Corn is planted earlier every year and this is one important component in maximizing grain yield. In 2009, 47% of the statewide corn crop was planted by approximately April 26. This was four days earlier than the previous 5-year average (USDA NASS, 2009). Earlier planting dates are attributed to several causes: larger acreage per producer, less spring tillage, advancements in hybrids, increased tile drainage, and improved seed treatments. The start of corn planting is generally related to the date when the soil temperature reaches 50°F (10°C) or greater. Previous Iowa State University (ISU) recommendations for 99% maximum yield, relative to planting date, were identified as April 20 to May 19 statewide. We believe that this planting window can and should be earlier to achieve high yields. Research has been conducted at seven sites in Iowa since 2006. This report compiles the data and statistical results observed during this multi-year and multi-location study aiming to provide more precise planting date recommendations by region.

Core Ideas• Three distinct site-groupings resulted, with different recommended planting windows.• Two planting windows were developed for each site-grouping: 98-100% grain yield or 95-100% grain yield.• The north-central and northeast grouping had the earliest recommended planting window to maximize grain yield. Three distinct site-groupings resulted for Iowa, which is different than previous statewide research: north-central (NC) and northeast (NE); northwest (NW) and central (C); and southwest (SW) and southeast (SE). Two planting windows were developed for each site-group based on the maximum yield on the response curve and a subtraction of 2 or 5% relative yield to develop yield windows of 98-100% or 95-100%, respectively. The response curves for each site-grouping identify locations that exhibit a stronger grain-yield response to planting date, especially in the northern and southern locations. The NC-NE grouping had the earliest 98-100% planting window (12-30 April) whereas the NW-C grouping (15 April-9 May) and SW-SE grouping (17 April-8 May) were later. Impact of Planting Date in Corn SystemsGrain yield potential is maximized with the earliest spring planting dates for a particular region, assuming that other abiotic and biotic stresses are minimized (Gaile, 2012). Specific calendar dates that produce the optimum grain yields for a particular location vary due to differences in climate and the length of the growing season (Bruns, 2003). A trend toward earlier corn planting during the past several decades has occurred and is due to improvements in technology, along with temperature and precipitation shifts from climate change (Pryor et al., 2014;Urban et al., 2015). Technological advancements include the increased early-season stress tolerance of hybrids, use of improved seed treatments, increased herbicide use, reduction in tillage operations, larger equipment, tile drainage, and mechanical improvements in planters that distribute seed more uniformly in suboptimal conditions (Shaw, 1988;Kucharik, 2006;Lenhart et al., 2012). Increased farm size also necessitates

Corn is planted earlier every year and this is one important component in maximizing grain yield. In 2009, 47% of the statewide corn crop was planted by approximately April 26. This was four days earlier than the previous 5-year average (USDA NASS, 2009). Earlier planting dates are attributed to several causes: larger acreage per producer, less spring tillage, advancements in hybrids, increased tile drainage, and improved seed treatments. The start of corn planting is generally related to the date when the soil temperature reaches 50°F (10°C) or greater. Previous Iowa State University (ISU) recommendations for 99% maximum yield, relative to planting date, were identified as April 20 to May 19 statewide. We believe that this planting window can and should be earlier to achieve high yields. Research has been conducted at seven sites in Iowa since 2006. This report compiles the data and statistical results observed during this multi-year and multi-location study aiming to provide more precise planting date recommendations by region.

Corn is planted earlier every year and this is one important component in maximizing grain yield. In 2009, 47% of the statewide corn crop was planted by approximately April 26. This was four days earlier than the previous 5-year average (USDA NASS, 2009). Earlier planting dates are attributed to several causes: larger acreage per producer, less spring tillage, advancements in hybrids, increased tile drainage, and improved seed treatments. The start of corn planting is generally related to the date when the soil temperature reaches 50°F (10°C) or greater. Previous Iowa State University (ISU) recommendations for 99% maximum yield, relative to planting date, were identified as April 20 to May 19 statewide. We believe that this planting window can and should be earlier to achieve high yields. Research has been conducted at seven sites in Iowa since 2006. This report compiles the data and statistical results observed during this multi-year and multi-location study aiming to provide more precise planting date recommendations by region.

Research was conducted to equip Iowa producers with more accurate corn planting date information based on their specific location. Objectives for this research project addressed during the presentation include:1. What are the optimum planting windows for maximum yield? 2. How do new recommendations (2006-2009 data) compare to previous ISU recommendations (1998)(1999)(2000) data)?3. What are the risks associated with planting outside the optimum window?4. How is plant growth and development impacted by varying planting dates?5. Are grain yield differences correlated with differences in growth and development?