Soybeans can compensate for in-season stresses, however, in general, stress has a negative effect on yield potential.
Cutting, Cobra® herbicide and dicamba herbicide treatments frequently reduced yields in these trials.
Crop stage and environment play a critical role in the ability of soybeans to overcome stress events.
Soybean Growth Response Influence on Yield Potential
The apical meristem or growing point directly influences the growth habit and architecture of a soybean plant. Soybean plants will predominately produce a single main stem from the original apical meristem if not damaged. Any environmental or production practice that stresses or stunts the plant can affect its growth habit. Soybeans can compensate during stress or damage to the apical meristem by producing branches at axillary buds which can produce trifoliate leaves and reproductive structures. Branching could potentially have positive or negative influences on grain yield that is hard to quantify and predict.
Soybean Growth Habit Trial
The NK Agronomy Research team conducted trials at Blue Earth, MN, Grundy Center, IA, Malta, IL, Slater, IA, and Waterloo, NE to evaluate how changes to soybean growth habit can affect yield potential. Either soybean variety NK21-C2E3 brand or NK30-B2E3 brand was planted depending on the geography at a target rate of 140,000 seeds/acre. Five treatments specifically chosen to alter plant growth or physiology were compared to an untreated control:
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Figure 1. Soybeans rolled with drum roller at Slater, IA (top) and ran over with the tire of a utility vehicle at Malta, IL (bottom). 1.Rolling at V1: Soybeans were rolled at the V1 growth stage using either a drum roller or tires of a utility vehicle (Figure 1). Rolling soybeans to press rocks into ground and level soil surface has become a common practice in areas of the Midwest with a goal of improving soybean harvestability. Rolling soybeans can also lay plants over, delay growth and in some cases break the main stem resulting in branching.
2.Cobra at V3:Cobra was foliar applied at rate of 8 oz/acre combined with 2 pts/acre of crop oil at 20 gal/acre spray volume. Cobra is a PPO Inhibitor herbicide which may result in soybean injury (burn or necrotic leaf tissue) when applied postemergence. Injury symptoms are typically worse when applied during hot and humid conditions in conjunction with crop oil. In some cases, the meristematic tissue can become injured resulting in branching.
Figure 2. Soybeans with visual leaf burn from an application of Cobra. 3.Dicamba at V3: Dicamba was foliar applied to non-dicamba-tolerant soybeans at 1/1000th the labeled rate for effective weed control to simulate off-target movement. Herbicides containing the active ingredient dicamba are considered synthetic auxin herbicides. Auxin is one of the five major plant growth regulator (PGR) groups. When non-dicamba-tolerant soybeans are exposed to dicamba, injury symptoms such as cupping and strapping of newly emerged leaves occurs. Depending on the degree of exposure, plant height reductions and injury to the growing point can occur, creating branching.
4.Cut at V3: Scissors were used to physically cut the main stem off directly above the unifoliate leaves to mimic hail or deer damage. Removing the growing point causes the soybean to branch (Figure 4).
5.Ascend2® at V3 and R1: Ascend2 was foliar applied at a rate of 3.4 oz/acre at V3 and again at R1. Ascend2 contains three categories of PGRs (cytokinin, gibberellin, and abscisic acid) that effect plant hormones to stimulate plant growth.
Visual In-Season Growth Habits
Malta and Waterloo used a utility vehicle to mimic rolling soybeans which tended to provide more down pressure than the drum roller used at Blue Earth, Grundy Center, and Slater. Plants were laid over after rolling but gradually sprung back up during the following days. There were no broken plants using the drum roller. Using the utility vehicle method resulted in an average of 20,000 plants/acre that were broke off and dead
Figure 4. Soybean plant 4 weeks after being cut at the V3 growth stage resulting in a “Y-branch” at Malta, IL in 2023. Final stands were still above 100,000 plants/acre so yield potential was not affected. Around the R1-R2 growth stage the plots that received the rolling treatment looked visually the same as the untreated plots.
Depending on the location, the severity of the Cobra leaf burn symptoms differed.
Figure 3. Stunted soybeans from either dicamba or being cut caused a longer duration before canopy compared to the surrounding untreated plots at Malta, IL in 2023. Brown and necrotic areas of the leaf were visual the next day following the application (Figure 2). Plant growth was slightly delayed for a couple days while the soybeans recovered from the stress. After 4-6 days new trifoliate leaves emerged with #06936E tissue. By the R1-R2 growth stage, plots that received Cobra were visually the same as untreated plots.
Figure 5. Plots labeled with dicamba or cut treatment for each rep at Slater, IA in 2023. Plots are 4 rows wide. Plants in the dicamba and cut treatments were stunted and behind in growth all season resulting in delay in maturity compared to other treatments.The low dose rate of dicamba significantly stunted plant growth. These soybeans were behind in height and maturity all season long. Dry weather following the application at Blue Earth, Grundy Center, Malta, and Slater delayed the ability of the soybean to grow out of the stress. At Waterloo, plants were stunted but adequate moisture and growing conditions accelerated the recovery process. Nodes were stacked closer together on the main stem and it took weeks longer for stunted plants to canopy compared to untreated plots (Figure 3).
Similar to the application of the dicamba, physically cutting off the main stem of the soybean plant directly above the unifoliate leaves at the V3 growth stage resulted in significant stunting. The original growing point was removed resulting in two new main stems growing from apical buds creating a “Y-branch” (Figure 4). These plants were behind in biomass and maturity the entire growing season compared to untreated plants (Figure 3).
No visual above-ground plant growth effect was noticed in the soybeans that received multiple applications of Ascend2.
Figure 6. Difference in soybean senescence between untreated (left) and cut soybeans (right) at Slater, IA in 2023.Yield Results
Soybeans at Malta, IL experienced heavy pressure of white mold. The level of severity was spatial dependent within the trial leading to variability in yield. Despite the variability in yield, plots at Malta that received the dicamba or cutting treatment tended to increase yield (Table 1). Soybeans in these plots were significantly behind in growth all season and canopied much later than untreated plots. The delayed canopy increased air flow within the rows and slowed the development of white mold. White mold presence in these plots were visually lower than plots that received the other treatments. In this unique situation, the typical negative effects of delaying soybean development was beneficial by reducing the severity of white mold in these plots ultimately resulting in higher yield. The rolling and Cobra treatments also stunted soybean growth, however, the delay in canopy was not enough to effect white mold development.
Malta, IL was excluded from the multi-location analysis due to the unique environment of heavy white mold pressure. When averaged across Blue Earth, Grundy Center, Slater, and Waterloo, soybeans treated with Cobra, dicamba, or were cut significantly reduced yield by 2.4, 4.2, and 7.1 bu/acre, respectively. Applying the low dose rate of dicamba or cutting off the growing point of a soybean plant stunted the development of the plant likely reducing pod number and seed weight. These plants matured later than soybeans in all the other treatments (Figure 5 and 6).
Soybeans that received the rolling treatment were able to recover quickly, even the more aggressive rolling method at Waterloo, NE did not impact yield potential (Table 1). Rolling soybeans early at the V1 growth stage is optimal for plants to spring back and recover. Rolling too early during emergence can break off the cotyledon and kill the plant.
Ascend2 applied at V3 and again at R1 did not significantly affect yield at any location. Only at Blue Earth, MN did Ascend2 tend to increase yield by 1.3 bu/acre (Table 1).
At Waterloo, NE, the low dose rate of dicamba significantly increased soybean yield compared to the untreated (Table 1). Under certain environmental conditions dicamba injury during early growth stages can increase yield potential, however it is an uncommon occurrence. The vast majority of peer reviewed research studies show a negative or no yield response when a low dose of dicamba is applied to non-tolerant soybeans during early vegetive growth (Griffin et al. 2013, Foster et al. 2019, Osipitan et al. 2019, and Scholtes et al. 2019). However, there are cases when a yield increase has been observed in certain environments (Castner et al. 2021a, Castner et al. 2021b, and Meyeres et al. 2021). In these published articles, most concluded that as soybeans reach mid-vegetative into reproductive stages, yield decreases from dicamba exposure becomes greater. A study conducted at the University of Missouri found that irrigating and keeping adequate soil moisture levels significantly reduced injury symptomology and yield losses following a low dose rate of dicamba (Dintelmann et al., 2022). Waterloo, NE was the only location in this study that was irrigated, likely accelerating the recovery time of the dicamba injured soybeans. It is important to remember applying dicamba herbicides off label is illegal.
Summary
Soybean is an indeterminate plant meaning the plant continues to grow vegetatively while simultaneously in the reproductive stages. The indeterminate nature of soybeans is why yield is determined over an extended period of time within the growing season. Flowers and pods can abort while new flowers and pods are formed. The ability to form new flowers and pods makes soybeans able to compensate or overcome stresses throughout the season.
Results from this study demonstrate that the environment and growth stage play a critical role in the ability for soybeans to overcome different levels of stress. In general, most stresses have a negative impact on yield. However, given the correct environment and level of stress, soybeans can compensate for early-season stress and potentially increase yield potential.
Reference:
Castner, M., J. Norsworthy, L. Barber, T. Roberts, and E. Gbur. 2021a. Interaction of contact herbicides and timing of dicamba exposure on soybean. Weed Technology, 35(5), 725-732. doi:10.1017/wet.2021.37.
Castner, M. C., J.K. Norsworthy, T. Barber, E. Gbur, and T. Roberts. 2021b. Does dicamba exposure elicit a hormetic response in sensitive soybean? Crop, Forage & Turfgrass Mgmt. 7:e20121.
Dintelmann, B., S. Farrell, and K. Bradley. 2022. Influence of recovery treatments on dicamba-injured soybean. Weed Technology, 36(1): 1-7. doi:10.1017/wet.2021.63.
Griffin, J., M. Bauerle, D. Stephenson, D. Miller, and J. Boudreaux. 2013. Soybean Response to Dicamba Applied at Vegetative and Reproductive Growth Stages. Weed Technology, 27(4): 696-703. doi:10.1614/WT-D-13-00084.1.
Foster, M.R. and J.L. Griffin. 2019. Changes in Soybean Yield Components in Response to Dicamba. Agrosystems, Geosciences & Environment, 2: 1-6 190026.https://doi.org/10.2134/age2019.04.0026.
Meyeres, T., S. Lancaster, V. Kumar, K. Roozeboom, and D. Peterson. 2021. Response of non-dicamba-resistant soybean (Glycine max) varieties to dicamba. Weed Technology, 35(5): 718-724. doi:10.1017/wet.2021.4.
Osipitan, O.A., J.E. Scott, and S.Z. Knezevic. 2019. Glyphosate-Resistant Soybean Response to Micro-Rates of Three Dicamba-Based Herbicides. Agrosystems, Geosciences & Environment, 2: 1-8 180052.
Scholtes, A., B. Sperry, D. Reynolds, J. Irby, T. Eubank, L. Barber, and D. Dodds. 2019. Effect of soybean growth stage on sensitivity to sublethal rates of dicamba and 2,4-D. Weed Technology, 33(4): 555-561. doi:10.1017/wet.2019.39.
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