High-yield areas in a farm field this year may not be high-yield areas next year. And low-yield spots may not be the lows. So says Tom Colvin, ag engineer at the National Soil Tilth Lab in Ames, IA.

Since 1989, Colvin and his colleagues have harvested and yield-checked 224 individual areas in a uniform 40-acre, central Iowa field that has been rotated between corn and soybeans. They've used a small combine and weigh tank.

"We've found that the relationship between yields at various locations within that field are not static," Colvin reports.

"There have been certain areas of the field that have had predictable yields - high, low or average - but they make up less than 10% of the total. The rest of the field locations have not had a stable pattern and continue to change in yield rankings."

Colvin and co-workers have found as much yield variation within soil types as between them. Also, 30% of the field always shows a negative net return. But the loss is not always at the same locations, and probably not from the same causes.

How does Colvin explain this lack of consistency?

"While we expect that the spatial structure of yield is controlled primarily by soil properties, other factors such as weeds, insects, diseases and management practices also alter the structure," he says. "In addition, yearly weather patterns influence each of these factors, with one being more strongly expressed in one year and another in a following year."

Another complication, Colvin says, is that water stress - either too little or too much - can vary considerably from year to year due to rainfall patterns and differing degrees of soil drainage.

In essence, there appears to be an interaction between climate and soil that's responsible for the yield instability by location, he notes.

How should you respond?

"Accept the fact you won't have absolute consistency in yields from year to year across a field," Colvin replies. "But over a period of years, you can find areas of a field with relatively strong tendencies. Go with the odds. Use yield maps in combination with your scouting notes and adjust your input rates accordingly."

He says certain things will be obvious from the start, such as the yield-depressing effects of chronic wet spots. Those can be corrected early on.

Gary Grimm, Agri-Data Management, Catlin, IL, reports that one of his clients has made the types of adaptations Colvin suggests.

"We have been yield-mapping a 74-acre field, which is in a corn-soybean rotation, for five years," says Grimm. "The yield data have been interesting and at the same time confusing.

"It's still not possible to predict with total accuracy which areas will be high in yield and which will be low. However, we have observed trends in the field and our client has managed those trends accordingly."

For example, ponding in certain areas of the field in wet years affected yields. The farmer tiled the field and those areas are now more predictable.

The field has been soil-sampled, using GPS technology, and the farmer has based fertility management on the sampling. That, too, seems to have had a leveling effect on yield, Grimm says.

Crop consultant Jay Johnson, Beaman, IA, has worked with clients on yield-mapping since that technology was introduced.

"We definitely have seen extremes in yield from one year to another in the same areas of a field, he reports. "It has been as much as 40 bu/acre on corn."

Johnson says water-holding capacity is generally what makes the difference, especially on rolling terrain.

"If it's a dry year, the bottoms do better. If it's a wet year, the hills do better," he points out. "And since we can't predict the weather, we don't know ahead of time where the best yieldswill be.

"Tiling normally will reduce the yield variability considerably and increase yield predictability. We had a tremendous amount of tiling last fall in this area," he says.

Irrigated fields also are prone to yield variability. High- and low-yield areas are not necessarily the same from year to year, says crop consultant Mark Wooldrik, West Point, NE.

Wooldrik has seven clients who have yield-mapped a total of 10,000 corn-soybean acres for the past few years. He has scouted those fields even longer. About 40% are irrigated.

"We've not seen much repeatability in high- and low-yield spots on either irrigated or dryland fields," reports the crop consultant. "Rainfall amounts - and the way in which different soil types and terrain react to high or low moisture - have been the major factors.

"It seems that tiling will make yields more predictable, especially on irrigated fields in wet years," Wooldrik says.