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Source: Iowa Soybean Assn's On-Farm Network news release

Corn planting will soon begin in earnest. But Iowa Soybean Association (ISA) On-Farm Network replicated strip trials show high seeding rates aren't always better.

More than 200 corn population trials from 2009-2013 indicate lower seeding rates are economically better than high rates the vast majority of the time, especially in dry years. Monday's U.S. Department of Agriculture Crops and Weather Report said topsoil moisture is rated 38 percent short or very short statewide, while subsoil is rated 60 percent short or very short.

For most corn population trials, farmers compared a difference of 5,000 seeds, or 2,500 higher and lower than the grower's typical seeding rate. For example, if the rate is usually 32,500 seeds per acre, the trial compared rates of 30,000 and 35,000 seeds per acre.

According to data from On-Farm Network trials, the average yield difference between low and high rates is only 0.9 bushels per acre. And, higher seeding rates don't necessarily mean higher profits.

Trial data can be found on the On-Farm Network's Replicated Strip Trial Database.

Using $5 per bushel corn and $18.75 for 5,000 seeds ($300 per bag with 80,000 seeds), the database's return on investment calculator shows high population rates did not provide a return on investment about 80 percent of the time in on-farm trials.

Figure 1. Aerial imagery with soil map units (SMU's). Notice the amount of variability within SMU's.
In dry years, it's even less. In 2012, higher seeding rates were only more profitable 5 percent of the time. Individual results will vary depending on corn and seed prices entered.

"It was surprising to me that in the majority of planting rate trials over the last five years, yield increases from the higher planting rates did not pay for the extra cost of the seed," said Pat Reeg, On-Farm Network director. "Based on this research it seems the biggest potential from variable rate planting is in seed savings."

The goal of this trial, and hundreds conducted by the On-Farm Network each year, is to help farmers become more productive and profitable in a sustainable way.

Reeg said there's two questions population studies help answer: What is the ideal planting rate from a uniform seeding standpoint? Are there places in fields where it makes sense to plant with variable rate seeding?

"Over the last few years, many have said if we want to increase yield we have to increase population. With the price of seed, it's important that farmers verify that," Reeg said.

Productivity often varies within fields for a number of reasons. Some areas have better soil and higher water holding capacity and others don't.

Figure 2. Yield map showing variability within a Black Hawk county field. The areas colored green out yielded the red areas by over 115 bushels per acre.
Technology and equipment --- yield maps, global positioning systems (GPS), variable rate planters, etc. --- allows farmers to assign seeding rates to zones within fields.

"Yield maps, aerial imagery and other precision agriculture layers are great at showing the level of variability within fields so it may make sense to vary the (planting) rate," Reeg said. "However, predicting where to vary planting rates and by how much has proved challenging."

By testing two populations within the range farmers would consider planting, farmers can find out where low rates do better than high and vice versa.

"If you're going to build zones, shouldn't you know where you get a better return on investment?" Reeg concluded. "I often see zones created from soil map units that do not align well with the actual soil transitions in the field as shown below in figure 1.

A corn planting rate trial last year in Black Hawk County is a good example, Reeg said. Strips of 28,000 and 34,000 seeds per acre were planted across the field. After emergence, nine areas were checked for uniform stand.

The yield varied by 115 bushels per acre between good soils with high water holding capacity compared to sandy areas. The higher planting rate outperformed the lower rate by 1.2 bushels per acre across the field.

But after the cost of the extra seed was factored in and corn at $5 per bushel, the high population lost about $16.50 per acre compared to the lower rate.

The farmer found out the low population out yielded and financially outperformed the higher population in the less productive area (12.6 acres) by about $15 per acre, while the high population in the high productive area (41.4 acres) lost about $1 per acre.

"There's no doubt changing population can impact yields," Reeg said. "Whether it's financially good is another thing. Validating the yield difference of planting rates is an essential step in developing variable rate planting prescriptions."

Reeg encourages farmers looking to improve profitability to test products and management practices on their farms using replicated strip trials.

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