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The interactions between corn plant population, genetics, soil type, fertility, crop rotation, pest control, tillage and weather are very complex, says Joel Wipperfurth, master agronomy adviser for Winfield, Owatonna, Minn. That’s why it’s important to evaluate the effectiveness of a variable-rate seeding program on a farm-by-farm basis, he says.
One way to do that is to plant check strips of higher and lower seeding rates alongside the prescribed rate in each management zone within a field. If a lower seeding rate produced the same yield as the prescribed rate, for example, you might want to adjust the prescription the next year.
To make it easier to measure the results of VRS, Bob Gunzenhauser, DuPont Pioneer, suggests that growers try three or four different seeding rates, each differing by about four thousand seeds/acre.
How to adjust seeding rates
Allen Walter, a farmer from Groton, S.D., has used VRS for three years. He starts with an average planting rate of 33,000 seeds/acre, bumps it up by 3,000 in above-average areas of the field, and drops it by 3,000 in below-average areas, which tend to be wet. But he admits that when it comes to assigning seeding rates to different management zones, “I’m just kind of winging it.”
Reeg suggests that, before you adjust seeding rates, consider both the cost of the seed and the expected yield differences. “Raising or lowering seeding rates can impact the bottom line, and not always in a positive way.” As a first step, he suggests planting two different uniform population rates in alternating strips across the field. “This can help you identify if there’s enough difference in yield response to justify changing rates as you go across the field.”
At Advanced Cropping Systems, Mieth says they’ve developed their own algorithm for yield response to population across environments. A typical seeding prescription calls for eight to 16 different rates in a field. However, a good part of what goes into the Rx are the agronomist’s and grower’s experiences, he adds. “We work with each grower to determine what he’s comfortable with, as far as minimum and maximum seeding rates.” Because population response is influenced by the entire cropping system, “we take into account hybrid, tillage, fertility, rotation – everything.”
Advanced Cropping Systems’ strip trials suggest a 7- to 8-bu. average yield increase over the whole field using VRS, Mieth says, and sometimes more. He points to one example where the grower was seeding corn at a low, uniform rate of 22,000 plants/acre because the field had a lot of sandy pockets. Yet, that low rate was still too thick for the bad areas, where plants were often stunted, and not thick enough for the good areas, he says.
“So we cut rates to 15,000 seeds/acre in the poor areas, and in good areas, we increased from 22,000 to 28,000. The whole-field average yield came up by 15 bu./acre,” he says. The whole-field average seeding rate also went up by about 2,000 seeds/acre, or about $6/acre. “Population was allocated to where it would get the best return.”
On his South Dakota farm, Walter says it has been hard to nail down the return from VRS. “To put a value on it – to say I gained 3 or 5 bu./acre – I can’t do that. But I already had the equipment, software and zone maps to do variable-rate seeding. So I’m not investing any additional money,” he says. “I see only upside potential.”