May 23, 2008

When planting shallow, the seed-vee may not properly close. This may lead to drying of the seed zone or allow pesticides to reach the germinating seed.

 

Insects, Herbicides May be Wrongly Blamed

This spring presented less than ideal conditions for planting corn in many areas. With the cool weather, some producers may have decreased planting depths in an effort to facilitate quicker emergence. However, soil temperature and soil moisture are less buffered at shallow planting depths than at deeper ones and resulting stands may be less uniform in emergence. A number of problems can develop due to shallow planting and some can mimic insect and herbicide injury. Identifying the actual cause can aid in management and decision-making.

Planter Mechanics and Shallow Planting

Most corn planters were designed to plant 2 to 3 inches deep, so the plant develops a good root system. When planting shallower than that, especially when using a planter with angled closing wheels, producers risk sidewall compaction as the press wheels compact the soil below the seed rather than around the seed. (See the CropWatch story, Avoiding Sidewall Compaction at Planting - Don't Plant Too Shallow) Also, when planting shallow, the press wheels may not properly close the seed-vee, allowing it to dry out, affecting germination or root development. Problems may be blamed on insects or herbicide injury, but actually result from planting, so scout carefully to determine the true causes of problems.

Injury typical of seedling growth inhibitors (metolachlor, alachlor, acetochlor, dimethenamid, flufenacet): Swollen shoots or improper leaf unfurling (buggy whipping).

Injury typical of pigment inhibitors (isoxaflutole, mesotrione): Bleaching or chlorosis of plant tissue.

Herbicide Injury Potential

Injury potential from pre-emergence herbicides may be higher where inadequate seed furrow closure occurs. In this situation, herbicide may contact the germinating seed or be washed into the seed furrow. Scout for emergence problems where seedling growth inhibitors or pigment inhibitors were applied (Dual II Magnum, Harness, Surpass, Balance, Define, and many other products). Cool, wet conditions also can reduce a plant's ability to metabolize these herbicides, exacerbating injury.

This planting depth demo plot was sprayed with a growth regulator herbicide at the V8 growth stage, later than labeled. The rows were planted at 1 inch, 1.75 inches, 2.5 inches, and 3.25 inches, from front to back, respectively. Standability improved with the better root systems developed with deeper planting.

If the soil surface is hot and dry when the nodal roots are developing, rootless corn syndrome may develop if the corn was planted too shallow.

Poor Nodal Roots

Even after corn has emerged, root development problems may occur due to shallow planting, but be blamed falsely on insect feeding. As an example, the nodal roots develop about an inch above the seed or when the emerging shoot senses daylight, about 1 inch below the soil surface. When corn is planted too shallow, these nodal roots will form right at the soil surface. If the soil surface is hot and dry, the nodal roots don't develop properly, sometimes resulting in rootless corn syndrome. A timely rain or irrigation will help roots develop and the corn straighten. Planting deeper can alleviate this problem because the roots will develop in moist soil.

Rootworm feeding is characterized by dark feeding scars on the roots as well as roots that have been chewed off.

Standability problems usually decrease with deeper planting, as a stronger root system will develop with more sets of nodal roots in the soil, anchoring the plant better. If the root system is weakened later in the season by insect feeding or herbicide injury, the plant has a better chance of recovering. Too often, plants with a poor root system may lean over if the soil is very wet when high winds occur. While they may straighten up, insect feeding (corn rootworms) may be blamed, particularly in late June and early to mid-July when rootworm feeding occurs. The actual cause can be determined by digging plants and washing the roots off with a hose or pressure washer. Rootworm feeding is characterized by feeding scars that often are black and roots that have been chewed off.

Avoiding the Problem

Producers can minimize many of these problems by simply planting deeper. A deeper seed depth allows a better root system to develop, with more nodal roots below the soil surface. When scouting poor stands or standability problems, producers must carefully observe the planting depth and effective root development.

Remember that what happens at planting can have an impact throughout the growing season. It is important to scout now, note any issues, and make management decisions that will minimize potential problems later in the season.

Lowell Sandell
Extension Educator, Weed Science
Paul Jasa
Extension Engineer
Keith Jarvi
Extension IPM