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