(Click here to see tables associated with this article)

As soybeans move from flowering into beginning pod (R3) in Indiana, questions have arisen about applying fungicides and insecticides to soybean to increase or protect yield. Currently there is low foliar disease pressure in Indiana soybeans.

However, many growers are interested in applying fungicides for other benefits, including retention of green leaf area, which may lead to an extended period of seed fill and higher yields. In addition, insecticide applications have been included in this concept and are promoted as increasing yields, even in the absence of significant insect pressures. The question remains whether such a management practice will result in consistent yield increases that will offset the added cost of the chemicals.

Fields trials were established in 2009 and 2010 at three locations in Indiana: the Pinney Purdue Agriculture Center (PPAC), in LaPorte County, the Agronomy Center for Research and Education (ACRE) in Tippecanoe County, and the Southeastern Purdue Agriculture Center (SEPAC) in Jennings County. All fields received Canopy DF pre-emergence herbicide about one month prior to planting.

Our soybean management program consisted of 22 fl oz glyphosate at growth stage V3 or R2, 6 fl oz of the strobilurin fungicide Headline® fungicide at R2 or R4, and 3 fl oz of the insecticide Warrior® at R4 (Table 1). Trials were rated for disease and insect pressure throughout the season, however, no significant pressure was found in any trial location in 2009 or 2010.

In 2009, significant yield benefits were only observed in soybean receiving Headline at growth stage R4 (Figure 1). This treatment yielded, on average, 2 bu/A higher than the control group, which was 43 bu/A. The treatment of Headline at R2 followed by Warrior at R4 also resulted in mean yield of 45 bu/A, but this treatment was not significantly greater than the control due to higher variability in the results. Warrior insecticide did not increase yield beyond a herbicide application.

Yield increases were observed when insecticide was included in the soybean management program in 2010 (Figure 2). However, the yield from a Headline treatment was the same as a yield from glyphosate alone.

When both Headline and Warrior were used together, yields were about 10 bu/A greater than glyphosate alone. These results suggest that insect populations were greater in 2010 than in 2009 at the field locations used in this study, but were not measurably detectable.

The decision to incorporate additional pesticides, like fungicides or insecticides, into a management program is ultimately based on economics. Of the treatments included in our analysis, a single application of glyphosate gave the highest average net return of $435/A (Table 2).

While additional treatments like a tank mixture of Headline and Warrior at R4 could result in higher net returns (1128 $/A), these treatments were not always profitable because of the added pesticide and application costs and the lack of consistent yield increases. Keep in mind that these numbers do not take into consideration other farm-related costs such as rent, taxes, labor, etc.

An individual grower's net return will not always be the same as those returns presented in Table 2, but the take home message is that fungicides and insecticides were not as cost effective as the glyphosate alone due to the inconsistent yield response.

Our results indicate that yield increases due to fungicides and insecticides are possible under conditions of below pest thresholds but consistent yield increases were not observed. Likewise, the additional cost of these pesticides and the variability of soybean market prices can influence the profitability of their use.

According to our analysis, soybean growers will rarely economically benefit from their application. Therefore, we recommend that growers base their decision to apply a fungicide or insecticide on the presence or absence of significant fungal or insect pressures in their fields.