"In evaluating yield maps, it's important to look for repeating or definite patterns rather than focus on small differences," says George Rehm, University of Minnesota extension soil scientist.

"Unless there is an obvious reason for yield differences, too much attention to the small differences can lead to errors in management decisions," Rehm cautions.

He says straight-line patterns of yield differences that occur in the direction of planting are likely the result of management. The cause may be planting date differences, change in hybrid or variety, skips in herbicide application or soil compaction.

Straight-line patterns that occur across the rows may be the result of tile lines, old traffic patterns, previous manure applications, old fencerows or buried cables.

Irregular yield patterns may be due to natural factors over which the grower has little control. These include changes in soil type, insect damage, differences in disease pressure, or non-uniform manure application.

If yield differences appear on the map as irregular patches, the cause could be changes in soil type, natural drainage patterns, weed infestations, insect damage, disease pressure or major changes in fertility.

Collect yield maps from a field for more than one year before using the information to make a management change, Rehm says.

"Keep an open mind and be willing to consider several factors that could cause yield differences," he says.