Evaluating early corn populations

Accurately assessing corn stands is one of the first crop scouting exercises Ohio corn growers should conduct on their fields. The 1/1000th acre method is commonly used to evaluate emerged corn seedlings. Count the number of seedlings in a length of row equal to 1/1000 of an acre based on row width (Table 1). Multiply the number of plants by 1,000 to get plants per acre. Repeat this at several locations throughout the field to determine an average.

I like to make a rope 17-feet 5-inches long and make a knot at both ends and drag it through the field making several counts along the way to get an accurate evaluation. Another method is to count 150 plants and measure the distance from start to finish with a measuring wheel. Divide the number of feet traveled into the appropriate factor in Table 1 to determine plant population. Because a longer row length is counted, the samples are more representative and fewer locations are required. This should be done randomly at approximately five locations across a field. Concentration on low population areas should be avoided; however, a measurement or two in these areas should be completed and used to evaluate the overall field average.

If gaps exist in the stand, try to determine the cause by digging up seeds and nearby soil. Insect larvae, such as seedcorn maggot or wireworm, may have fed on the seed and destroyed the germ. Compacted sidewalls within the seed slot could be preventing the seedling root radicle from becoming established. Planting too wet, crusting, dry soils, disease, fertilizer and herbicide injury are other factors that can inhibit emergence. Knowing the cause can help with management decisions on possible replanting.

Replanting can be a tough decision to make; however, economic gain is needed to justify a replant. New yield potential based on planting date and population should exceed the expected yield potential of the current crop enough to cover replant costs, which include removal of the current crop, new seed, fuel, and pest control.

As seedlings emerge and begin their growth process, it is important to know what to look for. The first stand assessment is a good time to evaluate tillage and planting systems. This is best done by close observation as opposed to that done from the seat of a pickup truck or tractor. When trying to get a good feel for the true planting conditions of a field, avoid the end rows due to the compaction created by constant traffic. Be sure to get far enough into the field to ensure the planter was moving at normal speed and remember that areas around windbreaks and field edges may not be typical of the rest of the field. Look at both high and low areas to get a feel for the range of conditions.

Investigate areas of doubles and skips in the sampling process for each field. Dig in the skips to see if a kernel is present and note its condition.

Measure the planting depth by using a trowel to carefully remove a seedling with the roots intact. Locate the mesocotyl, which is the area that extends between the seed and the point where the permanent nodal roots are forming. Adding approximately .75-inch to the length of the mesocotyl gives an estimate of the planting depth.

Uneven seeding depth exposes deeper planted seeds to slightly cooler seed zones than shallower placed seeds and can lead to uneven emergence. Measure planting depth in several areas and note the range of depths.

A corn seed needs about 110 GDUs, or heat units to emerge from the soil. Once the corn plant accumulates about 350 GDU’s the growing point of the seedling begins to rise above the soil surface and is susceptible to frost injury, mechanical injury, or the elusive 4-legged animal eating the tops out of the plant.