Source: University of Illinois
Should later-maturing varieties be planted first in order to take maximum advantage of the longer time in the field? There’s no problem with doing that, although early planting moves up harvest date some, so works counter to the goal of spreading harvest time by using different maturities. In 2018 we ran a trial at Urbana, supported by a seed company, to see how varietal maturity affected response to planting date. The first planting date was April 26, the last was June 6, and varieties ranged in maturity from MG 2.3 (very early for this location) to MG 3.6, which is a little later than average for this location.
For all but the earliest-maturing variety in this trial, the planting date response was almost perfectly linear, with the loss of nearly 7/10ths of a bushel per day of planting delay—a total of more than 27 bushels—over the 41 days from the first to the last date (Figure 2). This loss rate accelerated a little for the latest-maturing variety between May 24 and June 6. The earliest-maturing variety lost only 17 bushels from first to last planting, but only because its yield at the earliest date was so much lower than yields of the later-maturing varieties.
The month of May 2018 was much warmer than normal, and this got the soybean plants off to a very fast start. Warm nights are conducive to early flowering, and this was especially notable in 2018. In the early-planted crop, first flowers appeared in early June, well before the longest day of the year, and unlike the interruption of flowering that often takes place under normal night temperatures for about a week before and after the longest day, flowering was early and continuous in 2018. As a result, nearly half of the Illinois soybean crop was flowering by July 1. The warm May probably affected the yield response to planting date as well; with warm temperatures, early-planted soybeans as fast as late-planted ones, and this widened the developmental gap between the different plantings.
Planted on April 26, the earliest variety reached first flower on June 9 and matured on August 28, compared to June 15 and September 17 for the latest-maturing variety. When planted on May 24, the earlier and later varieties flowered on June 15 and July 2, and matured on September 12 and September 25, respectively. So when planted late, both varieties flowered very early in their life cycles, both spent less time in reproductive stages than when they were planted early, and they ended up yielding about the same. While in this case it’s accurate to say that the later-maturing variety benefitted more from early planting, that’s only because the early-maturing one was physiologically less able to use the longer growing period allowed by early planting to produce high yield.