Over the last decade, researchers in the U.S. Corn Belt states have updated soybean planting date guidelines based on research showing maximum yields with late April to early May planting dates.

Additionally, researchers have designed experiments to test if management strategies such as row spacing, seeding rate and the decision to use seed treatments should change with these new planting date guidelines. However, few research efforts have been established to test if varietal maturity selection should change with the earlier planting date recommendations.

A three-year field research study as part of the DuPont Pioneer Crop Management Research Awards (CMRA) Program with Dr. Emerson Nafziger at the University of Illinois was established. The objective of this study was to test whether or not there is a need to change varietal maturity recommendations based on when soybeans are planted.

Replicated small-plot research trials were conducted from 2012 through 2014 at several DuPont Pioneer and University of Illinois research farms. In total there were 12 site-years (research locations) and 26 different Pioneer brand soybean varieties used in these trials—six site-years in northern Illinois and two in central Iowa plus four site-years in central Illinois.

There were two planting dates used at each site, an “early” (targeting late April) and a “normal” (targeting late May) planting date.

For the purposes of data analysis, the eight northern Illinois/Iowa and 4 four central Illinois site-years were grouped separately. At the northern locations, the varieties ranged in maturity from MG 1.9 to 3.8, with a “baseline” of 2.9. At the central locations the varieties ranged in maturity from MG 2.5 to 4.5, with a baseline of 3.5.

In both the northern and central regions, there were highly significant interactions between planting date and varietal maturity. So, we know that varietal maturity affected soybean yield, but we also saw that the effect of varietal maturity on grain yields was different for the early compared to the normal planting date.

Northern Region (N. Illinois/Iowa)

In the northern region, maximum yields were produced by varieties 0.4 and 0.2 maturity units later than the mid-maturity (MG 2.9) baseline varieties at the early and normal planting dates, respectively.

The interaction between varietal maturity and planting date in the northern region was the result of:

  • Higher yields with early planting for the mid- and full-season varieties, but no such increase in yields for short-season varieties planted early.
  • Those varieties that were 0.5 maturity units shorter to 1.0 unit longer than the mid-maturity baseline (2.9) varieties had higher yields when planted early, while those that were 0.5 to 1.0 units shorter than the mid-maturity varieties did not have higher yields with early compared to normal planting.

Central Region (Central Illinois)

In the central region, maximum yields were produced by varieties 0.1 maturity units longer and 0.3 maturity units shorter than the mid-maturity baseline (MG 3.5) varieties at the early and normal planting dates, respectively.

The interaction between varietal maturity and planting date in the central region was a result of:

  • Mid- and full-season varieties produced higher yields from early planting, but short-season varieties did not.
  • The fullest-season varieties lost more yield when planting was delayed than did the short-season varieties.
  • Those varieties that did not have higher yields with early planting were 0.3 maturity units shorter to 1.0 unit shorter than the mid-maturity baseline (3.5) varieties.

Conclusions

On average, yields within 1 bushel per acre of the maximum were produced by varieties over a range of about 0.9 maturity units.

Among individual site-years in the northern region, maximum yields were produced by varieties from as early as a 1.9 to as late as 3.8 relative maturity. Taken together, these two observations reconfirm that the focus should remain on selecting top-yielding genetics, and that these top-yielding varieties can be found over a modest range of maturities relative to the latitude of production.

These findings suggest that growers who are often able to plant starting in late April or early May should consider making a small shift toward varieties later than MG 2.9 in the northern region, with less response expected from doing this (relative to the baseline of MG 3.5) in the central region. Any such shift should be small, perhaps only 0.2 or 0.3 units longer (e.g. from 2.9 to 3.1 or 3.2), and the emphasis should remain on choosing top-yielding varieties, not only on changing to longer maturity.

Shorter-season varieties showed little yield increase from early planting in the central region, similar to what we found in the northern region. Though the interaction between planting date and maturity was less striking in this region compared to the northern region, fuller-season varieties lost a little more yield when planting was delayed than did earlier-maturing varieties. Data from more site-years in the central region would help strengthen these findings.