Three management decisions that need to be optimized
Palle Pedersen, Soybean Extension Agronomist
Originally published in the Iowa Soybean Review, March 2007

It has now been three years since the Iowa Soybean Association and I initiated the checkoff-funded “Soybean Yield Potential in Iowa” project. We just finished the last experiments and are wrapping up the final report.

The major reason why we initiated the project is because our soybean yield over the last decade has not consistently met our expectations. Many believed we were hitting a “yield plateau,” and the glyphosate tolerant soybean varieties were the main cause of this. That is not the case. Glyphosate tolerant soybean do not influence the genetic yield potential and there is not a yield plateau for soybean. Another reason for the initiation of the project was to determine the yield potential in Iowa – what is realistic and where should we focus our management decisions?  Based on this research we found three management decisions that should be optimized to maximize yield potential every year.

Planting date. I changed our planting date recommendation two years ago in Iowa based on the information I gathered in this and other projects. Our new recommendations are based on three variables that do not take soil temperature into consideration at all. Soybean planting should be initiated based on the calendar date (April 25 for the southern 2/3 of Iowa and May 1 for the northern 1/3 of Iowa), seedbed conditions and the weather forecast for the next couple of days. Since 2003, I have conducted 24 experiments across Iowa and there is a 79 percent probability that you will get a higher yield by planting earlier and following our new recommendations.

The disadvantage of planting date is, of course a late spring frost. That is why we should not plant earlier than recommended to minimize that risk. Another reason is variety selection is even more critical when we plant early since many pathogens prefer cool, wet, and high yielding environments. Finally, the thing that most growers in central and southern Iowa are facing with early planting is bean leaf beetles. The bottom line is that they should not hold you back from planting early, since bean leaf beetles are very easy to manage. You can easily pick up around 1.5 bu more per acre by planting earlier.

Row spacing. Since 2003, I have conducted 19 row spacing studies across Iowa, and on average we see a 4.3 bu per acre advantage of using 15-inch row spacing compared to 30-inch row spacing. That follows the trends and recommendations from our neighboring states that anything less than 30-inch rows will yield greater than 30-inch row spacing. Despite this, close to 55 percent of our acres in Iowa are still planted in 30-inch row spacing. We are currently finishing up an economic analysis comparing a split-row planter to a regular 30-inch row planter and the bottom line is that you will be surprised how few acres it takes to pay off a split-row planter with our current price of soybean.

There have been cases where we have not been able to find the strong yield advantage to narrow rows. Often it is related to variety selection, like planting a soybean cyst nematode (SCN) susceptible variety in an environment with SCN. Often the stresses on the canopy from the pathogen will outweigh the canopy differences by planting in narrow rows. Small responses are found in very high yielding environments (more than 70 bushels) since you will often get a quick canopy development no matter what row spacing that you are using.

SCN management. Start managing SCN as soon as they have been confirmed in your field. The threshold for managing SCN today, where there is no yield drag between SCN resistant and susceptible varieties, is 1 egg per 100 cc. Our top yielding varieties were all SCN resistant since we had SCN at all locations. We also saw increased root development using SCN resistant varieties in addition to lower levels of seedling diseases and improved yield stability across locations.