Soybean yield seems to be correlated to planting date. Dramatic changes and better technology have moved the soybean planting date earlier and earlier. However, what is the optimal time to plant soybean in Iowa?
Many farmers still consider May 15 as early when planting soybean in Iowa. That was the old recommendation. Dramatic changes and better technology have moved the planting date earlier and earlier every year. There are two major reasons for the earlier planting. First of all, soybean responds favorably to early planting dates if soil conditions are ideal for planting. Second, the potential risk of stand reducing, late-spring frost is offset by the opportunity to capture maximum soybean yield potential when early-season growing conditions are favorable.
Soybean can easily germinate at soil temperatures around 50ºF at 2 inches - but germination is slow
The ideal soil temperature for soybean germination and emergence is 77ºF. However, soil temperatures at a 2 inch depth in Iowa, do not reach these levels until late May or early June. Soybean can easily germinate at soil temperatures at 50ºF at 2 inches. It is not unusual for emergence to take 3 weeks at these low temperatures.
Do not plant when soils are too wet or cold
Early planting means colder soils and slower emergence than later planting, but this will in most cases not negatively influence yield. Our recommendation of optimal planting dates for soybean, if soil conditions are suitable, is during the last week of April for the southern 2/3 of Iowa and the first week of May for the northern 1/3 of Iowa (Figure 1). This is based on research conducted in Iowa over the last three years, funded through the soybean check-off and the Iowa Soybean Association. In 2003, where the yields were low we saw on average a yield loss of 0.25 bu/acre/day in Iowa. However, in 2004 and 2005 where the yields were higher the yield loss was closer to 0.40 to 0.80 bu/acre/day with some locations losing 0.90 bu/acre/day. The largest response to planting date was often found under high yield potential. In situations with lower yield potential, mainly because of poorly drained fields, high levels of soybean cyst nematode, and sudden death syndrome, the yield response to planting date was much lower.
The most important thing to consider when determining planting date is to plant into a good seedbed since little is gained by planting at extreme dates unless planting conditions are optimal. Planting into a field that is too wet early in the season will reduce emergence, plant population, and will often lead to a reduced yield at the end of the year. Research from Iowa does not support planting before the last week of April since soil conditions in most cases are not suitable.
Concerns when planting early
Concerns about low soil temperatures is the increased probability of injury to the seedling from pathogens, insects, and also the risk of frost injury to newly emerged plants since the growing point is above ground. A prime example was the growing season of 2005.
Cold soils slow root development and make the stand more susceptible to root rotting pathogens. If there is a history of seedling diseases from Phytophthora, Pythium, Rhizoctonia, or Fusarium, fungicide seed treatment is recommended.
Early planting can also coincide with high populations of bean leaf beetles. In fields with a history of of high populations of bean leaf beetles or other insects, it is recommended to manage them since they can negate the benefit of early planting. Bean leaf beetles can easily be managed by foliar insecticide or an insecticide seed treatment, so that should not hold you back from a higher yield potential. Controlling early season problems helps maximize the yield benefit from planting the last week of April through the first week of May.
A fact sheet can be downloaded on soybean planting date.