Management Categories for Variable Rate Soybean Planting.

The increased interest in managing seeding rates within soybean fields is being driven by rapid advances in precision agriculture technologies and the need to increase productivity and economic returns. The objective of this study was to identify field and within-field factors that affect soybean yield responses to seeding rates that are slightly above or below the common planting rates used by farmers in Iowa. Across the state in 2009 and 2010, farmers conducted 58 field-scale replicated strip trials with high (on average of 395,000 seed ha^-1) and low (on average of 340,000 seed ha^-1) seeding rates. The two treatments were replicated at least four times in each trial. Yield responses to the higher seeding rates within each trial were estimated using 30-m grid patterns. Hierarchical modeling and Bayesian analysis were used to identify factors that affected yield responses to the higher seeding rates across seeding management categories that consistent at least six trials. For the field-level factors, we considered soybean row spacing (narrow vs wide), soybean planting dates (early or late), and monthly and cumulative growing season rainfall. For the within field-level factors, we used relative elevation, slope, soil drainage class. On average in both years, the higher seeding rates produced profitable yield responses (i.e., yield increases above the marginal seed cost) in less than 40% of the trials. Using estimated predictive  probabilities of profitable yield response we are developing a seeding rate management-aid tool that should help farmers and agronomists make better decisions where to increase or decrease seeding rates within management categories that include both within and field-level factors.