More corn growers are interested in using variable-rate seeding technology to optimize return on their annual seed investment.

A recent survey showed that among Corn Belt farmers with more than 1,000 acres, more than one-third have varaible-rate seeding capability on their planters and 20% actually use this capability. The percentage of farmers using variable-rate seeding doubled from the previous year.

Those using the technology expect it to help increase yields as well as reduce seed costs, says Pioneer agronomists Steve Butzen and Bob Gunzenhauser.

"Many new planters today come with variable-rate capability, making it easier than ever before to deploy a variable-rate seeding strategy," Butzen says. "But growers still need a reliable method to both select variable-seeding rates and evaluate whether their strategy ultimately worked."

1. Vary Corn Seeding Rates by Management Zone

Extensive field research has documented the value of planting higher seeding rates in more productive areas and decreasing rates where productivity is lower, Gunzenhauser says. In addition, many growers have their own data on variable planting rates gathered using yield monitors.

Pioneer also conducts numerous field trials annually in North America to help understand grain yield response to planting rate across yield levels. More than 100 hybrids were included in studies at dozens of Pioneer Agronomy Sciences research locations between 2004 and 2008.

They found the recommended planting rates by corn grain yield level were:

• 35,000 to 38,000 seeds per acre on irrigated ground yielding more than 190 bushels per acre
• 34,000 to 37,000 seeds per acre on non-irrigated ground yielding more than 190 bushels per acre
• 32,000 to 35,000 seeds per acre on ground yielding 160 to 190 bushels per acre
• 29,000 to 33,000 seeds per acre on ground yielding 130 to 160 bushels per acre
• 24,000 to 28,000 seeds per acre on ground yielding 100 to 130 bushels per acre
• 16,000 to 18,000 seeds per acre on ground yielding less than 100 bushels per acre

2. How to Designate Management Zones in Fields

Designating management zones within a field is the first step in developing a variable-rate seeding strategy. This process is usually based on one or more of the following.

• Yield map results over several years

• Crop productivity ratings based on soil type

• Topography, landscape, slope or drainage

• Grower knowledge of yield history, cropping history and general productivity of field areas

• Soil electrical conductivity and/or soil color

• Remote imagery (aerial and satellite), NDVI, bare soil and crop vigor

"The grower is best qualified to identify management zones that will be stable from year to year, based on consistent performance trends over years," Gunzenhauser says. "For example, low-lying field areas may perform best in dry years and poorly in wet years like 2009, and the grower is most familiar with such nuances."

3. How to Implement a Variable-Rate Seeding Strategy

Growers new to variable-rate seeding may benefit by partnering with someone knowledgeable in this area, such as a farmer group, consultant or Pioneer sales professional. Once the management zones in a field have been designated and the proper corn hybrid selected for the field, Butzen says growers should consider these steps:

• Select the seeding rate to be used in each zone. Typically designate three to four seeding rates within a field. To be meaningful, seeding rates should differ by at least 3,000 to 4,000 seeds per acre.

• Create each field prescription using mapping software and export the prescription to a storage device (USB device or memory card). This should include check strips or blocks to evaluate prescription effectiveness.

• Upload information to your variable-rate controller. Set parameters correctly, including offset distance between GPS receiver and planter units. Make sure the controller is set to record as-planted information.

4. Evaluating Variable-Rate Seeding Prescription Effectiveness

Setting up Checks: There are two common methods of evaluating the effectiveness of variable-rate seeding prescriptions — planting “check” blocks or strips at rates higher and lower than those prescribed for the management zone. "By comparing performance of the prescribed rates vs. these 'check”' rates, the effectiveness of the prescription can be determined," Butzen says.

Blocks: Place blocks of higher and lower planting rates inside at least one management zone of each prescription rate. "Make sure the check block is well within the zone rather than on the edge, and that it does not comprise most of the area of the zone," Gunzenhauser says. "Each check block typically encompasses about 1 to 2 acres in area and is relatively square."

Strips: These are typically field-length strips of a single planting rate that pass through several management zones. "A strip should be placed so that it crosses management zones of most or all other rates," Butzen says. "There should be at least one strip for each designated seeding rate. Strips are typically one planter pass wide."

In-season Monitoring:  -After stand establishment, take stand counts in the different planting rate zones and check areas (blocks or strips). "It's important to verify that target populations were actually attained to assure the validity of the test," Gunzenhauser adds. "Pay special attention to high-stress areas, such as poorly drained spots or high crop-residue areas.

5. Interpreting the Results

• Use yield mapping or Geographic Information System (GIS) software to compare yield results. Compare yields within check blocks or strips to yields in areas immediately adjacent to these checks.

• Did higher seeding rates produce greater yields in the low productivity zones? Did lower seeding rates produce equal yields in the high productivity zones? "If yes, then adjust the prescription planting rates accordingly the next time corn is planted in the field," Butzen says.

• Evaluate profitability by comparing yields and accounting for seed costs for any two rates in question. If there are additional costs to implementing the variable-rate seeding strategy, include these costs in your equation.

• Repeat planting comparisons in different fields over several years to better understand the value of variable- or uniform-rate seeding strategies.