By Kyle Poling, Pioneer field agronomist in West Central Ohio
“You have 50 times to get it right the first time” says the old-timer offering advice to a young farmer preparing to plant their very first corn crop. Generally, growers maximize corn yield if they plant in late April or early May. However, advantages of planting before mid-May can disappear if growers plant when soils are too wet.
Seed germination is the initial step in plant growth and is triggered by absorption of water. Corn kernels must absorb ~30% of their weight in water before the germination process begins. A seeding depth of 2 inches has often been found to provide the most consistent combination of moisture, temperature, and seed-to-soil contact for uniform germination and emergence. Inadequate seed-to-soil contact, a dry seedbed, or a rapidly drying seed zone may provide less than optimum absorption of water, causing the germination process to slow or stop completely.
It is important to evaluate every field for soil moisture conditions before starting any field work. Use the simple “ribbon” test to determine soil conditions and fitness. Soil that is too wet to plant will form a ribbon when squeezed between your thumb and forefinger. Planting into wet soils can cause sidewall compaction from the seed disk openers and a seed trench that does not close. Uneven germination, variable plant growth, and restricted root development are all consequences of planting into wet soils that will result in lower yield potential.
If soil conditions are fit or getting very close to fit, there are other factors to consider before planting, including:
- Ideal soil temperature is 50 degrees F at a 2-inch depth and PREFERABLY a warming trend in the 3- to 5-day forecast: Germination and root development will not occur below 50 degrees F (root growth will be extremely slow even in the low 50s). Corn seed is particularly susceptible to cold stress during imbibition — the uptake of water by a dry seed. The risk of chilling injury decreases incrementally as the soil temperature increases above 50 degrees F. Prolonged exposure to low temperatures slow seed and plant metabolism/vigor, thus increasing the sensitivity to herbicides and seedling blights. Plant death from freezing conditions after corn planting in Ohio is rare because the growing point is below the soil until about the V4 growth stage. The slim risk of pre-emergence freeze injury can occur if sustained air temperatures of 20 to 25 degrees F or below enabling penetration of injury-causing cold temperatures to the depth of the growing point.
- Minimum of 24-hours of rain-free: Planting just before a stress event such as a cold rain or snow can result in a reduced stand. When a dry seed imbibes cold water (typically 50 degrees F or below), imbibitional chilling injury may result (causing corkscrewed shoots, fused coleoptiles, premature leaf emergence underground & other germination oddities). Cold water can also cause cell walls in the germinating seed to rupture. Ruptured cells can have ill-effects on developing seedlings and leaked cell contents can also attract disease pathogens and insects. Corn seed is most susceptible to imbibitional chilling injury in the first 24-48 hours after planting.
- Avoid planting right before a period of large temperature swings: Even if the “average” soil temperatures are above optimum, seedlings can be adversely affected by wide swings in soil temperatures. Affected seedlings will have stunted and distorted leaves and may or may not emerge from the soil. Research has shown that a swing of soil temperatures of more than 27 degrees F (soil high temperature minus soil low temperature > 27 degrees F) may adversely affect mesocotyl growth. The effect of adverse conditions can and does vary from seedling to seedling, causing erratic and uneven stands.
The ideal planting date is not simply determined by the calendar date. The best opportunity to “get it right the first time” is timely planting when favorable weather is forecasted and proper soil conditions (temperature and moisture) are met.