Corn Seed Bed Preparation

Soil preparation for corn should follow the same general good management practices as for most other crops. The goal should be to achieve a firm seed bed with minimum moisture loss and no wind erosion.
Germination of corn is very temperature dependent, not occurring when soil temperatures drop below 10°C. Optimum germination and emergence occurs when the soil temperature at a depth 4 inches reaches 12 to 15°C, usually during the third or fourth week of May in Manitoba.
Zero-till planting can be used successfully on fields that have little trash cover, e.g., corn fields that had been harvested for silage the previous year. Zero-till planters are normal planters equipped with fluted coulters that till a narrow strip ahead of the planter units. Nitrogen fertilizers can be applied by broadcasting ammonium nitrate (34-0-0) on the surface and relying on rain to move it into the soil. Nitrogen can also be applied in the form of anhydrous ammonia using narrow 'knives'. Phosphorous fertilizer should be banded to the side and below the seed, as is done in planting under all conditions. Weed control would have to be achieved by fall application of atrazine or postemergence sprays.


Seeding Date

Ideally, corn should be seeded between May 1 and May 15. Earlier seeding when possible is acceptable as emerged corn recovers readily from frost injury. When seeded late, yield potential is reduced significantly and the risk of crop failure increases. Research has shown that, on average, a yield reduction of 1 bushel per acre per day occurs when the date of seeding is delayed beyond mid-May. Planting earlier than the first week in May may be warranted when an open spring occurs. Under these circumstances a 5% increase in plant population would be advised to allow for losses that may occur due to climatic and biological factors.
For silage, the conditions are similar. The amount of grain in the silage is dependent on silking and pollination occurring with a minimum moisture stress. Early planting produces the best quality silage.
When seeded early to mid-May, corn is able to tolerate spring frosts fairly successfully because the growing point is normally below the soil surface during the seedling stage. Stands can be reduced by frost or other factors by as much as 5% without an appreciable loss in yield. The recommendation of adding 5% population to a very early spring planting should compensate for freezing losses. There is a greater risk to final yield from planting too late than from planting too early. Although corn leaves can easily be frozen in the spring, corn is rarely killed by spring frosts. The growing point of corn generally remains below ground until the danger of frost is over.
An early date of planting is important for maximum yield, top quality (test weight) and low percent kernel moisture in grain corn at harvest. In silage, the seeding date influences the maturity. Since maturity influences fermentation and thus silage quality, planting date is crucial for optimum silage quality.
An additional effect of planting date on final grain yield is the soil moisture deficit that normally occurs at silking. Rainfall and evapo-transpiration patterns on the prairies result in an average moisture deficit of 4 inches on July 31 and 8 inches on August 31. Early planting, resulting in early emergence, usually allows silking to occur during the best possible moisture conditions. An additional five to six weeks are normally required for grain to mature following anthesis, which for early hybrids is reached in the last week in August. After grain maturity, fall frosts are no longer a significant factor.
However, there are risks to planting corn too early since as planting dates are moved earlier, soil temperature becomes a more important consideration. When soil and air are cool, germination and growth can take significantly longer, during which time micro-organisms and insects can cause damage leading to stand establishment problems. As well, there is the possibility that plants can be damaged from late spring frosts if the growing point emerges above ground level.


Corn should be placed in close contact with warm, moist soil and with enough covering for reasonable protection against pests such as birds and mice and surface drying. Shallow planted 1 inch corn will be exposed to warm daytime temperatures, but also to surface drying. Deep-planted corn, below 3 inches, will be exposed to cool soil temperatures that may result in a higher incidence of seed rot, but the soil surrounding the seed is unlikely to dry. Consequently, under most conditions, a planting depth of about 1.5-2 inches is recommended- slightly deeper in sandy soils and slightly shallower in heavy soils.


Many factors affect plant population including soil type, fertility, drainage, planting dates, location, and the purpose of the crop. Seeding rates are usually given in 1000 kernels/ acre; however, the preferred method to determine optimum seeding rates for corn is to consider final plant stands.
Using this method, the producer needs to take into account:
  • Desired plant population in the field
  • Expected plant survival rate- determined from the expected germination rate less an allowance for seedling mortality. In general, an 85% plant survival rate would be achieved under most planting conditions. If conditions are cool and moist, seeding rate should be increased by approximately 3-5% to account for increased seed mortality.


For grain, a density of 24,000-26,000 plants/acre is recommended. Under reduced yield conditions, the recommended density is 22,000 to 24,000 plants/acre. Under optimum conditions, up to 30,000 plants/acre can be used. For silage, the stand should range from 24,000 to 30,000 plants/acre. Optimal plant populations vary with the choice of hybrid, management practices of the producer, and the growing environment. The only sure method of obtaining the desired seeding rate is to calibrate the planter at planting speed, using the current year's seed. Table 9 provides an indication of the number of seeds require to reach the desired plant population.

TABLE 9: Corn seeds required per 3 metre (10 ft) row for various populations
(assuming 85% of the seeds survive and produce a viable plant)
Target population (plants/acre)
30" row spacing
# of seeds per 3 m row (10 feet)
36" row spacing
(85% expected survival)
18,000 12.2 14.6
20,000 13.5 16.2
22,000 14.9 17.8
24,000 16.2 19.4
26,000 17.6 21.1
28,000 18.9 22.7
Under good growing conditions, an increased plant population results in higher yields because of the increased number of ears per unit of land. However, increased plant density also places plants under more stress through inter-plant competition for light, moisture and nutrients. This results in smaller ears, more barren plants, thinner stalks and increased stalk breakage. If the population is pushed too high, these factors combine to cause a yield reduction and higher harvest losses.
Hybrid selection is a critical factor in determining the optimum plant population. The population selected must be suitable for the hybrid being grown and the growing conditions likely to be encountered. It is also important to look at the stalk lodging resistance of the hybrid. Under high plant populations, there is often increased incidence of lodging due to a reduction in average stalk diameter. Hybrids with a good reputation for stalk strength can generally handle the shift to higher populations.
An increase in plant density is most likely to result in improved grain yield in fields that already have a history of high yields. These will be fertile, well-drained fields with good moisture-holding capacity.
The benefit from higher plant densities will most likely be obtained when corn is planted early. Later plantings result in larger, leafier plants and more inter-plant competition. Late-planted corn will also silk later when the risk of drought or heat-stress is greater. Therefore, plant density should be reduced as planting is delayed.
Producers should ensure growing and environmental conditions in their area can handle an increase in plant population. Some factors to consider include soil moisture, expected CHU accumulation, and evaluation of potential pest issues (i.e. increased chance of corn borer infestations).
Increased plant densities are not appropriate for all situations because of the many factors that interact with plant population to place the crop under stress. Factors such as drought, weeds, insects, diseases, soil compaction, inadequate fertility and poor drainage all serve to exaggerate the stressful effects of increased plant populations. The more severe these stresses are, the lower the optimum plant population.
Because lodging is of less concern with corn grown for whole-plant silage, higher plant populations can be used than for grain corn.

Row Width

Producers in Manitoba generally use row width's ranging from 30 to 36 inches. Some producers are using row widths of 20 inches. Experiments in Ontario and the U.S.A. indicate very little difference in grain yield for corn grown at this range. The small differences found favoured the narrower spacing when the same populations were used. Research is currently being conducted in Manitoba to see if similar results are obtained. Therefore, it is advisable to select a row spacing that can be handled with the available equipment, making field operations such as planting, cultivating and harvesting manageable.

Spacing Within the Row

Corn can tolerate some variability in seed spacing. Yield is not significantly affected by small gaps as long as the proper seeding rate is delivered. Uniform seed distribution within the row should be the goal of the seeding operation. Therefore, producers should consider evaluating their planter performance to ensure uniformity of plant spacing. A well-tuned planter operating at a reasonable speed should help to minimize non-uniformity of plant spacing within the row. Planting at high speeds with a poorly maintained planter can result in a large number of doubles (two-plant hills) and skips (missing plants). Doubles can result in barren stalks and skips can cause significant yield loss, both resulting in lost yield potential for the field. Producers can also do some crop scouting once the crop is up and growing to determine if plant spacing is acceptable.
Producers should also consider increasing their target plant populations. Studies at the University of Guelph suggests that at relatively high populations (28,000 to 36,000 plants per acre), populations with less uniform corn plant spacing have generally not yielded lower than plots with more precise planting. Simply put, higher population may compensate for sloppy spacing - but at the cost of more seed per acre.


Seed Treatment

Seed corn is sold treated with a fungicide to protect the seed against decay organisms. An insecticide may also have been added to control insects that attack seed in storage. At planting, additional insecticide could be added to the seed to give protection from soil insects such as wireworms and seed maggots. For seed treatment recommendations, consult the Manitoba Agriculture, Food and Rural Initiatives publication Guide to Crop Protection.

Seed Quality

Canada has a Seeds Act to assist producers in purchasing high quality seed. In addition, companies have seed quality control programs. Farmers should, therefore, encounter few problems with vigor and germination level of corn seed. However, if emergence problems occur it may be necessary to check the quality of the seed involved. Tags on each bag of seed corn indicate the germination level, the date it was tested and the seedlot from which it was obtained. Unless these tags are saved it is impossible to identify the seedlot that was sown. Therefore it is also impossible to recheck the quality of the seed if a problem arises. A set of tags from each seedlot along with a small sample of seed should be saved to assist in tracing the cause of such problems.
It would be wise to check the germination of carry-over seed before planting. One approach is to place 100 kernels of seed between wet paper towels. The towels should be kept moist and at room temperature. At the end of a week, the number of kernels with healthy sprouts represents % germination. A more reliable test involves planting the seed in a flat of soil. This test should be conducted at a lower temperature (12 to 15°C). The number of healthy plants that emerge within two weeks represents percent emergence. Planting rate should be increased to compensate for reduced emergence.

When to Replant

Poor seed quality, cool weather, wireworms, seed corn maggots, seed rots, incorrect seed placement, herbicide damage, fertilizer burn or extremes in soil moisture content can all result in reduced emergence. With poor emergence, the question of replanting arises.
A very significant reduction in stand is needed before replanting can be justified. The yield potential of replanted corn will be reduced because of the later planting date. There also will be additional costs for tillage, seed, planting and perhaps chemicals. Replanting corn will usually be less profitable than accepting the reduced stand if it is over 16,000 plants per acre. Even at lower populations, replanting is not always advisable. Replanting should be undertaken only after carefully weighing the costs against any potential gain.