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Making variable rates work

The ability to vary seeding rates across the field with some of the newer planters is another reason growers are interested in participating in the Iowa Soybean Association’s On-Farm Network plant population study. With the rising cost of seed, there’s a lot of interest in reducing seeding rates per acre — shaving them if you can — without hurting corn and soybean yields.

In the 2009 On-Farm Network study on working farms at a number of locations across Iowa, strip trials were conducted for both corn and soybeans at different seeding rates.

For corn, growers compared a difference of about 5,000 seeds per acre, dropping their normal planting rate 2,500 seeds per acre in the low-rate strips and increasing their normal planting rate by 2,500 seeds per acre in the higher-rate strips.

Key Points

• Variable-rate planting boosts return on seed investment.

• You must target the different management zones in a field.

• Information is key to finding the right populations for those areas.


For soybeans, farmers were asked to compare a difference of 30,000 seeds per acre. They planted 15,000 fewer seeds per acre than they normally plant, and also planted 15,000 more seeds than they normally plant.

“By alternating the two population rates across the whole field, we can evaluate yield spatially within the field,” explains Tracy Blackmer, research director for the On-Farm Network. “Looking at yield monitor data from the side-by-side strips, we can see which planting rate is more profitable and where. This is a very important first step in determining whether variable rates might be feasible.”

Writing a prescription

“The real question,” notes Blackmer, “is what information you should use to determine how and where to increase or decrease the planting rate.”

Some of the commonly mentioned information options for this include soil type, historic yield records, soil electrical conductivity and elevation of the field. Some experts suggest using a combination of two or more of these factors, which increases the difficulty of writing the planting rate “prescription.”

Blackmer says based on his and his staff’s analysis of the 2009 studies, the jury is still out on which of these can be a reliable predictor for where growers might profitably increase or decrease planting rates of corn and soybeans per acre. “We’ve seen that seeding rate is important,” he adds. “But the difference between what people expected and what we saw suggests we need more studies to improve and increase the amount of information we have, so we can accurately determine just when and where we should vary the planting rate.”

— By Kacey Birchmier and Mick Lane

Do higher corn populations need more N?

A long history of basing nitrogen fertilizer rates on yield goals makes it easy to assume farmers must apply more nitrogen to achieve higher corn yields.

“The reality is, most of the time, we can’t accurately predict the correct nitrogen rate for a field within 25 pounds of nitrogen per acre,” says Tracy Blackmer, director of research for the Iowa Soybean Association. “Predicting how natural influences like rainfall amount, frequency and intensity; soil and air temperatures; and biological activity in the soil can affect yield is beyond our ability at this time. Yet these make a big difference in nitrogen availability, uptake and final yield.”

While it seems reasonable that higher yields can’t be achieved without adequate nitrogen, the reality is that nitrogen does not limit corn yields most of the time. While 5 bushels per acre may seem like a significant yield increase — at $3.50 per bushel, it’s an extra $17.50 per acre in income — it needs to be put into perspective. Corn contains about 0.7 pound of nitrogen in each bushel. In 5 bushels of corn, there’s 3.5 pounds of nitrogen.

In nitrogen rate studies with a single population, there is a big yield increase when adding 50 pounds of nitrogen compared to not adding any fertilizer. As more fertilizer is added, the yield increase is less and less. At some point, you’ll see that the last 50 pounds of nitrogen increases yield by only 5 bushels per acre. Although it varies, 50 pounds of nitrogen is often equal in value to about 5 bushels of corn.

“So, if we increase the planting rate by 5,000 seeds per acre and, in anticipation of a higher yield, add 50 pounds of nitrogen per acre, only to increase the yield by 5 bushels, the combination of the extra nitrogen and extra seed makes it unprofitable,” Blackmer notes.

If you’re considering higher corn populations, it’s important to make sure the whole system is being managed as well as can be. Simply applying more nitrogen to go with more seed does not guarantee a higher profit. Based on many of the “theories” being discussed, acting without evidence can be expensive.

Corn yields are much higher than they were 20 years ago, when farmers used more nitrogen per acre and planted lower populations. This has occurred largely due to nitrogen management initiatives in Iowa that have collected a wealth of data, allowing farmers to fine-tune nitrogen use.

“While you don’t want a crop to be short, you don’t want to buy extra nitrogen you don’t need,” says Blackmer. “Just as it’s important to test different corn plant populations before changing your seeding rate, it’s also important to evaluate your nitrogen management. Based on the difficulty of making accurate nitrogen rate predictions, the small yield difference resulting from a subtle shift in plant population seems to be pretty small. But see for yourself. Test before you invest in more nitrogen.”

Trials in strips

Participants with variable-rate drive planters simply changed their seed drop rates from pass to pass in these replicated strip trials.

Those using planters with two transmissions were able to use a split-planter configuration by setting the low planting rate on one side and the high rate on the other.

Farmers who used mechanical-drive, single-transmission planters either stopped after planting each strip and manually adjusted seed drop rate, or used high-accuracy GPS/RTK technology to plant strips with one rate and leave unplanted strips between them. Then they changed the rate and planted the unplanted strips with the second rate.


corn planting rate study image 1.tif

SNAPSHOT: This aerial photo shows replicated strips across a cornfield in the plant population study. It helps identify areas that differ in yield potential and could be managed differently.

This article published in the April, 2010 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.