Understanding Soybean Varietal Responses to Fungicide and Enhanced Fertility

• Fungicide application increased soybean yield by 1.3 bu/acre across all sites under low disease pressure.
• Enhanced fertility did not significantly increase yield, especially when soil phosphorus (P) and potassium (K) were sufficient.
• All soybean varieties responded similarly to fungicide and enhanced fertility.
• It is more probable that fungicides will provide greater return than enhanced fertility when soil P and K are sufficient. 

Introduction

The 2021 Agronomy In Action Research trials found that soybean fungicide application followed by enhanced soil fertility were the two most responsive management variables out of multiple agronomic and plant nutrition variables evaluated. However, the magnitude of those responses was found to be location dependent. Previous trials were also not designed in a way to understand if responses can be variety specific. Historic data continues to support corn hybrid-specific responses to both fungicide and zone-placed fertility, although less is known about variety specific soybean responses. Agronomy In Action researchers carried out trials in 2022 to better understand if soybean fungicide and fertility management practices should also be based on variety.  

2022 Soybean Management Trial Details

Research trials were conducted at eight Agronomy In Action sites in 2022. Replicated trials were designed to evaluate response to fungicide and enhanced fertility as well as the two factors combined. The host farmer standard fertility practices were applied across the entire trial and additional fertilizer was applied to specific blocks within the trial to create “enhanced fertility” treatments. Enhanced fertility blocks consisted of the baseline fertilizer in combination with NACHURS Triple Option^® at 15 gal/ac (22 lbs/acre P2O5, 29 lbs/acre K2O, and 2 lbs/acre S) applied through the planter. Planter fertilizer applications were applied using a 2×2×2 placement to provide nutrients in proximity of developing roots while also avoiding direct contact with seed and avoiding potential germination issues from high salt content.  Higher-than-normal starter fertilizer rates were meant to mimic zone fertilizer placement used in strip-till or other precision placement practices. Soil test P and K levels at each site are provided in Table 1. 

To test for varietal response to fungicide and fertility, 10 varieties with ranging agronomic characteristics, disease tolerance scores, and herbicide tolerance traits (Enlist E3^® soybeans and XtendFlex^® soybeans) were selected for each maturity group (MG) region. Fungicide application blocks were established in both normal and enhanced fertility areas and received a broadcast application of Miravis^® Neo fungicide at the R3 growth stage at a rate of 13.7 oz/acre.

Enhanced Fertility Response

Enhanced fertility did not have a statistically significant effect on soybean yield at any site, and numerical increases were less than 1 bu/acre at seven of eight sites. Although soil P and K were sufficient at six of eight sites, those with very low or low soil P or K still did not respond to the enhanced fertility. The lack of yield response at sites with high soil test P was not surprising, as university research supports this trend.^1,2,3 Though the lack of response at the low P or K sites was surprising, it may have been caused by random field variability.

Fungicide Application Response

Averaged across all locations there was a 1.7 bu/ac response to R3 fungicide application, although responses ranged from 0 to 3.8 bu/ac depending on location and fertility levels (Graph 1). This average response was smaller than expected but was likely related to low disease pressure at most locations throughout pod fill stages. In comparison, fungicide applications produced a 3.7 bu/ acre increase across nine Agronomy In Action sites in 2021. Although disease pressure was generally low, delayed plant senescence within treated plots was observed at many locations which likely extended pod fill and potentially contributed to smaller yield gains (Figure 1).

Soil fertility level did not appear to affect the response to fungicide at most locations, as both standard and high fertility blocks averaged a 1.7 bu/acre response to fungicide when comparing across all locations (Graph 1). Locations with medium to very high base fertility levels did not show a clear trend that further enhancing the fertility would help improve overall fungicide response. Inconsistent differences observed in responses between individual locations was likely noise resulting from natural field variability rather than fungicide. There was a numerical yield advantage to fungicide response in enhanced fertility blocks as compared to base fertility rates at Jefferson, WI (NS), and Waterloo, NE (P≤0.10), where soil P test levels were low to very low at 11 and 5 PPM, respectively (Table 1). Observations at these two locations could suggest that the magnitude of a fungicide response may have been limited by insufficient base soil fertility.

The response of individual soybean varieties to fungicide application was consistent in all MG regions (10 varieties within each zone, 30 total tested) (Table 2). Individual variety responses to fungicide were inconsistent within each MG region, indicating that variety-specific responses are difficult to pinpoint, if they exist. For example, GH2722XF brand had a -0.2 bu/ac average response to fungicide across all locations, yet two of three sites exhibited ≥1.9 bu/acre yield responses. If one chooses to prioritize fungicide applications, it should be done based on field disease pressure, and not variety driven.

Conclusions

Fungicide application increased soybean yields by 1.7 bu/acre across eight Agronomy In Action sites with low disease pressure. Overall fungicide response did not appear to increase with enhanced fertility, but may play a role in response when adding supplemental fertility if base nutrient levels are deficient. Enhanced fertility by itself did not increase yield, likely due to sufficient soil P and K levels at six of eight sites. These results indicate that fungicide application has a greater likelihood to produce more return than additional P and K fertilization when soil test levels are already sufficient.

References

¹   Dodd, J.R. and A.P. Mallarino. 2005. Soil-test phosphorus and crop grain yield responses to long-term phosphorus fertilization for corn-soybean rotations. Soil Science Society of America Journal 69:1118-1128.
²   Hankinson, M.W., L.E. Lindsey, and S.W. Culman. 2015. Effect of planting date and starter fertilizer on soybean grain yield. Crop, Forage, & Turfgrass management. DOI: 10.2134/cftm2015.178.
³   Webb, J.R., A.P. Mallarino, and A.M. Blackmer. 1992. Effects of residual and annually applied phosphorus on soil test values and yields of corn and soybeans. Journal of Production Agriculture 5:148-152.