irrigation and Sub-irrigation

Numerous irrigated corn studies have shown that 29,000 plants/acre is the optimum plant population for corn when soil moisture is not limiting. When supplemental moisture is available to corn via irrigation, seeding rates should increase proportionally to the quantities of water that can be supplied. In extremely hot, dry years, most growers using hose reel machines can not deliver the quantities of water needed by corn. Growers with sub-irrigation capabilities should use the same seeding rates as those used by growers irrigating with hose reel machines. In contrast, center pivot irrigators who water aggressively can plant to obtain a the ideal plant population of 29,000 plants/acre. Under center pivot irrigation, plant population should not change with soil type. However, it should be obvious that center pivot operators tending corn on sandy soils will have to irrigate more often.

Plant Population and Drought

Many growers producing corn on drought-prone soils strive for final stands of 16,000 to 19,000 plants/acre. Reduction of plant population to that level is worrisome if adverse seedbed conditions or insect problems are encountered. When corn stands fall below 14,000 plants/acre, satisfactory weed control is seldom attainable. However, the probability of drought is far greater than the probability of insect or seedbed problems. Growers using low plant populations as a hedge against drought should regularly scrutinize hybrid information to ensure that the hybrid they choose will still produce adequate yields at low plant densities when rainfall is ample. Those same hybrids should be among the highest yielding when soil moisture is lacking and yield levels are low.

Prolificacy

Prolific hybrids are corn lines that bear more than one ear per plant. At normal seeding rates, the second ear on a prolific hybrid may not contribute significantly to final yield. At low plant populations, the contribution of a second ear to grain yield can be significant. Prolific corn hybrids use nitrogen efficiently and tolerate stress better than single-eared hybrids. However, it is important to recognize that, to date, commercial hybrids producing more than one ear per plant in low population scenarios have not out-performed single-eared hybrids that responded to low seeding rates by "flexing" their ear size to produce more grain.

Silage and No-Till Considerations

Silage producers generally should increase corn seeding rates by 2,000 to 4,000 plants/acre. The elevated plant population increases tonnage and total digestable nitrogen production without undue risk of lodging because silage harvest occurs as soon as plants are physiologically mature. Experienced no-till farmers with up-to-date planting equipment should not increase corn seeding rates. Before planting techniques were perfected, it was customary for no-till growers to compensate for inexperience and poorly performing planters by dropping up to 15% more kernels. Improvements in no-till equipment and increased residue management experience enables today's no-till corn producers to use standard seeding rate recommendations.

Table 2-5. Planting population guidelines for different soil types with dryland and irrigated corn.

Soil Type	Texture	Water-Holding Capacity (inches/inch)	Kernel Drop* (x 1000)	Final Stand (x 1000)
Dryland Corn
Wagram	loamy sand	0.09	19,800 (10.6)	18,000
Norfolk	sandy loam	0.11	22,000 (9.5)	20,000
Goldsboro	sandy clay loam	0.13	24,200 (8.6)	22,000
Hyde	shallow muck	0.20	30,000 (7.0)	27,000
Ponzer	muck	0.22	30,000 (7.0)	27,000
Irrigated Corn (with hose-reel or cable-tow machines)
Wagram	loamy sand	0.09	22,000 (9.5)	20,000
Norfolk	sandy loam	0.11	24,200 (8.6)	22,000
Goldsboro	sandy clay loam	0.13	26,400 (7.8)	24,000
Irrigated Corn (center pivot or linear move machines)
Wagram	loamy sand	0.09	31,900 (6.5)	29,000
Norfolk	sandy loam	0.11	31,900 (6.5)	29,000
Goldsboro	sandy clay loam	0.13	31,900 (6.5)	29,000

* Based on 30-inch row spacing and 90% germination

H Numbers in parenthesis indicate number of inches between seeds

Planter Speed

Planter speed is critical in obtaining optimal seeding rates in conventional and no-till corn production systems. Most planter types function best at 4 1/2 miles per hour; successful no-till growers plant slower. Excessive planter speed will manifest itself in erratic stands, poor weed control and low yields. Corn growers must recognize that planter performance has the greatest influence on their ability to produce uniform stands of the desired density. The potential yield for a given field is highest when corn plants are evenly distributed. Row widths and seeding rates that combine to distribute plants uniformly across a field ensure that individual plants have maximum access to available light, nutrients and soil moisture. The ability of growers to select the proper plant population and to achieve that plant density with precision spacing and uniform emergence within rows determines, to a great degree, the profitability of corn enterprises.

Table 2-6. Recommended final plant populations for corn.

Location	Hybrid Maturity
	Early to Mid	Late
Tidewater	27,000	25,000
Coastal Plain	24,000	22,500
Piedmont	22,000	20,000
Mountains	25,000	23,000

Selecting Row Spacing for Corn Cropping Systems

The choice of row spacing is one of the most fundamental components of a corn cropping system. Narrow rows permit more uniform plant distribution and reduce the inter-plant competition for moisture, nutrients, and light. However, as row width decreases, the difficulty in managing weeds, insects, and fertility increases and there is an increase in machinery costs. In choosing row width, the corn grower must balance the potential increase in yield that comes from narrower rows against the additional machinery cost and management that a narrow row system demands.

Yield Response to Row Width

Fig. 2-3. Corn yields at three different row widths in 1996 Camden County."

Tidewater Area:

Coastal Plain:

Piedmont Area:

The potential yield increase from changing from wide to narrow rows must be weighed against the cost of that change, both in terms of equipment modification and an increase in the annual input costs. Table 2-7 shows a comparison between potential yield increases and the returns to a narrow row corn system. Costs of converting equipment to narrow rows are considered (figured at $20,000) as well as the interest cost and the increase in fertilizer. Additional insecticide costs are not figured in this chart. Given an 8.5% yield increase between 36 and 30 inch rows, corn growers with over 250 acres of corn with a yield history of 125 bu/acre or higher can justify moving from 36 to 30 inch rows. Growers considering moving from 30 to 20 inch rows must grow over 350 acres of corn to justify the expense. Note that the difference in overall profit between wide and narrow rows is small. Even a grower with 500 acres of corn at a 6% yield increase will only make an additional $2,000 by converting to narrow rows.

Narrow Row Corn Cropping Systems

It is clear that although there is a yield advantage to narrow rows in North Carolina, the profit margins are slim. However, if a grower can reduce the cost of the equipment conversion and maximize yield increases, he/she can increase long-term profit. To do this requires a consideration of the components needed to implement a profitable narrow row cropping system.

Crop Sequence.

Crop Management.It appears that hybrid selection, planting dates, or plant populations do not impact the yield increases found in narrow row systems. Therefore, the grower should not change current practices. Increasing plant populations is not required or recommended to obtain the desired yield increases from narrow row systems.

Soil Management.

Table 2-7. Economic return from changing to narrow rows at a range of yield increases.

Narrow-Row Increase	Return Per Acre Yield Level (bu/acre)
%	125	150	175
250 Corn Acres
2	-$14	-$13	-$12
4	-$10	-$8	-$6
6	-$6	-$3	-$1
8	-$3	-$1	-$5
350 Corn Acres
2	-$8	-$7	-$6
4	-$5	-$3	-$1
6	-$1	-$2	-$5
8	-$3	-$7	-$11
500 Corn Acres
2	-$4	-$3	-$2
4	-$0	-$2	-$4
6	-$4	-$7	-$9
8	-$8	-$11	-$15

Costs Assumed:

Convert corn head $5000
Convert planter $6500
Convert Sprayer $ 500
Purchase New dual tires and rims $8000
Total machinery modification of $20,000 carried over 5 years at 10% interest - annual cost of $5000
Fertilizer costs for narrow rows increase at the same percentage as yield.
Cash price for corn - $2.40; Herbicide and seeding rates stay the same.

a limited area; therefore, there is less concentration of fertilizer or insecticide in the root zone. Studies have shown that per-acre rates of starter fertilizer need to be increased as row spacings narrow. Starter fertilizer rates should increase 1 to 1.5 pounds per acre for every inch that row spacing is decreased. Sidedress nitrogen should be applied earlier (14-21 days after emergence) to avoid problems with crop damage from the application operation.

Weed and Pest Management.

Equipment Management.

Narrow row corn systems show promise for increasing corn yields conservatively from 4 to 6%. For growers who raise over 350 acres of corn this would result in a small profit. A complete corn management system as outlined above should be used to maximize yield increases and minimize costs.

Seedbed Preparation and Planting Considerations

Seedbed Preparation

Improvements in planter design have reduced the emphasis on seedbed preparation for corn, but the basic fact still remains that uniform and consistent stands are the result of seed placed in an optimal environment for germination and emergence. The optimal environment for a corn seed has three requirements: adequate moisture, firm seed to soil contact, and temperatures above 55^oF. The moisture content of the soil at the depth that the seed is placed should be from 2 to : of field capacity. This means that the soil should be moist enough to show visible signs of moisture, but not so wet that it can be molded into a ball without easily crumbling. In dry conditions, planting depths should be increased to insure adequate moisture at the seeding depth.

For germination to occur rapidly and uniformly across the field the seed must be surrounded by soil. The planter should be adjusted so that the seed is placed at the proper depth with good soil contact and cover. This is the key to success in no-till systems. No-till planters frequently use fluted coulters to cut a trench or slot into which the double-disk seed openers place the seed. If these coulters cut too deep or if the flute design allows too much soil to be disturbed they leave a deep ragged slot which allows the seed to fall deeper than intended. Good seed-soil contact is almost impossible in these situations. On the other hand, no-till planters working in heavy trash tend to ride up on the trash mat resulting in seed placement that is uneven and inconsistent. Planting into a conventionally tilled seedbed which is cloddy from being worked too wet or, in the case of heavy organic soils, which is too fluffy, will also result in poor seed-soil contact. Farmers using conventional tillage should avoid multiple tillage passes which lead to puddling and compaction of the soil surface. Modern planters have the capability to work in rough seedbeds and do not require excess tillage to be successful in placing the seed correctly.

Soil Temperature

Seed germination and seedling emergence are affected by many environmental conditions, particularly soil moisture and temperature. Wet soils or soils with heavy amounts of residue will be cooler than soils that have ideal moisture or no cover. On all but the heavy organic soils of eastern North Carolina, Atrash whippers@ attached to a no-till planter have been shown to be effective in promoting better germination and emergence by reducing surface cover over the seed and by stirring the top 2@ of soil. For best results, corn growers should avoid planting into cold, wet soils if at all possible. Corn should be planted when soil temperatures reach 55^oF at a 2 inch depth and the weather forecast is favorable. In tidewater, piedmont, coastal, and mountain areas of North Carolina, 55 degree soil temperatures are usually reached just prior to March 20, March 25 to April 5, March 20 to March 25, and April 5 to April 20, respectively.

Planting Depth

Depth of planting will influence several factors associated with stand establishment and should vary with planting date. In general, the deeper the planting depth, the cooler the soil and the slower the emergence. Temperature in the first 2 inches will be greatly influenced by air temperature and can fluctuate as much as 10 to 15^OF during a single day. Soil temperatures are fairly consistent at the 2 to 4 inch depth, and warm gradually as the season progresses. Normal planting depths range from 1 1/4 to 2 inches. Since cold temperatures can reduce germination, planting depth should be 1/2 to 1 inch shallower at very early planting dates.

Another impact of planting depth is associated with the process of germination and seedling growth. One of the first structures to emerge from the seed during the germination process is the coleoptile (co-lee-op-tile), a sheath like structure that protects the young shoot (see Figure 2-4). Without the coleoptile, the tender shoot or plumule, would be shredded before it could penetrate the soil surface. Planting corn seeds too deep can result in the coleoptile growth terminating well below the soil surface. As the shoot grows through the coleoptile tip, it will continue to grow unprotected towards the surface. In heavy soils, crusted or compacted soils, an unprotected shoot will be torn apart before it can emerge.

Research has shown that planting too shallow or too deep can adversely affect corn yield. Planting depths greater than 2 inches result in seedlings with less vigor, slower growth and development, and less yield. Conversely, planting depths less than 0.5 inch lead to poor seminal root development, shallow rooting depth, and poor drought tolerance. Special care should be taken not to plant corn too shallow on landscapes prone wind erosion. Strong winds can uncover seed and destroy stands. It is, therefore, important to regularly check the seeding depth during the planting operation.

Harvest and Storage

An efficient corn harvest is the result of attention to management throughout production and harvest seasons. Decisions, such as the selection of hybrids that mature at different times (see the section on hybrid selection), can help improve harvest conditions and insure that the corn produced makes to the bin. Other details, such as combine preparation and repair, require careful planning, but payoff in less down time and better combine performance. Estimates have put average corn harvest losses in North Carolina anywhere from 5 to 10 bushels per acre, with expert operators and managers reducing this to about 1 to 2 bushels per acre. The added income from an efficient harvest is almost pure profit, so a timely harvest and the few minutes spent on careful combine adjustment can be extremely profitable.

Harvesting early is the key to successful corn production. Those who harvest early can: