Assistant Professor, Soybean Extension Agronomist

Ever since I moved to Iowa in 2003 I have worked extensively on the importance of soybean growth and development and its correlation to yield. Through the soybean checkoff and the Iowa Soybean Association we have been able to address what it takes to raise high yielding soybean, and where we need to focus our research in the future to move the yield potential up even further. One thing that has been interesting to work on is the importance of early vegetative growth. Previously, we had never really paid much attention to what is happening prior to flowering, but that seems to be a mistake.

I do not believe that there are any farmers in Iowa that have not heard me talking about the importance of soybean planting date. And there is a reason. Soybean planting date is in some way our “soybean yield stimulus package”. By planting early we will stimulate the plant to achieve a higher yield potential. Unfortunately, we are not doing a good enough job. Our data shows that we need to be done planting soybean anywhere in Iowa by May 15 but based on USDA and the planting progress data over the last 5 years we often only have 50% of our acres planted by that day.

One of the major reasons why early planting is so important is we want to be sure the plant intercept as much solar radiation as possible since a soybean plant is a source limited plant. That means there is a close relationship between seasonal canopy photosynthesis and yield. So, by shorting our growing season we will reduce the yield. That is also why I do not recommend you to plant in 30 inch rows – the canopy closure takes too long time if there is some kind of a stress.

Thus, we want to plant early so we can get an early start and capture as much sun light prior to flowering as possible. This will help you build the foundation fast growing plant that we know will have less aborted flowers. Any sunlight that hit the ground is wasted and will not contribute to any yield. Early season stresses are therefore extremely detrimental to soybean yields. Weed competition is probably our biggest headache and that is why I am a huge believer in the use of a preemergence herbicide in our soybean production system to reduce the risk of early season weed competition. Weeds compete directly with soybean for nutrients, moisture and light, thus reducing yield.

In addition to increasing the seasonal canopy photosynthesis we will also add more vegetative nodes on the main-stem. On average we will get a new node for every four to five days, so planting the last week of April (or first week of May in northern Iowa) can give us two to three more nodes compared to planting in mid to late May. Many farmers have told me over the years that they cannot see a difference between planting early vs. planting late when they go out in their fields in late June. Their beans planted late are as tall as the ones that they planted early. That is true. Late planted beans often will get longer internodes from the warmer temperatures in late May, equalizing the height differences but they have less nodes and therefore less areas where they can set pods.

Another thing that cannot be mentioned enough is that variety selection is even more critical when you are planting early since you will be planting into cooler and wet soils that will favor some diseases. Planting a variety without any yield information and disease ratings is therefore very risky since early planting will make the plant more vulnerable to diseases such as sudden death syndrome but also to many seedling diseases. However, it should not hold us back since we can manage them very easily by variety selection and the use of seed treatments. Planting early, just to plant early, is not recommended. Early planting to achieve higher yield therefore takes more time and management. For more information on soybean management practices please visit my Web site at www.soybeanmanagement.info.

Palle Pedersen, palle@iastate.edu

*Content Funded by the Soybean Checkoff