Preparing for Plant 21 (PFP21): Soybean Planting Date 

Using the optimal soybean planting date is critical to maximize yield for our full season soybeans. Earlier planting dates typically result in more main stem nodes; more nodes result in more flowers and pods and subsequently higher yields. This national publication on soybean planting dates thoroughly explains the benefits of earlier soybean planting. There is much energy across the United States on planting soybeans earlier than historical for the associated yield benefits. In a recent analysis of 877 North Carolina Soybean Yield Contest entries, one of the strongest predictors of high soybean yield was the use of planting dates earlier than mid-May. 

What is the optimal soybean planting date in North Carolina? The optimal planting period in North Carolina was historically defined as May 1 to June 10, however as we shift to using earlier maturing varieties and more aggressive management strategies we need to redefine the optimal planting period. It will take several years of research to do this. The Soybean Extension Program started this research in 2019 investigating the impact of soybean planting date on soybean yield and quality and will continue to do so for several years. We have data from 2019 and 2020 from 8 North Carolina locations that include in 2019 Currituck Co., Hyde Co., Sampson Co., Union Co., and Yadkin Co. and in 2020 Beaufort Co., Robeson Co., and Rowan Co. We had variable response to planting date across years/yield environments, so we broke this data into high (Beaufort Co. 2020, Currituck Co. 2019, Rowan Co. 2020) and lower (Robeson Co. 2020, Sampson Co. 2019, Union Co. 2019, Yadkin Co. 2019) yield environments. In the higher yield environments, soybean yield was highest at the earliest planting dates (mid-April) for maturity group ≤5 and declined as planting date was delayed (Figure 1). For the lower yield environments, yield was highest for soybeans planted from late April to mid-May regardless of maturity group (Figure 1). There was some yield penalty for late March to mid-April planting dates in the lower yield environments (Figure 1), which is likely driven primarily by the hot, dry May/early June in 2019. Regardless of high or low yield environments, there were benefits of planting soybeans by mid-May. Over the past five years, soybean acres planted behind wheat have comprised 15-35% of our overall soybean acres, however, our planting progress by the end of May has only been 50-60% of our soybean acres (USDA-NASS). This means a lot of full-season soybeans could benefit from earlier planting dates. Rotational complexity does play into decision making about soybean planting date, but soybeans are less sensitive to most of our row crops planted into cool conditions and for this reason, growers may consider planting soybeans earlier than some of the other crops in their rotation for the associated yield benefits. We had drastically different conditions in May 2019 (hot and dry) and May 2020 (cool and wet) which inevitably influence the impact of planting date on soybean yield across the two years and underscores the importance of doing this research over multiple years before redefining the optimal planting period for soybeans in this state.

Figure 1. Impact of planting date and maturity group at high (Beaufort Co. 2020, Currituck Co. 2019, Rowan Co. 2020) and lower (Robeson Co. 2020, Sampson Co. 2019, Union Co. 2019, Yadkin Co. 2019) yield environments. 

What about seed quality? When maturity group ≤5 soybeans are planted prior to mid-May, we can run into seed quality issues as these soybeans approach maturity. We are in the process of investigating how we can minimize seed quality issues for earlier maturing varieties planted before mid-May through management, but this is a concern with the weather conditions encountered as these maturity group and planting date combinations approach maturity in hot and humid months. In our research trials, the earlier maturing varieties at planting dates mid-May or later had minimal seed damage and purple seed stain. Seed damage and purple seed stain was generally low for MG≥5 across all planting dates. 

How early is too early to plant? Soybeans will germinate at temperatures just below 50°F, however, when planting into cool soils (50-60°F), emergence will be delayed and can take 15-25 days. This slow emergence can make the plants more susceptible to seedling diseases. Freeze damage can occur on emerged soybeans if emerged tissue is exposed to 28°F or below for ≥4 however even if tissue is burned off at the growing point that plant does have an opportunity to recover from freeze damage at the meristems. This video from the University of Wisconsin does a good job discussing this. Our data so far does not show yield benefits of planting prior to mid-April, however, we need more years of research to confirm if these findings are consistent across environmental conditions. Beyond agronomic considerations, if you want RMA replant coverage for your soybean crop, depending on your location in the state, your replant coverage at current does not start until April 15-21. 

What other things should I consider if planting early? In 2019 and 2020, the Soybean Extension Team investigated the necessity of using fungicidal seed treatments at planting dates earlier than mid-May. We had variable results over the two years based on differing weather patterns, varieties used, field history, and disease pressure, but on average over the two years, there was value in protecting both soybean stand and yield with a fungicidal seed treatment at planting dates earlier than mid-May. We have a lot of data generated across North Carolina at later planting dates (Mid-May through early July) that would indicate a fungicidal seed treatment is not necessary to protect yield at these later planting dates. A PFP21 post on soybean seed treatments is forthcoming. If we are going to push soybean yields higher, we need to be committed to season-long soybean scouting. If you are planting soybeans earlier than most other growers in your environment, you may see early-season insect pressure you might not encounter in later planted soybeans. In our research plots over the past few years, we have seen bean leaf beetle and kudzu bug pressure in the earliest planting dates at some locations that we did not see in later planting dates. When soybeans are planted earlier, they are more likely to encounter hot and humid temperatures during late reproductive development that can intensify seed quality issues. We need to be scouting for late-season foliar disease and insect pressure in an effort to minimize the impact of these seed quality issues. 

If you are interested in predicting when a certain maturity group will move into reproductive development based on soybean emergence, there is an excellent tool called SoyStage developed out of the University of Arkansas that can be used to make these predictions. Remember that depending on soil conditions at planting, soybeans can emerge in several days or up to a few weeks after planting. 

We will discuss the optimal maturity group to use across the diverse soybean planting dates employed in North Carolina in our PFP21 topic next week.