Development of Regional Corn Planting Date Recommendations for Iowa.

U.S. corn producers plant earlier now due to an increased land base, larger equipment, more stress tolerant hybrids, improved seed treatments, and reduced tillage systems. Research was conducted to equip Iowa producers with accurate corn planting date recommendations on a regional basis. Overall objectives of this project were to (1) establish optimum planting windows for maximum yield, (2) understand how growth and development are altered by planting date, and (3) identify primary factors influencing grain yield. Multi-year (2006, 2007, and 2009) and multi-location (seven Iowa State University research farms) field research was conducted for a total of 21 site-years; 2008 data was not included due to atypical weather conditions. Five planting dates were arranged in a randomized complete block design (RCBD) with 3-4 replications at each site-year. Target planting dates were between 1 April and 1 June in 15 day intervals; interval adjustments were made for weather. Data collected varied by site-year and included: plant population, plant height, leaf area index (LAI), internode spacing, grain moisture, grain yield, and kernel weight. Recommendations for percent maximum yield were developed based on day of year (DOY) with three distinct regions for Iowa: (1) north central and northeast Iowa, (2) northwest and central Iowa, and (3) southern Iowa. Each region has a recommended “window of time” for achieving 95-100% or 98-100% maximum yield. Strikingly, the data is consistent in the start date for all three regions but varies in the length of time available for planting as well as the severity of yield loss beyond the designated optimum window. Developing recommended “windows of time” better takes into account the inherent variability that exists in weather and soil conditions year-to-year. Our approach varies from past methods employed which typically focused on the yield reduction after a specific point in time. The previous approach likely caused an incomplete understanding for producers and agronomists of the inherent relationships among the environment, growth, development, and yield determination.