So, what kind of yield advantage does a producer gain by planting soybeans early? In Nebraska, research reported in the Agronomy Journal demonstrated that for each day soybean planting was delayed after May 1, the yield penalty per day was as much as 5/8 (0.63) bu./acre in a great soybean year (like 2004), and still a substantive ¼ (0.25) bu./acre in a not-so-great soybean year (like 2003). Multiplying these yield penalties by the current soybean price provides a clear indication of the importance of planting date in terms of optimizing the net profit potential in a soybean production system.
The yield penalties accruing from delaying soybean planting beyond early May in Nebraska have also been documented in other states ( Wisconsin, Indiana and Iowa).
The yield reward arising from early planting should not be used as a reason to plant seed into seedbeds that are too wet to plant. Other than trying to plant early, exercise good judgment relative to the other seed planting practices.