Precision ag research on three Iowa farms may not rival world-famous Mayo Clinic thoroughness, but it's in the same league.

For three years, Iowa State University scientists on a multi-discipline team have been scrutinizing fields by half-acre management cells. What have they found?

"We simply need to better manage yield variability in fields," says Keith Whigham, extension agronomist, who has summarized and presented the results so far. "We've found, for example, 20-bu/acre differences from one area of the soybean field to another, just due to soil variation."

Some take-home messages:

* Soil variations in fields can be even more startling than indicated by yield monitors, which reveal variations that totally amaze growers.

* Due to prohibitive costs of sampling cells smaller than one acre, results show that producers may be overestimating the usefulness and profitability of grid soil sampling and variable-rate fertilization.

* The extensive data collected from each management cell provided soybean cyst nematode (SCN) numbers that indicated the probable cause for yield loss. The grower discovered a staggering yield and profit drain from SCN.

* Results in the same field may differ from year to year because of weather and management differences.

The On-Farm Site Specific Crop Management For Iowa project, funded by the Iowa Soybean Promotion Board, began in 1996 on the Ron Heck farm in central Iowa at Perry.

The project was expanded in 1997 to include the Barry Kusel farm in western Iowa at Manning and the Don Keiper farm in eastern Iowa at Palo.

Both soybeans and corn are being studied each year, and those crops are rotated on each field. The soils represent three soil associations, different topography and different weather regimes.

The research team includes a biosystems engineer, several agronomists, an economist, an entomologist, several plant pathologists and a statistician from Iowa S tate. A soil scientist from the USDA-ARS Soil Tilth Laboratory and an agronomist from West Central Cooperative at Jefferson, IA, are also on the team.

Each team member evaluates data for his or her own discipline and consults with other team members and the cooperating producers. Data is collected for each management cell during weekly scouting visits to the fields.

Plant observations and measurement are recorded for growth stages; plant population; plant heights; insect, disease and weed presence; any unusual plant symptoms; and other variables. Aerial photos are taken of the fields at different plant growth stages to compare with observations taken on the ground during scouting.

The scientists found through this project - and research at other precision-ag sites - that sampling of larger cells (3-4 acres) usually underestimated soil-test variability and tended to overestimate nutrient levels.

In most fields, the scientists concluded, sampling by soil type would have provided similar and less costly estimates of P, K and pH than large-cell grid sampling.

"Both methods, however, grossly misrepresented soil nutrient levels compared to 1/2-acre sampling," Whigham explains. "The reliability and cost of the soil sampling are the most important factors that determine the effectiveness and economics of variable-rate fertilization."

The bottom line: Many farm fields will need to be subdivided into a few management areas in order to effectively utilize the benefit of variable-rate application of nutrients, especially nitrogen.

"The use of smart sampling techniques can significantly reduce the costs associated with soil and tissue testing," explains Whigham. "Smart sampling means only collecting samples in areas of the field which characterize the field's most important variability."

In the project, which covered farms in western, central and eastern Iowa, cyst nematodes were found only on one farm, the one in central Iowa. In northern Iowa and southern Minnesota, high-pH soil frequently occurs and results in iron deficiency chlorosis on non-resistant soybean varieties. The scientists found high SCN cyst counts were frequently associated with high-pH management cells in the central Iowa fields in this project.

Depressed yields in those areas lead the scientists to speculate that the stress effect of the two may be cumulative. The same areas of the fields were heavily infested by bean-leaf beetles and weeds, which further stressed soybean plants.

"Those producers who take advantage of these precision ag technologies and utilize the data collected will become more familiar with their fields and have a better understanding of the variability within and among fields," Whigham says. "Once the producer has a better understanding of the variability, decisions can be made about possible changes to improve profitability."