Variable-rate planting

With seed costs rising and technology allowing growers to easily vary seed planting rates within their fields, many growers are interested in doing replicated strip trials on plant populations to see what would give them the best return on their investment. Yield and profit are keys in the decision-making process when growers are determining their planting rates.

For the past two years, a number of growers have participated in the Iowa Soybean Association’s On-Farm Network strip trials to compare different seeding rates. During the 2010 growing season, there were 67 planting rate trials for corn and 37 for soybeans across Iowa.

Dean Reynolds of Maxwell in central Iowa participated in the 2010 corn planting rate trials. For the trials, he adjusted his seeding rates by 2,500 higher and lower than his normal planting rates. For all of the On-Farm Network corn planting rate studies, rates varied by 2,500 or 3,000 seeds per acre below and above the grower’s normal planting rate, creating a spread of 5,000 to 6,000 seeds per acre between the upper and lower seeding rates.

“For corn we liked planting 33,500 to 34,000 seeds per acre across all acres before we had access to variable-rate technology,” says Reynolds. “For the past two years we varied our seeding rate from 36,000 seeds per acre on the most-productive soil down to 28,000 seeds per acre on least-productive soils. The On-Farm Network seeding rate study gave us an opportunity to see if our top seeding rate was close.”

Corn seeding rate results

In the statewide corn trials, the average yield for the higher planting rate was 190.6 bushels per acre compared to 188 bushels per acre at the lower rate. That’s an average yield difference of 2.6 bushels per acre. The yield difference range was from minus 5 bushels to plus 16.7 bushels per acre.

“One thing that stood out is some of the most responsive sites had lower target rates [27,000 to 29,000 seeds per acre] for the low rate,” says Pat Reeg, On-Farm Network technology manager.

Unfortunately, the environment in 2010 wasn’t favorable. “We had so much rain it made some of the light soils do much better than normal, and some of the heavier soils did worse,” says Reynolds. At a $4.50-per-bushel market price for corn and a cost of $230 per bag of seed, 58% of the sites had a loss and 42% showed a profit. The average across all sites was minus $1.89 a bushel.

Soil types, yield history, soil conductivity and elevation in the field can have an effect on whether you should increase or decrease the corn seeding rate for any specific area within a field. While viewing data from other growers’ trials can help determine whether or not you want to change your seeding rate, Reynolds says growers need to conduct their own replicated strip trials to see how crops will react in their fields.

Trials use grower practices

On-Farm Network trials are set up to work with the grower’s management practices. Plants grow differently in different areas of a field, so pay attention to the differences, says Reeg. Also, environment and current weather conditions play a role in responsiveness to different populations.

“We saw where we had the higher population on the lighter ground, corn yields were increased because there was plenty of moisture. For this reason we plan to continue to look at our seeding rates. We expected to see higher corn yields on heavier ground and lower yields on lighter ground, but instead it was just the opposite,” says Reynolds.

More trials are needed to find trends and patterns that may provide improved insight into variable-rate planting. “Growers have the technology to vary populations across the field. The challenge is determining when to increase, decrease or plant at a uniform seeding rate,” says Reeg.

With advancing technology being used for planting, can growers today rely on their equipment to be accurate when varying corn seeding rates? The On-Farm Network trials are looking at that question, too.

There’s still more to learn

“In addition to the yield data collected, we also collected plant stand counts in 47 of the 67 trials in 2010,” says Reeg. “The actual stands were impressively close to the growers’ target rates. The planters were able to adjust rates accurately, and very few plants were lost in the early part of the growing season.”

However, Reeg says more study is needed. “Even though we hit the target rates in terms of seeds per acre, the spacing delivered by these newer planters with all this technology was not impressive to me,” he says. “For corn, only 67% of the plants were plus or minus 2 inches of the target spacing. That is a 4-inch range. Of the remaining 33%, it was split with about half the plants close together and half with gaps.”

Reynolds says with the hydraulic drives tied into the controllers, it’s easy to use variable-rate planting.

“You can have it completely automated or you can change it with a push of a button, which simplifies the process,” he adds.

Birchmier writes for the ISA On-Farm Network. Her colleague Mick Lane assisted with the articles.

Making trials more realistic

One of the major differences between the On-Farm Network planting rate studies and those conducted by seed companies and university researchers is the On-Farm Network trials are planted and harvested just as growers manage the rest of their crop.

Traditional research studies are conducted in small plots, planted with plot planters, and harvested with plot combines or by hand. In On-Farm Network trials, the replicated strips across the field help identify areas that differ in yield potential and could be managed differently. Also, these seeding rate trials were scattered throughout the state, covering a range of terrains and soil types.

“Our On-Farm Network replicated strip trials are designed to match individual grower’s management practices,” says Tracy Blackmer, director of research for the Iowa Soybean Association. “The trials are planted and managed by the growers throughout the fields with the growers’ equipment, allowing for real-world results.”

More information on these trials can be found at


PRESCRIPTION PLANTING: With higher seed costs, more farmers are looking for ways to determine the optimum plant population for areas of fields, based on yield potential of different areas or management zones.

This article published in the April, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.