The year was 1997. Some were pushing soybean planting dates into early April or even into late March. Others were still clinging to traditional May 10 to May 20 planting dates. Purdue University was still recommending May 10 to May 20 as the ideal planting window for soybeans.

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Beck's decided to start a soybean planting date study in their Practical Farm Research Plots to see how planting dates shook out for them. The trial wasn't replicated. However, they've done it now in the same manner for 18 years. It is replicated over time.

More long-term data indicates that timely planting of soybeans is key to higher yields. (Thinkstock/Darcy Maulsby)

Eighteen years later their evidence is solid. Their preferred window is April15 to April 30, although their 18-year data shows no difference in yield for planting dates from April 15 to May 15. Yields for that entire window were 106% compared to the average yield, benchmarked at 100%. Yields dipped slightly to 97% for May 16 to May 31 planting dates, then dropped sharply to 86% for June 1 to June 15.

The highest yield in the 2015 Beck's planting date plot at Atlanta came on April 15, but it was only about 1.5 bushels per higher than the next closest date. Ironically, that was April. Note, however, that after April 15, they couldn't plant until May 22.

Related: Planting date matters if you want top soybean yields

For the 18-year summary, soybeans planted from March 16 to March 31 averaged 99% of normal. If you believe their findings, odds of higher yield would be slightly better planting March 20 than May 20! Odds favor planting in late March vs. anytime in June, assuming soil conditions are right, by a large margin.

What that means, some say, is that if you have large amounts of acreage of soybeans to plant, you are better off planting some in early April, or late March, vs. having to plant in June.