As corn hybrids have developed, scientists' focus has been increasing the stress tolerance of individual corn plants. Corn yield potential per plant has not increased over the past 50 years, but the amount of yield per acre has.

A lot of the observed yield increase per year is directly correlated with increased plant populations. Iowa plant populations have increased about 425 plants/acre/year since 2001.

Although plant populations continue to increase, we must consider whether the yield advantage of planting more seed is economically productive.

Seeding rates were evaluated across 10 locations in Iowa in 2006 and 2007, ranging from 25,000 to 45,000 seeds/acre. The optimum seeding rate, averaged across all locations, was approximately 35,000 seeds/acre.

Location played a huge role in the final yield results. Yield at four of the 10 2006 locations significantly dropped off past 30,000-35,000 seeds/acre. Six of the 10 locations had the highest yields from 35,000 to 45,000 seeds/acre. It is interesting to note that there were 200-bu. yields in both of these groupings.

Early season vegetative growth and conditions surrounding pollination no doubt influenced which seeding rates were best. Therefore, a seeding rate of 35,000 seeds/acre is a good general recommendation based on 2006 and 2007 data. Yet, it is imperative to consider the field and environment when selecting seeding rate.

Can seeding rates be even more fine-tuned? Economically, optimum seeding rates may vary within fields but may be difficult to actually pinpoint. New variable-rate seeding (VRS) planter technology allows growers to change seeding rates within a field. A couple research papers were written in the past few years discussing the feasibility of using variable-rate technology to improve profit.

THE FIRST STUDY was conducted across the Midwest on producer fields during 1987-1996. More than 42,000 plots were thinned to plant populations ranging from 18,000 to 42,000 plants/acre. The economically optimum final plant population was approximately 27,500 plants/acre.

Some fields were more productive and responded favorably to higher plant populations. For every 16-bu./acre increase in average yield at a site, the optimum plant density increased 485 plants/acre. Yet, the economic value of variable plant populations within a field vs. one overall plant population was not evident.

The scientists determined that more detailed and costly site information was necessary to make VRS feasible and economic. During the late 1990s, scientists in Colorado investigated the rain-fed yield response to seeding rates in relation to their variable landscape. High yields were 120 bu./acre, and plant densities ranged from 10,000 to 25,000 seeds/acre.

Obviously, these variables are much lower than experienced in Iowa. The economically optimum plant densities changed approximately 2,000 seeds/acre between high- and low-yielding parts of fields; the higher plant densities produced higher yields. High- and low-yielding areas were related to characteristics such as elevation, soil brightness and soil apparent electrical conductivity. Placing variable populations in the fields was found to only save $2.50/acre in seed costs (although this is based on seed costs of $1.25/1,000 seeds).

A more recent article by some of the same authors suggests that the optimum plant population within a field can vary from 5,000 to 12,000 plants/acre in a given year based on the environmental conditions unique to that season. Thus, using VRS within fields at this time will not likely result in significant savings given current technology and the year-to-year variability that exists.