Optimize Corn Populations for Top Yields

Mike Rankin
Crops and Soils Agent - Fond du Lac County
University of Wisconsin - Extension

There are still corn producers who may be losing money because plant densities are not high enough to support optimum economic yields. Living in the upper Midwest, we benefit from owning snow blowers and keeping plant populations high compared to our southern corn belt counterparts. Most producers need to start thinking about final stands in the 28,000 to 32,000 plants per acre range. However, there are some considerations to be made for determining the optimum planting rate on a given farm or field.

Hybrid.....

Corn hybrids differ in their response to varying plant densities. Those with shorter maturities and height tend to be more responsive
to high populations than the tall, late-maturity types. Some hybrids are touted as having a "flex" ear trait. This means that they may compensate for lower plant densities by producing a larger ear or multiple ears per plant. In reality, every hybrid exhibits a degree of flex tendencies. Research does show that some hybrids yield better at lower populations than others but a majority of the best modern day grain hybrids will still put the most money in the bank at higher plant densities. Often, if a company suggests a lower planting rate for a particular hybrid it is because of standability problems at the higher density.

Soil type.....

Soil type impacts water holding capacity. Coarse-textured, sandy soils cannot hold as much available water and it is recommended to back populations off to about a 26,000 plants per acre harvest stand in these situations. However, there are very few soils that would be considered "sandy" in east-central Wisconsin. Conversely, we do have a number of clay soils in the area where emergence problems can be commonplace. In this situation, overplanting by more than the recommended 7 to 10 percent may be needed to obtain optimum harvest stands.

The percent stand loss from planting to harvest is an important number to know. On a poorly drained, cold soil it can be 5000 to
6000 plants per acre. On a well drained soil planted in late-May, it may only be 1000. The point is, if you don't know how many
plants that are normally lost from planting to harvest, there is no way to determine an optimum seeding rate for obtaining a given
stand density. When plant populations are referred to in research and seed company literature, these are almost always harvest
populations rather than seeding rates.

Management factors.....

For yield benefits from increased plant populations to be realized, other management factors cannot be limiting. Two that are
particularly important are soil fertility and optimum planting date. Early planting dates will result in the greatest return from higher
plant densities.

Phosphorus and potassium levels must be maintained at adequate levels for yield increases from high plant densities to be realized.  Additionally, recommended rates of nitrogen must also be applied or available to the crop from a previous legume crop or manure application.

Corn silage.....

In 30-inch rows or greater, plant densities for corn harvested as silage should not be drastically different than corn grown for grain
(30,000 to 32,000 harvested plants per acre). Although Wisconsin research shows some yield advantage to increasing plant
populations beyond this range, overall forage quality and milk per acre decline at excessively high densities. Corn plant populations
in the 35,000 to 40,000 plants per acre range may have merit where corn is planted in ultra-narrow rows (15 to 20 inches).

Economics.....

Boosting plant populations is not expensive. Seed generally costs about $1.00 per 1000 kernels. At this price, increasing planting rates by 5000 kernels per acre would only need a 2 bushel per acre return (corn priced at $2.50 per bushel). In the Fond du Lac County plant density study shown in the graph, the average three-year yield increase from 24,000 to 30,000 plants per acre was over
7.5 bushels per acre. This included the drought year of 1988.

Bottom Line.....

Don't let a controllable factor like plant population limit corn yield potential.   Remember, it's not ear size but rather kernels per acre that determines yield.

For more information contact Mike Rankin