A multidisciplinary approach

Within-field yield variability is readily observable on most crop producers' yield maps. With this evident variability comes the question, "What is the cause?" Once the relevant crop growth factors are determined spatially, economical assessments of site-specific management practices can be made. The On-Farm Site-Specific Crop Management for Iowa project, funded by the Iowa Soybean Promotion Board, is addressing these issues.

For the 1997 season, work continued on two 50-acre plots in the Clarion-Nicollet-Webster soil association in Boone County. Also in 1997, four 30-acre plots were added. Two of the additional plots are in the Dinsdale and Kenyon soil association in Linn County, whereas the other two plots are in the Marshall soil association in Carroll County. All plots are in a soybean-corn rotation and have been divided into half-acre management cells. A selection of current results from this multidisciplinary project follows.

Boone County Plot

Boone County plot map

Crop Growth and Development

Yields have been determined for each half-acre cell. The 1997 soybean yields differed by 10 bu/acre among half-acre cells in the Linn and Carroll County plots. The Boone County plots contained greater variation in soybean yield with a range of 48 bu/acre. In the Boone County plot, two areas of 12- and 17-half-acre cells contained soybean yields of 30 bu/acre or less. The ranges in corn yields among half-acre cells for 1997 were 51, 43, and 71 bu/acre at the Linn, Boone, and Carroll County plots, respectively.

Weekly scouting data for each half-acre cell provided timely accounts of crop growth and development. Harvest plant populations varied by as much as 104,000 plants per acre in soybean plots and 10,000 plants per acre in corn plots. Because plant populations were adequate, the variation in populations appeared to have little impact on yield.

Heaviest weed infestations at harvest occurred in areas with the lowest yields. These areas were similar to those in which weed infestations at crop emergence occurred. For the Boone County plots, heavy weed densities often arose where iron chlorosis was prevalent in soybeans.

The greatest reduction in crop plant height was in areas of course-textured soils where soil moisture was limiting. Plant stage development sometimes lagged in these areas but was mostly uniform within fields.

Using hand-held global positioning system equipment, we recorded areas with iron chlorosis. In the Boone County soybean plot in 1997, those areas with iron chlorosis yielded only 54 percent as compared with the rest of the plot. Iron chlorosis occurred primarily on calcareous soils.

Interrelated Factors

Yearly variations in rainfall and spatial variations in soil texture appear to be the major contributors to within-field soybean and corn yield variability. Although we have limited capability to change these growth factors, their impact on within-field yield variability can be better understood. This, in turn, will allow better determination of the extent that other variable crop growth factors can be managed to increase crop production profits.

The multi-disciplinary approach for this project provides the opportunity to view the various crop growth factors as they interrelate. At the Boone County plot, several crop growth factors appear to be hindering crop production. Many of these factors are occurring in similar areas of the plot. For example, the spatial arrangement of weed infestations might be a result of where iron chlorosis occurs, whereas the plant symptoms of iron chlorosis are primarily found on calcareous soils. Through understanding primary factors and subsequent reactions in these spatial areas, integrated crop management decisions can be better determined.

This article originally appeared on pages 9-10 of the IC-480 (4c Precision Ag Edition) -- April 9, 1998 issue.

Updated 04/08/1998 - 1:00pm