Planting Soybeans Behind the 2021 Flood--Seed Treatments and Yield

In June 2021, flooding of a large acreage in the Delta of Arkansas and Mississippi left soybean producers with a dilemma–how to manage a replanted crop that likely will result from this event.

In late spring of 2011, a similar flooding event inundated a large acreage in the lower Mississippi River Valley floodplain. The following narrative summarizes results from studies that were conducted following that 2011 flood to evaluate the effect of seed treatments and inoculants applied to a MG V variety, and the yield of MG IV and MG V soybean varieties, that were planted following this flood that occurred in the South Delta in 2011. The studies were conducted by the late Dr. Ernie Flint in cooperation with Mr. Brad Burgess, Operations Manager–MSU Variety Testing, and Mr. Philip Vandevere, County Director–Yazoo County Extension, following the 2011 flood on the farm of Mr. Bob Ragland, a soybean grower in the Satartia area of Yazoo County.

In both of the below studies, soybeans were planted on June 28. The study sites had been inundated with flood water for approximately 6 weeks to a depth of 6 to 8 feet before planting. Seeding rate was approximately 7 seed per ft. in rows spaced 18 in. apart, or about 200,000 seeds/acre. Soil moisture was near optimum at planting, and seedlings were emerged to a stand at 5 days after planting.

In one of the studies, a mid-MG V variety (REV 55R21) was planted, and seed treatments were: 1) No treatment (NONE); 2) FI–fungicide (F) + insecticide (I); 3) RM–Rhizobium (R) and mycorrhizal (M) inoculants; 4) FI + RM; 5) FI + R; 6) FI + M; 7) R only; and 8) M only. All treatments were applied to the seed at labeled rates 5 days prior to planting.

Results from this one-year study follow. Conclusions follow the table.

•    It is apparent from the data in the above table that use of a fungicide + insecticide (FI) seed treatment was valuable for ensuring the highest yield. Plants in this treatment were also the tallest numerically. This is further validation for the current recommendation to growers that seed be treated with these products.

•    The fungicide + insecticide (FI) seed treatment generally ensured the greatest plant population.

•    The study site had a high population of three-corned alfalfa hoppers soon after emergence. The insecticide seed treatment can likely be credited with suppressing this pest enough to allow for the greater plant populations, especially in the fungicide + insecticide (FI) treatment.

•    Soybean seeds planted following a flood where soybeans had been grown in previous years did not need a Rhizobium inoculant to achieve the highest yield. Thus, the existing native populations of Rhizobium in soil at this site were apparently sufficient to effect a level of nodulation that resulted in the highest yield in the fungicide + insecticide (FI) treatment following the inundating flood period.

In a second study, six varieties from each of the groupings of early and late MG’s IV and V were selected for planting. Results from this one-year study follow.

Conclusions from this study follow.

•    Mature height of varieties in the three earliest maturity groupings was less than 17 in., and this was too short to allow efficient harvest because of low pod set.

•    Number of nodes among varieties of the different maturity groupings was similar, which indicates that internodes of varieties in the earlier groupings were shorter than those of varieties in the late MG V grouping.

•    The later maturing varieties, in this case late MG V’s, yielded the most when planted late following flooding. In fact, the average yield of the best two varieties in this grouping was 48.6 bu/acre.

•    Variety selection within the late MG V grouping was important for these late plantings since yield of the best two of the six late MG V’s exceeded the average of the group by 12+ bu/acre.

•    The higher yields from the later varieties did not result from increased nodes, but were associated with taller plants of the better-adapted higher yielding varieties which enhanced harvest efficiency.

Again, these are results from only one year–2011–that was 10 years ago. However, this type of research that usually cannot be replicated in time provides valuable information for producers to consider when determining how to manage replanting behind the recent June 2021 flood.

Caveat: Some plots in this study were reduced to marginal plant populations by damage from three cornered alfalfa hoppers. This underlines the importance of more intense scouting in these late plantings to ensure that insects that do not usually cause a problem in early plantings are not adversely affecting plants in these late plantings.

This stand reduction had a more pronounced effect on the earlier maturing varieties (early and late MG IV’s and early MG V’s) because they were short and would not have compensated as well as the later-maturing varieties (late MG V’s). This was a “real” situation and does not detract from the take-home message of this study; i.e., varieties in the late MG V grouping produced the taller plants that resulted in greater harvest efficiency, and they produced a greater and very acceptable yield from these late plantings.

It is recognized that this research is 10 years old, and therefore should be viewed with that in mind. That is why it is important to use more recent tools such as SOYMAP and the latest variety trial results when making the decisions that will come with these late plantings.

Composed by Larry G. Heatherly, Updated June 2021, larryh91746@gmail.com