Golden Harvest and Pioneer have collected more data on the yield response to varying seed corn rates within a field. Findings reinforce what both these seed companies said last year: In most cases, this site-specific practice won't save you money or increase yields. (See September 1997 issue, page 16.)

Both seed companies found, yet again, that a single rate is best regardless of where you are in the field. The only exceptions are extremely adverse conditions that could reduce germination or stand establishment and poor yield environments of under 100 bu./acre, the researchers say.

"It doesn't look like variable seeding rates are going to pay off right now," says Bob Sabata, agronomy research manager, Golden Harvest. "There is more value in trying to find the optimum population for a field rather than each yield environment within that field, because it seems to stay the same." However, finding the optimum population is tricky, Sabata says, because it can vary by hybrid and by year. But in most yield environments, it should fall between 24,000 and 31,500 seeds/acre, based on the combined research.

Acres of data. Golden Harvest conducted its second-year tests on 40-acre dryland fields in eastern Nebraska and western Iowa. Researchers split the 40 acres into three distinct yield environments - high flat land, a steep slope, and a mild slope. Across all three environments they planted two top-selling elite hybrids - one or their own, the other from a competitor - at three rates ranging from 16,000 to 32,000 seeds/acre.

They used a weigh monitor at harvest to measure yields at each environment, and contrary to what they expected, yields were lower on the mild slope than on the steep slope. "That goes to show topography isn't the only factor determining yield environment," Sabata says. But regardless of relative yield, all three environments yielded highest at a rate of 24,000 seeds/acre for the Golden Harvest hybrid and 25,047 for the competitor. He says the findings will likely hold true for most fields in eastern Nebraska and western Iowa.

Pioneer has studied variable rate seeding for seven years on both small plots and larger fields, finding similar results. Last year, in particular, researchers planted over 70 elite hybrids on small plots in 35 to 40 locations scattered across the Corn Belt.

The findings? "Counts producing 26,000 to 30,000 plants/acre at harvest tend to be optimum, except at very low-yielding sites," says Tom Doerge, precision farming agronomist, Pioneer. The actual seeding count would be 5% higher than that range, or 27,300 to 31,500, he says.

Find your optimum rate. On acres yielding less than 100 bu., Doerge says you'll maximize yields by seeding below the recommended rate. These low-yielding areas are usually easy to predict from year to year. They include perennial wet spots, eroded knobs, corners of center pivots or very shallow topsoil less than 6 in. For areas yielding above 100 bu./acre, the only time you should increase rates above the recommended rate is when there is excessive surface moisture, cloddiness, compaction or expected soil erosion immediately after planting that may reduce germination or stand establishment, Doerge says.

Otherwise, in decent yield environments, both researchers say to plant one optimum rate across the entire field. That rate will vary by hybrid. To nail it down, look at your yield data collected over the long term. Seed corn companies that conduct a lot of population studies can also help you arrive at a number.

But, keep in mind that their counts are often based on super high-yield environments, Sabata says. "Their environments may yield 180 to 210, even for dryland," he says. "I don't know if you can take that number and decide what you should plant all the way across."