By Grace M. (Bluck) Looker, Laura E. Lindsey, Anne E. Dorrance, and James D. Metzger
From 2000 to 2013, the average soybean commodity price in the United States increased almost 300%, from an average price of $4.35/bu to $14.07/bu (NASS, 2016). Consequently, the rise in commodity price generated interest among farmers in agricultural inputs to maximize soybean grain yield. Inputs must be cost effective and associated yield gains need to withstand fluctuations in commodity prices as those observed in 2016, when the average soybean price dropped to $7.50.
A study was conducted to evaluate five agronomic inputs: Rhizobia inoculant, pelletized gypsum, foliar manganese (Mn) fertilizer, foliar insecticide, and foliar fungicide. The study was conducted at 16 field trials in Ohio, nine of which were studied in 2013 and seven in 2014. Counties where the trial was established include Clark, Clinton, Delaware, Erie, Henry, Mercer, Preble, Sandusky, Wayne, and Wood.
There were limited effects of inoculant, gypsum, foliar manganese (Mn), foliar insecticide, and foliar fungicide on soybean grain yield. However, these fields had established corn-soybean rotations, no sulfur (S) or Mn deficiencies, and limited insect pressure.
Foliar fungicide applied at the R3 growth stage was effective at reducing frogeye leaf spot and brown leaf spot disease severity. Soybean yield was influenced by fungicide application in 6 out of 16 site-years by 3.1 to 11.8 bu/acre.
High rainfall in June and July created an environment that was favorable for development of brown leaf spot and frogeye leaf spot, and disease severity appeared to be the driving force in determination of yield response to fungicide.
Crop scouting is a useful tool to identify fields where disease pressure levels are higher to determine when fungicide applications may be justified, as well as to scout for nutrient deficiencies and insect pressure to warrant the application of inoculant, gypsum, foliar Mn fertilizer, and foliar insecticide.
Methods. An omission trial treatment structure was used to evaluate the effect of:
- Rhizobia inoculant: TagTeam LCO Liquid MultiAction Legume Fertility (Monsanto BioAg)
- Gypsum: NutraSoft Pelletized Gypsum (The Andersons) applied at 2 tons/acre
- Mn foliar fertilizer: EDTA Max-In Ultra Manganese (Winfield Solutions LLC) applied at the label recommended rate during the R3 soybean growth stage.
- Foliar insecticide: Warrior II with Zeon Technology (Syngenta) at the label recommended rate during
- the R3 soybean growth stage.
- Foliar fungicide: Headline (BASF) at the label recommended rate during the R3 soybean growth
In an omission trial, two treatment controls are used, with one control having every input factor applied (enhanced production system) and the other control having none of the input factors applied (traditional production system) (Table 1).
Planting dates ranged from May 15 to June 3. Soybeans, ‘Asgrow 3231’ (maturity group 3.2), were planted in 15-inch rows. The previous crop was corn at all locations. All sites were no-till except for the 2013 Clinton County site which was minimally tilled.
Enhanced vs. Traditional Production System Results. There was very limited soybean yield response when comparing the enhanced production system (all inputs applied) to the traditional production system (no inputs applied). In only 2 out of 16 site-years was there a significant yield increase in the enhanced system compared to the traditional system (3.4 bu/acre increase at the 2013 Wood Co. location and 6.5 bu/acre increase at the Mercer Co. location). A yield increase of at least 8.1 bu/acre would be required to breakeven using the 2016 soybean price of $7.50/bu.
Rhizobia Inoculant Results. Omission of inoculant from the enhanced production system and addition of inoculant to the traditional system did not result in any significant yield changes at all site years. Lack of yield response from inoculant is likely attributed to all site-years being in a corn-soybean rotation for several years. These findings correspond to other studies where yield response due to inoculant was rare when soybean had been grown in a field within the previous 5 yr (Abel and Erdman, 1964; Boonkerd et al., 1978; De Bruin et al., 2010; Ham et al., 1971; Muldoon et al., 1980; Nelson et al., 1978).
Gypsum Results. Soybean yield was not positively influenced by gypsum application at any of the site-years. Ohio fertilizer guidelines recommend 0.21 to 0.40% S in the uppermost fully developed soybean trifoliate at the R1 growth stage (Vitosh et al., 1995). Soybean trifoliates collected had S within the recommended range and no visual symptoms of S deficiency were observed.
Manganese Foliar Fertilizer Results. Manganese foliar fertilizer application influenced soybean yield at 1 out of 16 site-years. Omission of the Mn foliar fertilizer from the enhanced production system was found to significantly reduce yield by 8.0 bu/acre at the Sandusky Co. location in 2014. Although the leaf samples collected at the Sandusky Co. location were within the recommended sufficiency range of 21 to 100 ppm of Mn and did not exhibit visual deficiency symptoms, the soil at this site-year had a large concentration of sand (63%). Due to the high sand content, the soil was very likely dry and may have lost moisture easily. In dry soil, Mn can be converted to form that is unavailable for plant uptake.
Foliar Insecticide Results. There was a yield response to foliar insecticide at 1 out of 16 site-years. A significant yield reduction of 4.8 bu/acre occurred at the Wayne County location in 2014 due to the omission of insecticide from the enhanced production system. At the Wayne Co. location in 2014, soybean aphid and bean leaf beetle were noted. Overall limited response due to insecticide may be attributed to low insect pressure and low defoliation severity. In general, trifoliate defoliation in the mid- to upper canopy was <15%.
Foliar Fungicide Results. Omission of fungicide from the enhanced production system resulted in significant yield decreases of 3.1 to 11.8 bu/acre in 5 out of 16 site-years. The addition of fungicide to the traditional production system resulted in a yield increase of 7.0 bu/acre at 1 out of 16 site-years. Site-years where there was a yield response to fungicide (either a yield loss when fungicide was omitted in the enhanced system or a yield increase when fungicide was applied in the traditional system) had an average of 11 inches of rainfall in June and July.
Site-years that were unresponsive to fungicide application had an average of 9 inches of rainfall in June and July (excluding two locations that experienced an intense rainfall event in July that caused standing water for 24 h). Brown leaf spot and frogeye leaf spot are favored by wet conditions and heavy rainfall, as rain splashes the fungus upward in the plant canopy, spreading the disease. Greater disease pressure created by wet growing conditions may have resulted in the yield responses from the fungicide input.