Estimating Probability of Economic Yield Response From Fungicide Applications On Soybean.

Fungicide applications (FA) on soybean (Glycine max (L.) Merr. ) have become common in the recent years in Iowa, and yield responses (YR) have been often observed, even when soybean leaf diseases are not visible. However, the current guidelines are not specific as to when and where growers might expect an economic YR from FA. The objective of this study was to estimate the frequency of economic YR from application of Headline (Pyraclostrobin) on soybean. About 280 on-farm replicated strip trials having three to 10 alternated strips with and without Headline in each trial were conducted across Iowa during the last five growing seasons. The strips received FA by ground or air at R1 through R3 soybean growth stages.  Soybean yields were measured by combine yield monitors. Yield responses to Headline were estimated as differences between mean treatment yields for each trial. Late-season color and near-infrared digital aerial imagery was used to verify treatment locations and identify possible application errors within the trials. The mean YR to Headline (0.12- 0.25 Mg ha^-1) was statistically significant in each year and across five years (0.16 Mg ha^-1). The probability of economic YR ranged from 0.41 to 0.65 among the years and was slightly affected by rainfall during each growing season. The fungicide treatments increased soybean grain moisture by 0.1% and decreased combine speed by 0.1 mph, but neither speed nor grain moisture had a significant effect on harvest time. Large scale on-farm evaluations should help identify potential site-specific factors (weather, method and timing of applications) that influence the frequency of economic YR to FA on soybean.