Tips for planting soybean 2006
Palle Pedersen, Soybean Extension Agronomist
Originally published in Iowa Farm Bureau Spokesman, February 8, 2006

Sitting here watching the snow, it is now time to make a “master plan” for the 2006 growing season. Over the last few years, we have conducted a lot of research throughout Iowa with support from the soybean checkoff and the Iowa Soybean Association to investigate what the yield potential is in Iowa and what we need to do to maintain it.

Soybean varieties have a genetic yield potential greater than 100 bu/acre. However, many farmers struggle to achieve yields greater than 30 to 35 bu/acre and question if soybean should be included in their future crop rotation.

This supposed “yield barrier” is largely the result of an oversimplified management system in part because of farm size that neglects basic agronomic principles. This oversimplification has been possible in our corn production systems where modern corn hybrids can handle many of the stresses through the season. Taking this approach to soybean production will, in many cases, limit the yield potential to 30 to 35 bu/acre during a normal growing season.

Understanding how a soybean plant develops can provide insight into selection of management practices that should lead to maintaining yield potential. Simply putting more inputs into a management system will not improve yield if the crop does not get the right start.

Here are a few guidelines to help optimize the basic agronomic practices
Variety Selection. Variety selection is the foundation to develop an effective and successful management plan. Despite the importance, variety selection is often the least considered management decision a farmer makes each year. The reason is that it is complicated, since many variables need to be considered and it is impossible to find a variety that will protect us against all yield limiting variables that we have out there and still achieve the high yield potential. Many farmers, therefore, give up and ask others, who do not understand the field, to select their varieties. For that reason, a significant yield loss can occur even before the season starts.

I could talk a lot about how to select a variety. In general, I would recommend a variety which is a “stable” high yielding variety that has been tested independently in replicated trials at numerous locations. It is better to get data from multiple years, but it is not easy since many varieties are only sold for 3 to 5 years and sales of a particular variety might already be declining by the time multiple-site and year data are available to the grower.

Selecting soybean varieties that have resistance or tolerance to major soybean diseases can be an effective and economical method to control diseases and maintain yields.

Since we can not get protection against every single pathogen then the question will be what should we focus on? That is easy.

Soybean cyst nematode (SCN) is the most yield-limiting pathogen in Iowa. Every year it costs us more than 50 million bushels in yield loss in the state of Iowa alone. Today, it is found in 75 percent of Iowa fields. The question that I always get is "when should we start to manage it and what is the threshold and is there a yield drag with SCN resistant varieties?"

Soybean cyst nematode should be managed as early as possible since it is easier to manage a low population of SCN than try to reduce a large population. Is there a yield loss with SCN resistant varieties? The answer is no. In our high yielding studies, our top 5 yielding varieties are all SCN resistant and that research is also conducted at locations where the level of SCN is very low. For more information on selecting the right SCN resistant variety, one with high yields that also drive your SCN population down, go to the ISU SCN variety trial website.

Planting. Many believe that planting date for soybean is not as important as it is for corn. That is not the case. There is a yield benefit for planting early despite cold soil temperatures that slow plant growth during the seedling phase. The yield response is a result of increased seasonal canopy photosynthesis, more nodes on the main stem, and a potential of early flowering. 

If soil conditions are suitable, soybeans should be planted during the last week of April for the southern two-thirds of Iowa and during the first week of May for the northern one-third of Iowa. In 2003, we saw on average a yield loss of 0.25 bu/acre in Iowa when planting was delayed by one day after the optimum window. In 2004 and 2005, it was closer to 0.60 bu/acre/day. Some locations lost as much as 0.90 bu/acre/day from delayed planting. Planting soybeans early, however, is not advisable until you have a good seedbed. “Mudding-in” soybeans just to plant early and causing soil compaction outweighs any benefit of early planting.

There is a risk associated with early planting, and this risk should be evaluated prior to planting. However, with the responses that we have seen in Iowa over the last 3 years, it should be a risk that we should consider. The most important thing is not to plant earlier than the dates that I recommended above. You are not going to gain anything planting too early and you will increase your risk of both replanting and a yield loss.           

Planting depth should be from 1 to 1.5 inches when planting early. Planting too deep when planting into a cold seedbed will reduce your stand significantly. It will always be good advice to go out and check your planter settings every hour.

Finally, seed quality may vary a little bit in 2006. We don’t know for sure yet, but please be sure to factor in seed germination variability when you calculate your seeding rates.