A study published by Kansas State University Extension in 2022 analyzed how later planting dates affected corn yields in Kansas. The trial was conducted at two research stations (Topeka and Ottawa) between 2018 and 2021, with several varieties planted across several different dates each year. Each trial plot was 30-feet wide and between 30 to 40-feet long, with yield data from the middle two rows of each plot used for data analysis. This was done to prevent skewed data from neighboring plots. The results were different at both locations. 

For 2018 and 2019 in Ottawa, corn planted at the end of May and early June yielded the same as corn planted earlier due to hot and dry conditions as well as rain events during pollination, while corn planted in late June only reached 60-70% of the maximum yields. The results in 2020 were very different due to above average rainfall in July. The highest yields in 2020 were planted in late April through the middle of May. Data also indicated that the full-season hybrid yielded more than short-season hybrids across all planting dates. The authors noted that switching to a short-season hybrid due to delayed planting will not increase yields. The highest yields from 2021 were similar to 2020, with full-season hybrids planted in early April having the highest yields. 

Rainfall played less of a factor at Topeka due to irrigation, with the authors noting that yield response was more traditionally aligned with planting date. The highest yielding corn for 2018 and 2019 was planted in the last half of April, with the highest yielding corn in 2020 and 2021 being planted in early April. The full-season hybrid planted in early April 2021 had the highest yields, with both short and full-season hybrids planted in early June having roughly the same yields. 

But what about Illinois? A recent factsheet published by the University of Illinois Extension outlines key points for managing corn in Illinois for 2025. For planting date and hybrid selection, the authors recommend considering a hybrid that reaches maturity ten to 14 days before the first killing freeze of the year. This date can vary across Illinois, with median date of the first fall freeze for many parts of Central Illinois being around October 25, and the average day for most of Northern Illinois being around October 18. On planting date, data compiled from studies between 2007 and 2024 show that the optimal planting date for Illinois being between mid-to-late April. Yield declined gradually as planting is delayed, with corn planted in early May reaching 93% of maximum yields, further decreasing to 85% if planted in late May and 79% in early June. 

An article published in farmdoc daily in May 2020 analyzed the impact of late planting on the average yield of soybeans across the United States. Using three different measures and analyzing average soybean yield between 1980 and 2019, the authors found that, on average, there was a 0.5 bushels/acre reduction in yield for a ten-percent increase in late planting, with the range between 0.4 and 0.7 bushels/acre. The authors also noted that soybeans planted after May 25 will yield close to the national average, barring major issues with the weather. 

A study published in 2023 from the Ohio State University studied the interactions between planting date and seeding rate on yield at two locations in 2020 and 2021. Four different periods of planting dates were used: April 25-May 10, May 11-May 31, June 1-June 15, and June 16- July 1. Data from three of the four site-years indicated a relationship between planting date and yield. At one location in 2020, yields across three of the four date ranges were the same, while the yields for the last range (June 16-July 1) were approximately 15.91 bushel/acre less than the earliest range (April 25-May 10). The results from the same location in 2021 showed no difference in yields based on planting date due to optimal weather conditions. At the second location in 2020, the soybeans planted later (after June 16) yielded more than the soybeans planted earlier (before June 16), likely due to rainfall and other weather conditions. For 2021, soybeans planted earlier and at higher seeding rates had higher yields than those planted after June 16. These results aligned with other studies, which showed earlier-planted soybeans yielded better than later-planted soybeans. A 2020 study from Iowa State University found that the greatest declines in yield were seen in soybeans planted in mid-June, with soybeans planted before late May achieving at least 92% of maximum yield. 

But, again, what does this mean for Illinois? A similar factsheet was published by the University of Illinois Extension on soybean management for 2025. The authors found that, for Central Illinois, soybeans in the 3.6 maturity group had the best potential for maximum yields, with soybeans between the 3.4 and 4.0 maturity group being within one bushel/acre of maximum yields. Like for corn, the authors used data from studies between 2007 and 2024 to determine the optimal planting date for soybeans in Illinois. Soybeans planted between mid-to-late April were within two percent of maximum yields. Soybeans planted in early May reaching 94% of maximum yields, which further drops to 88% by May 31, 80% by June 15, and 77% by June 20. As the planting date gets pushed further back, the plant will develop fewer leaves, which further limits yield potential. 

This two-part series has explored the critical aspects of preparing for a successful planting season and the significant impact of planting dates on crop development and yields. Part one dived into the essential steps for planting preparation, emphasizing the importance of soil testing and preparation and equipment maintenance. These foundational practices set the stage for a productive and efficient planting season, ensuring your crops have the best possible start. Part two examined the importance of planting dates, highlighting how timing can influence germination, growth stages, and overall yield potential. By understanding the optimal planting windows and adjusting practices accordingly, producers can maximize their crop's performance and resilience. Together, these insights provide a comprehensive guide to achieving planting success. By combining thorough preparation with strategic timing, producers can enhance their farm's productivity and ensure a bountiful harvest. 

For further reading, visit the following links:

• Effects of late planting on corn yield (Kansas State University Extension): https://newprairiepress.org/kaesrr/vol8/iss4/2/.
• Illinois Corn Management (Illinois Extension): https://extension.illinois.edu/sites/default/files/2025-03/illinois-corn-management.pdf.
• Illinois Soybean Management (Illinois Extension): https://extension.illinois.edu/sites/default/files/2025-03/illinois-soybean-management.pdf.
• Impact of late planting on U.S. soybean yields (farmdoc daily): https://farmdocdaily.illinois.edu/2020/05/the-impact-of-late-planting-on-u-s-average-soybean-yield.html.
• Soybean planting date effect on yield (Ohio State University): https://doi.org/10.1002/agj2.21434.
• Soybean yield response to planting date in Iowa (Iowa State University): https://doi.org/10.1002/agj2.20053.