With corn acres expected to be at record are near-record highs in the Southeast this spring, many growers are fine-tuning their production practices — tweaking things such as plant populations and planting dates — to guarantee the greatest return on their investment.

A study conducted by Auburn University researchers in 2011 and 2012 at the Gulf Coast Research Center in Fairhope, Ala., evaluated the impact of plant population on yield, and the impact of the interaction between plant populations and planting dates on corn yields in non-irrigated conditions.

It’s well known that plant population can positively or negatively affect corn yield, but yield potential also can be influenced by planting date, which is strongly linked to the at-planting and in-season weather and climatic conditions.

“When considering management changes, farmers need to keep in mind that the optimum plant population will not only vary between regions of the state, but from season to season, and from field to field on the same farm,” according to the research report.

Currently, there is a big push to increase plant population in order to increase production, states the report. However, this change is typically recommended under non‐limiting conditions, such as those with an ample water supply and adequate fertilization.

Corn genetics have evolved to the point to where farmers can delay planting to take advantage of favorable environmental conditions, while maintaining a good insect protection and water-use package.

Researchers say that a combination of the currently available corn hybrids along with a growing knowledge of climate variabilities within regions allow them to investigate how plant populations and/or planting dates should be modified in order to increase production and profitability and reduce potential production risks.

The Fairhope, Ala., research — conducted on Malbis fine sandy loam soils — consisted of four different plant populations of 18,000, 22,000, 26,000, and 30,000 seeds per acre planted at two different times during the growing season: mid‐March (standard date by farmers in the area) and three weeks later.

The corn hybrid planted was Pioneer 31P42 with a relative maturity of 119 days. Experimental plots were four rows wide by 30 feet long with 38-inch row spacing. Plots were fertilized with 150 pounds of urea ammonium nitrate. Yield data was recorded after harvesting the middle two rows of each plot.

Regardless of the plant population and planting dates chosen for the study, corn yield in the 2011 season was lower than in the 2012 season. The year‐to‐year yield changes could be associated with differences in the climatic conditions.

When comparing 2011 and 2012 monthly precipitation and maximum temperature deviations with respect to historic values (1970‐2000), the main differences were observed from May to July which corresponds to the months of flowering and grain filling.

Time of rainfall

Even though precipitation was lower than normal early in the season for both years, the low yields in 2011 could be a consequence, not only of the low precipitation during the flowering period, but also the permanent precipitation deficit and the elevated temperatures throughout the season.

During the 2012 growing season, above-normal precipitation values were observed during the months of May and June, and maximum temperature was below normal for the months of June and July. These specific climatic conditions could favor pollination and grain filling.

According to the research, plant population has a strong effect on final yield, but the interaction between populations and the environment defines final yield.

As plant population increases, competition between plants for resources (light, water, and nutrients) increases.

The 2011 and 2012 growing seasons are good examples of the plant response to the environment, states the report.

During the 2011 growing season, a negative yield trend towards high population was observed with yield decreasing as plant populations increased, and that was independent of the planting date.

The precipitation deficit and the high temperatures observed in the 2011 season strongly affected the high plant population treatments (26,000 and 30,000) compared to the low population treatments (18,000 and 22,000).

Under the 2011 environmental conditions, the best corn production choice was the 22,000 plant population.

In 2011, changes in planting date had an overall impact on yield, but did not influence the yield response to plant population.

Corn planted on the April 12 (three and a half weeks later than the standard planting date) performed better than the standard planting date (March 17) which might be due to the rainfall and temperature observed during flowering.

Corn planted on March 17 received less precipitation during the period of flowering than the crop planted on the April 12.

There was also a sharp increase in maximum temperature after flowering of plants corresponding to the first planting date (March 17) compared to the second planting date which may have affected pollination and grain filling.

The increase in precipitation around mid‐July certainly favored ear development of the plants from the second planting date (April 12) which were harvested at the end of August, compared to the first planting date harvested in the first week of August.

In 2012, yield increased as plant populations increased, but reached a maximum at 26,000 seeds for both planting dates. The positive yield response to increases in planting density could be due to water availability from precipitation during the months of May, June and July.

Although there were yield differences between planting dates for all plant population treatments, the greatest difference was observed for the 26,000 plant population treatment.

Reasons for lower yield

The lack of precipitation and the increase in maximum temperature during the last two weeks of May (flowering period) and the last two weeks of June (grain filling) for the first planting date (March 20) could explain the lower yield associated with the 26,000 plant population treatment compared with the second planting date (April 13).

Determining the economically optimum plant population and planting date combination is influenced by five pieces of information: yield, price of corn, price of seed, variable costs associated with corn production, and climate forecast, according to the report.

Each farming operation is different and producers should use their actual costs of production to make management decisions such as planting date and plant population.

If the corn price increased ($6 or $ 7.75), revenue from corn production covered direct expenses for the delayed planting and lower plant populations (18,000 or 22,000) treatments.

Data from 2011 show that if a dry season is expected, a plant population of 22,000 seeds per acre is the best economical choice for the conditions in Fairhope, Ala., and the estimated cost of production.

In 2012, higher plant populations increased returns over direct expenses independent of the price of corn. The later planting date resulted in higher returns over direct expenses. Higher returns over direct expenses were realized when corn was planted on about April 13, compared with corn planted on March 20.

Based on the conditions in Fairhope in 2012, and the estimated costs of production, a seeding rate of 26,000 seeds per acre planted on about April 13, provided the highest returns over direct expense, regardless of price.

Aside from returns over direct expenses, there are other factors to consider prior to making decisions on planting date.

When the planting of one crop is delayed, it may negatively impact the ability to plant additional crops due to available labor and machinery hours.

Furthermore, delaying planting may negatively impact harvesting of other crops and planting of winter commodity crops or cover crops.

The report recommends that decisions regarding the management of planting dates should be made considering the needs of the entire operation, not just one crop.

“There are financial tradeoffs to all production decisions when considering all crops grown on the operation and the role they each play in the profitability of the operation.”

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Lastly, land rent is not considered in the direct expenses due to the variability across operations. This cost should be considered a direct expense and considered as part of the decision-making process.

The overall results from this two-year experiment show that the yield potential of corn grown under multiple plant populations and seeded at two different dates can vary depending on the weather and climate conditions.

Yield and profitability data shows that, under dry conditions, the use of low plant populations, especially 22,000 seeds per acre, may be a good option if planted about the second week of April.

In comparison, if normal precipitation is expected, an increase in plant population will result in higher returns over direct expenses.

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