Is Brazil's competitive advantage in soybean production driving some U.S. farmers to reduce the amount of soybean acres in their crop rotation? Probably not.

However, it's easy to argue that agronomics and, ultimately, soybean yields are causing many farmers to question the proportion of acres devoted to soybean production on their farms. A 50/50 corn-soybean rotation may no longer be the best answer for some farmers.

A quick glance over the last several years clearly shows that soybean yields have not been keeping pace with corn yields. The trend line for corn yields is increasing at a steeper slope than soybeans. If you use the rule of thumb of a corn/bean yield ratio of 3:1, in states like Iowa that ratio has been widening. (See graphs, pg. 36)

The yield scales are on a 3:1 ratio and if the corn and bean yields were maintaining that ratio, the yield lines would be on top of one another. Iowa's corn and bean yields closely followed that pattern until the last four years.

As farmers, we keep reading about the competitive advantage South America has over the U.S. in soybean production. In general, this may be true.

It's been suggested, for that reason, that the U.S. should concentrate on corn production instead of soybeans. What is going to drive more farmers to plant a higher percentage of corn is yield.

You can do all kinds of economic analysis. But increasing yield is the key to profitability. It's the one factor that has the most influence on the amount of revenue generated per acre and the cost of production on a per bushel basis.

There are a handful of factors that have negatively impacted soybeans in the U.S. over the last five years. They include soybean cyst nematodes (SCN), viruses, Sudden Death Syndrome (SDS), and bugs. I would argue that the introduction of Roundup Ready soybeans has also slowed the genetic advancement for yield in the soybean industry.

The biggest yield robber in soybeans is SCN. It has been estimated that SCN has reduced yield in recent years by 5-10% of the total crop. In those areas where cyst populations have skyrocketed, the yield impact is even more severe.

SCN continues to increase in severity in those fields where they are present. Even with a heightened awareness of planting SCN-resistant varieties on infested fields, the SCN problem does not go away, as resistant varieties still allow for some reproduction of cysts.

Recent research is also showing an interaction of pH and SCN. The cysts seem to thrive under high-pH conditions and rapidly expand populations.

In fields with both of these problems, bean yields are struggling or going downhill. Under lower pH, SCN seems to be less of a problem.

Is keeping soil at a lower pH a way of preventing SCN populations from expanding? Are the low pH soils of Brazil a natural defense mechanism for them in preventing SCN from being a yield robbing problem on that country's continuous soybean fields? See research on the Web at http://nutcracker.ae.iastate.edu/Research/Yields_1_Project/body_yields_1_project.html.

In the last three to four years, bean leaf beetles have had an impact on the Midwestern soybean crop to varying degrees. Research has discovered that the defoliation these bugs have been doing isn't causing as much of a problem as the virus they transmit.

Prior to the onslaught of the bugs, viruses were an insignificant disease problem in soybeans. It is estimated that viruses can reduce yields up to 30% or 10-20 bu/acre

In the last couple of years, soybean aphids have also started showing up across the Midwest. The aphids were first found in the eastern Corn Belt states and have quickly progressed west. The aphids transmit soybean mosaic virus while the bean leaf beetles transmit bean pod mottle virus. Viruses have quickly moved into the top four for disease problems and are estimated to be the second biggest yield robber in Iowa behind SCN.

The challenge with the bugs in soybeans is that they should be managed more for virus control than for defoliation. Most treatment threshold levels have been established based on damage done by defoliation and don't apply if virus transmission is the target.

Aggressive spraying of emerging soybeans for bean leaf beetles, along with treating the first generation in early July, is essential to controlling the bean pod mottle virus. If spraying for insects doesn't occur until August, the damage has already been done by the viruses.

The virus decreases yield and results in green-stemmed plants at harvest. Managing for virus requires a totally different strategy to maintain yield potential than treating for defoliation and pod feeding.

Diseases that once were not a problem seem to becoming more widespread and occur more frequently. It was not too long ago that SDS was a Southern problem. Now SDS can be found throughout much of the Midwest.

In 2002, SDS showed up in a good portion of Iowa while previously it had been isolated to small pockets. SDS can be a devastating disease in those pockets of the field where it shows up early enough in August to kill off the plants before seeds have a chance to fill.

These disease and pest problems facing soybeans are difficult to manage. To control bean leaf beetles, it takes one to three applications of insecticide if you want to avoid yield losses from defoliation and virus transmission.

There are no varieties with genetic resistance to diseases like SDS, only differing levels of susceptibility. There is no easy fix for high-pH soils. If you have them you are stuck with them. Once cysts infest your fields, they are there for a long time.

Roundup Ready (RR) soybeans have not necessarily been a yield-enhancing trait for soybean production. The insertion of the RR gene did not increase yield potential, nor does it help eliminate yield robbing pests or diseases.

In corn, the Bt gene has been a yield-enhancing trait by eliminating the yield-robbing impact of the corn borer. In the presence of corn borer pressure, the Bt gene can easily add 8-12 bu/acre vs. the non-Bt version of the hybrid.

The Bt gene has helped corn maximize its yield potential by allowing the plant to maintain its health as late into the season as possible.

The RR gene in soybeans may increase yields in those fields where weed populations have been so heavy that they were cutting into the yield potential. However, for those fields where weeds have been kept in check, the RR gene provides an excellent tool to simplify weed control management.

I would argue that quick adoption of RR soybeans has caused soybean yields to temporarily flatten out while the gene was quickly incorporated into seed companies' product lines.

Instead of developing new soybean varieties that had a yield advantage over the existing product line varieties, breeding efforts went into adding the gene to existing varieties. Instead of making incremental gains in yields, the initial varieties were maintaining yield while adding the RR trait.

How much did we lose in genetic gains in yield over the last five years as soybean product lines were converted to RR?

So is the miracle crop, soybeans, losing some of its luster? In some areas of the U.S. it is and is facing some difficult problems with no easy management solutions other than to switch to alternative crops or rotations.

If acres shift to corn or other alternative crops, it will not be because of South America's competitive advantage, but soybeans' lack of competitive advantage in yield relative to other crops.