Preparing for Plant 21 (PFP21): Soybean Maturity Group 

Soybean maturity groups (MG) are latitudinal zones developed to define where a soybean genetic package best fits based on photoperiod and temperature. Historically, most of NC has been slated for a MG6 zone, but we have growers interested in producing a wide range of MGs (3-8) for a variety of reasons. In a recent analysis of 877 NC Soybean Yield Contest entries, the single strongest predictor of high soybean yield was MG, with MG≤4 averaging 73 bu/A (54% of entries) and MG>4 averaging 55 bu/A (46% of entries). The majority (81%) of these analyzed yield contest entries were planted before June. Optimizing yield through appropriate MG selection is important not only for high yielding situations, but also for average-yielding situations (Figure 1). Below we will discuss some preliminary findings on optimal MG across planting date based on small-plot research conducted across seven NC locations in 2019 and 2020. This research will continue for several years allowing us to more robustly make recommendations based on yield environment, latitude, region, etc., but for now we can discuss preliminary results. 

Figure 1. Soybean MG and planting date impact on soybean yield in high and lower yield environments across NC in 2019 and 2020. 

Early Planting (before May): There is a lot of interest in early soybean planting dates as a mechanism to increase soybean yield in NC and across the US. When planting before May, our data from the past two years indicates that in high yield environments, yield was maximized with MG 3-5 varieties (81.9-84.8 bu/A) at mid-April planting dates and MG 4-5 varieties (80.4-81.8 bu/A) at late April planting dates. These results are consistent with what was derived from the yield contest analysis, showing a strong advantage of earlier maturing varieties (MG≤4) for maximizing yield in early planting, high-yield situations. In lower-yield environments, yield was highest with MG5-7 in mid-March to late April planting dates (Figure 1). 

Full Season Planting (Mid-May): In our standard mid-May full season planting dates, yields were highest with a MG4 (77.1 bu/A), MG5 (79.3 bu/A), and MG6 (78.1 bu/A) variety in a high yield situation. In the lower-yielding environments, the highest yields were observed with MG5 (60.3 bu/A), MG6 (64.2 bu/A), and MG7 (61.5 bu/A) varieties. If you look at the data from the full season (mid-May planting) soybean tests from the NC State Extension  Official Variety Testing program from 2017-2020, where many varieties are evaluated in each MG, there is a yield advantage to the earlier maturing varieties (MG4=69.6 bu/A, MG5=64.5 bu/A, MG6=61.9 bu/A, MG7/8=54.2 bu/A).

Double Crop Planting (Mid-June): In both our high yield and lower yield environments, we observed tight soybean yield ranges from our MG4-7.2 varieties, at 61.3-68.0 and 51.6-57.4 bu/A, respectively. Yields were lower with MG2-3 varieties in a double-crop situation because the very early maturing varieties have limited time for vegetative growth prior to flowering cues when planted thIS late. If you look at the data from the double-crop (mid-June plating) soybean tests from the NC State Extension Official Variety Testing program from 2017-2020, where many varieties are evaluated in each MG, the yield range between MG4-6 varieties is also tight (56.2-59.8 bu/A) with lower yields for the MG7-8 varieties (52.1 bu/A). This indicates that growers have flexibility selecting a MG to plant in a double-crop situation as long as very early (MG2-3) and late (MG>7.5) varieties are avoided. 

Late Planting (Mid-July): We have growers who plant soybeans behind corn or a vegetable crop in the same season resulting in soybeans being planted in mid-July. When planting this late, our data so far indicates that similar to a double-crop situation, growers have flexibility planting a MG4-8 variety this late. In our high-yielding locations, we have seen yields between 33.9-39.0 bu/A with MG4-8 varieties in July planting situations, with yields <33 bu/A for MG 2-3 varieties. In our lower yield environments, we have seen yields between 32.5-37.5 bu/A with MG4-8 varieties in July planting situations, with yields <30 bu/A for MG2-3 varieties. Based on these results, the majority of our growers planting this late have flexibility to use whatever varieties they have remaining seed from or can get access to at that late in the season. 

What about seed quality? In our high-yielding situations, we see a clear yield advantage to planting earlier maturing varieties. However, this can coincide with some seed quality issues. The early planted, earlier maturing varieties are coming into maturity earlier in the season when hot and humid temperatures often coincide with wet weather. Our data over the past few years indicates that seed damage and purple seed stain are most often encountered in soybeans planted prior to May for MG2-4 varieties. Across planting dates, typically damage and purple seed stain have been low for MG≥5 varieties. To capitalize on the yield benefits of earlier maturing varieties, growers must be committed to timely harvest and season-long scouting in an effort to minimize encountered seed quality issues.