Profitability of prophylactic R3 pesticide applications in soybean when pest pressure is low

Abstract

Identifying and implementing management strategies that maximize soybean (Glycine max) profitability is one of the most important decisions soybean producers consider each year. One management strategy that has received considerable attention is the prophylactic application of foliar insecticide and fungicide tank mixes applied at the R3 (beginning of pod development) growth stage. Anecdotal accounts that a synergistic effect occurs when a prophylactic fungicide and insecticide application occurs at R3, resulting in greater profitability, are likely contributing to the interest in this management practice. Interest may also be due to the inconsistent results documented in scientific literature. When low pest (disease and/or insect) pressure exists, every possible scenario has been reported. There have been reports of synergistic effects where soybean seed yield was greater for the fungicide and insecticide tank mix applied at R3 than when the fungicide and insecticide were applied separately or for the untreated control (Henry et al., 2011). In contrast, fungicide and insecticide tank mixes have been reported to be similar to the untreated control and the individual fungicide and insecticide treatments (Nelson et al., 2016). There are also many reports that the tank mix produces greater seed yield than the untreated control (Barro et al., 2023; Dorrance et al., 2010; Rod et al., 2021a), the fungicide treatment (Henry et al., 2011; Kandel et al., 2016), or the insecticide treatment (Dorrance et al., 2010; Henry et al., 2011). Finally, there are reports that soybean seed yield following the fungicide and insecticide tank mix were similar to that of the untreated control (Barro et al., 2024).

Even though considerable inconsistency is reported, investigations of the synergistic effect of prophylactic insecticide and fungicide tank mixes applied at R3 in full season soybean (spring-planted soybean preceded by corn [Zea mays] production the previous calendar year) have not been completed in Kentucky within the last 10 years. Furthermore, work in double crop soybean (soybean planted in early summer which was preceded by winter wheat [Triticum aestivum] in the same calendar year) found that prophylactic fungicide plus insecticide applications at R3 increased soybean seed yield by 5 bu acre⁻¹ compared to an Integrated Pest Management (IPM) based (Villanueva, 2023) R4 (fully developed pod) insecticide application that occurred (Rod et al., 2021a).

Full-season soybean trials were evaluated at three locations and double crop soybean trials were evaluated at two locations (Table 1). Beginning at R1, plots were evaluated once per week to determine insect defoliation and disease incidence and severity (Culman et al., 2014; Dorrance & Mills, 2010). Four treatments were evaluated: fungicide (Miravis Top; pydiflumetofen at 0.067 lb a.i. per acre + difenoconazole at 0.111 lb a.i. per acre, Syngenta) applied at R3 growth stage; insecticide (Warrior II with Zeon Technology; lambda-cyhalothrin at 0.03 lb a.i. per acre, Syngenta) applied at R3 growth stage; fungicide plus insecticide applied at R3 growth stage; and IPM-based pesticide applications, which were according to University of Kentucky Cooperative Extension recommendations (Bradley, 2019; Villanueva, 2023). All locations had six replications of each treatment. A CO₂-pressurized backpack sprayer (Model T; R&D Sprayers; 40 psi with 20 gal water per acre) equipped with Turbo TeeJet Induction Nozzles (TTI110015; TeeJet Technologies) spaced 15 inches apart on a 5-ft-wide spray boom was used to apply the pesticides, which included a 0.125% v/v non-ionic surfactant (Ad-Spray 80; Helena Chemical Company).

The experimental design was a randomized complete block design with six replications. Data were analyzed separately for the full season and double crop trials. The ANOVAs were determined with PROC GLIMMIX in SAS v9.4. Replications within each location were specified as a random effect. All other effects were specified as fixed effects. A significant (P < 0.05) location × pesticide treatment interaction was found for the full season soybean trial. Therefore, the three full season locations were analyzed individually.

Whole soybean seeds were analyzed with near-infrared reflectance (NIR) spectrometry at University of Minnesota's Soybean Breeding Laboratory to determine seed protein and oil concentrations. Seed protein and oil concentrations are reported on dry-weight basis.

Net economic benefit for each treatment was determined using partial budget analyses (Kay et al., 2020; Rod et al., 2021b). The “t-test: two-sample assuming unequal variances” function in Excel 2013 (Microsoft) was used to determine statistical differences between the net economic returns of the IPM-based treatment and each of the three pesticide treatments. The costs for fungicide, insecticide, and fungicide plus insecticide was obtained from local retailers and were $16.05, $4.49, and $20.53 per acre, respectively. The application cost ($8.50 per acre) was obtained from Halich (2024). Soybean prices used were obtained from USDA (2020), where October prices were $9.69 per bushel and November prices were $10.20 per bushel.

The results of each net economic return assessment were evaluated to determine the risk, or probability, of each treatment having a positive net return with the @RISK Excel Add-in (Palisade; Data Viewer’ option). In total, six observations per treatment were included for the full season trial at Lexington. At Princeton, 12 observations per treatment were included for both full season and double crop trials.

Seed yield and oil and protein concentration did not differ (P > 0.05) among any of the pesticide treatments for either soybean production system (Table 2 and 3). This was not surprising given that insect defoliation and incidence and severity of frogeye leaf spot (caused by Cercospora sojina) were less than 5% at the Princeton locations and less than 20% at the Lexington location (data not shown), which is below the IPM-based threshold for pesticide applications. Although this study and Nelson et al. (2016) found that seed yield was similar among the untreated control and pesticide treatments, it is more common that inconsistent results are found, even within the same study (Barro et al., 2024; Dorrance et al., 2010; Henry et al., 2011; Kandel et al., 2016).

Profitability of each treatment and the probability that each pesticide treatment would increase yield enough to pay for the cost of the treatment was examined, that is, profitability being ≥$0. In the full season trial, mean net economic returns were similar (P > 0.05) between the IPM-based treatment and each of the three pesticide treatments at all locations (Figure 1 and 2). Others also found that profitability was not increased when prophylactic pesticide applications were made in full season soybean (Henry et al., 2011; Kandel et al., 2016). The probability that each of the pesticide treatments would result in a net return ≥ $0 per acre ranged from 37% to 45% at Princeton and 45% to 78% for Lexington (Table 4). Other reports are similar to the Princeton findings that the probability of a net return was less than 50% (Barro et al., 2024; Kandel et al., 2016).

It was interesting that at Lexington the mean net benefit was positive for all pesticide treatments (Figure 2) and the probability that the net return would be ≥$0 was as great as 78% (Table 4). One potential reason for this could be cultivar resistance. The cultivar at Lexington had a lower frog eye leafspot resistance rating than the full season cultivar grown at Princeton (Table 1). Another possible explanation could be differences in pest pressure. At Lexington, qualitative assessment of “less than threshold,” which is typically less than 20% insect defoliation, were measured. In contrast, it was documented that insect defoliation and frogeye leaf spot incidence and severity were less than 5% at Princeton. Given that thresholds were not exceeded, this suggests that the insect defoliation and/or disease severity at Lexington was greater than the 5% measured at Princeton. These findings illustrate that the inconsistent results reported in both anecdotal and scientific communications may be due to different management systems, such as cultivar resistance, and/or environmental conditions, such as timing of pest infestation and/or differences in pest pressure.

For the double crop soybean trials, the net economic return was least (Pr > t = 0.0169) for the treatment that received a fungicide application: −$66.26 per acre (Figure 3) and the range in probability that the pesticide treatments would result in a net return ≥$0 per acre for double crop soybean was 20% to 50% (Table 4). Although the insecticide and fungicide plus insecticide treatments also resulted in a negative net return, neither differed significantly (P > 0.05) from the IPM-based treatment. In previous work, Rod et al. (2021a) found that an R3 prophylactic fungicide plus insecticide treatment was more profitable than IPM-based foliar pesticide applications, which resulted in a foliar insecticide treatment at the R4 growth stage. These results further support the fact that inconsistent reports of the benefit and profitability of prophylactic fungicide and insecticide applications are likely confounded with management systems and environmental conditions.

Prophylactic pesticide applications not only have the potential to decrease profitability, but they also can have extremely negative biological effects on natural enemies. In particular, the potential for developing pest populations, insect pests and plant pathogens, that are resistant to pesticides is a major concern as is the potential damage that can occur to off-target insects, such as natural enemies (Torres & Bueno, 2018), and pollinator (Pecenka et al., 2021) species.

This work provides further evidence that when insect pests and foliar diseases are below threshold levels, the likelihood of increasing profitability with a fungicide plus insecticide foliar application is low. These findings highlight the difficulty in developing generalized recommendations for pesticide applications based upon conditions at a single point in time. However, monitoring and scouting for diseases and insects can help growers implement management strategies based on IPM based tools rather than preventative measures.

Carrie Knott: Conceptualization; formal analysis; funding acquisition; writing—original draft. Carl Bradley: Investigation; writing—review and editing. Chad Lee: Investigation; writing—review and editing. Raul Villanueva: Investigation; writing—review and editing.

The authors declare no conflict of interest.