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Right seeding rates

Seed is expensive, and adding traits to provide insect control and herbicide resistance make it even more costly. But, it’s a necessary part of what you do as a row-crop producer in Iowa.

With planting time just around the corner, you might want to take a few minutes to re-evaluate your planting rate strategies for both corn and soybeans. Can you save money by cutting back on soybean seeding rates per acre? Can you make money by increasing your corn rates? Would variable-rate planting (changing the number of seeds you plant per acre based on soil type or area of the field) increase your profits?

Key Points

• The first year of a planting rate study has surprising results.

• Corn, soybean trials compared yields at different seeding rates.

• Results for both were opposite of what most growers expected.


These questions were addressed by farmers participating in seeding rate studies in the Iowa Soybean Association’s On-Farm Network trials last year. On-Farm Network participants conducted 72 replicated strip trials (36 for corn and 36 for soybeans) around Iowa. At the end of the season, five from each category were lost due to weather and other causes, leaving 31 studies in each crop for evaluation.

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

These trials were scattered throughout the state, covering a range of terrains and soil types.

Corn populations. For the corn studies, growers compared a difference of about 5,000 seeds per acre. All trials had at least three replications of each rate, for a total of no less than six strips at least the width of the combine header and running the full length of the field.

Rather than purchasing additional seed for the higher rates, most growers dropped their normal planting rate by 2,500 seeds per acre in low-rate strips, and increased it by 2,500 seeds per acre in the higher-rate strips. The average low planting rate across all studies was 31,500 seeds per acre, with a range of 28,000 to 35,000. The average high planting rate was 36,700 seeds, with a range of 33,000 to 42,000 seeds per acre.

Soybean populations. For the soybean trials, growers compared a difference of about 30,000 seeds per acre. Again, to avoid purchasing extra seed, most dropped their seeding rate by 15,000 seeds per acre for the low-rate strips, and increased it by 15,000 for the high-rate strips. The average low was 129,600 seeds per acre, with a range of 114,000 to 145,000. The average high rate was 160,100 seeds per acre, with a range of 139,000 to 190,000.

Surprising results

The results of these 2009 studies were the opposite of what you might expect. Most growers in the corn trials thought increasing the seeding rate would increase yields and profit per acre. Based on this first year, the conclusion is there was no significant difference between the yields from low and high planting rates.

Over all 31 trials that were completed, the lower planting rate yielded 186.9 bushels per acre, while the higher rate averaged 186.7 bushels per acre. That’s a decrease of 0.2 bushel per acre. Statistically, the difference is not significant.

At $230 for an 80,000-kernel bag of corn seed, an extra 5,000 seeds per acre adds $14.38 to the cost of production. If market price for corn is $4.50 per bushel, a 0.2-bushel-per-acre drop in yield cuts income by 90 cents per acre. The higher rate results in an average net loss of $15.28 per acre.

The top table shows another way to look at it. While there was a yield increase in the higher-rate strips at 14 of the sites, the increase was enough to be profitable in only three of the 31 trials. This suggests less than a 10% chance of profiting from raising your corn planting rate by 5,000 seeds per acre.

What about the soybean results? Most growers who participated in the bean trials did so because they thought they could maintain yields at lower seeding rates. They were looking to shave seed costs.

Based on analysis of the data from the 31 completed soybean trials, the average yield from the lower planting rate was 55 bushels per acre, while the average yield from the higher rate was about 56 bushels per acre.

If a 140,000-count bag of soybean seed is $40, cost is about 29 cents per thousand seeds. This means the extra 30,000 seeds will cost $8.57 per acre. The average yield increase from the higher seeding rate, at 1 bushel per acre, is profitable for any soybean market price above $8.57 per bushel. This isn’t the whole story, though. The bottom table shows that the higher seeding rate was profitable for 17 of the 31 sites. Odds are there’s better than a 50-50 chance you’ll get a profitable yield increase from a 30,000-seed increase in planting rate for soybeans.

Lane is communications director for ISA’s On-Farm Network; Birchmier is a student assistant.

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.


2009 soybean population economics2.eps

CORN RETURNS: There was a yield increase in the higher plant population strips at 14 of the 31 sites, but the increase was enough to be profitable at only three of the sites.

2009 soybean population economics2.eps

BEAN RETURNS: Average yield increased about 1 bushel per acre in the higher plant population strips. The higher seeding rate was profitable for 17 of the 31 sites.

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

All rights reserved. Copyright Farm Progress Cos. 2010.