Successful corn production is the product of optimum planting and preparation. Francis Childs, an Iowa farmer who won the National Corn Growers Association yield contest with a world record corn yield of 442 bushels per acre, says, “…root systems and uniform stands are the foundation of high corn yields.” That is sound advice.
Wet spring conditions are common in the Mid-South, but growers should resist the temptation to push planting when soils are marginally wet. Corn seed should be planted 1.5 to 2 inches deep to assure normal root development. Even though the soil surface may be dry and a tractor can get across the field, the planting zone may contain excess moisture, which can lead to germination and developmental problems.
One such problem too common the last couple of years is “rootless corn syndrome,” which encompasses several scenarios capable of inhibiting normal root development, specifically the nodal root system (originating from the base of the stem). Nodal roots begin developing at emergence, providing structural support and the bulk of nutrient and water uptake during the remainder of the plant's life.
Poor nodal root development often causes plants to fall over when the plants get 1 to 2 feet tall. Normal shoot development often precedes this scenario, although nutrient deficiency symptoms due to poor root exploration may be observed.
The ability of corn plants to recover from this condition depends upon favorable environmental conditions and the reason nodal roots are failing to develop.
My observations indicate shallow seeding depth and planting into excessively wet soils are the most common factors leading to rootless corn syndrome. The origination point of the nodal root system is moved upward when corn seed is not planted deep enough. Corn seed placed less than 1 inch deep will develop nodal roots at or above the soil surface, rather than below the soil surface. That exposes the roots to factors that impede root development, such as hot dry soil, herbicide injury, and insect predation.
That often leads to stand problems, nutrient deficiencies, and drought stress throughout the year. Birds are more likely to hurt stands of shallow-planted seed. Corn planted at the recommended seeding depth (1.5 to 2 inches deep) develop nodal roots below the soil surface, avoiding potential problems.
Planting into excessively wet soils causes similar root development problems. Though planting depth might be sufficient, the seed furrow openers will compact the seed furrow walls. The soil shrinks when it dries, particularly in clay soils, causing the seed furrow to open and expose the nodal roots — just like shallow planting. The hard, compacted seed furrow walls also prohibit root penetration.
Growers should strive for 24,000 to 32,000 plants per acre. The exact number depends mainly on the field's yield potential, the planter row width and the planting date.
If a yield goal of 200 bushels per acre (50 bushels per acre of soybeans or 2 bales per acre of cotton) is realistic, particularly under irrigation, then strive for 28,000 to 32,000 plants per acre. If this goal is unrealistic, lower the seeding rate accordingly, but generally do not exceed 28,000 plants per acre in dryland culture.
Row-width changes optimum plant population and yield potential because it ultimately affects plant spacing. In fact, Childs found his 20-inch rows out-yielded his 30-inch rows by 28.5 bushels per acre. Seeds spaced closer than 6 inches apart normally increase competition for light, water and nutrients, which weakens stalk quality without increasing yield potential, particularly under stress. Thus, the optimum plant population in 38- to 40-inch rows should be 2,000 to 4,000 plants per acre fewer than 30-inch rows.
Over-plant the desired plant population about 5 to 10 percent, depending upon seed germination and planting conditions. Early-planted corn (soil temperature 50 to 55 degrees F) should be seeded slightly thicker than normal because cool spring conditions cause higher seedling mortality and shorter plants at tassel, meaning more plants are needed to intercept light. Conversely, growers should lower the seeding rate with later planting dates since warm soils enhance seedling establishment, taller plants are produced and yield potential decreases.
Poor corn plant spacing and seeding depth are common problems that can affect yield potential as much as or more than actual plant population. Tuning up and calibrating the planter meter system are important, but performance also depends upon the planter's operation in the field.
One common cause of seed distribution problems is excessive planter speed. A 40-inch-row planter operating at 6 mph drops seeds at a rate 60 percent greater than the firing rate of a Thompson sub-machine gun. Childs recommends planting at 2 to 4 mph. I suggest keeping planter speed below 5 mph with vacuum planters and even lower for finger-pickup and plate planters.
Fast speeds contribute to much poorer seed spacing and less-uniform seed depth because seeds roll and/or bounce in the seed furrow.
Corn plants are extremely sensitive to spacing because they do not tiller or produce branches to alter size like most other row crops. Crowded or late-emerging plants produce small ears and spindly stalks due to intense competition for light, water and nutrients.
Erick Larson is a grain crops specialist with Mississippi State University. e-mail: firstname.lastname@example.org.