Planting file photo

Editor’s note: The following was written by Emerson Nafziger, retired University of Illinois Extension agronomist, for the university’s Pest Management and Crop Development Bulletin March 11.

Most people have already selected varieties for 2019. One issue that continues to attract interest is how varietal maturity affects yield.

In the University of Illinois variety trials, entries at each location are separated in roughly equal numbers into two sets: one with longer- and one with shorter-maturity varieties. Averaged across five regions and 13 sites in 2018, the “early” varieties averaged 74.8 bushels per acre and the “late” ones averaged 73.5 bushels aper acre.

Across a range of maturities within and among regions running from north (average MG 2.7) to south (average MG 4.2) in Illinois, maturity was not consistently related to yield.

Planting date results

Response to planting date by five soybean varieties differing in maturity. Data are from a trial conducted at Urbana, Ill., in 2018.

We might want to choose a mixture of earlier and later varieties to spread harvest, but should concentrate more on yield potential than on maturity.

Planting date

We continue to hear a great deal of talk about the need to plant soybeans early to get high yields.

This is hardly a new discovery: Ever since we saw major improvements in seed quality and seed treatments several decades ago, we have known that early planted soybeans were capable of emerging without the need to wait until soils had warmed up to 55 or 60 degrees before planting.

In recent years some have taken “early” planting to an extreme, however, with claims that soybean planting should come before corn planting, as early as March if possible.

For 26 planting-date trials conducted in central and northern Illinois between 2010 and 2016, our target planting dates were in mid-April and then about every two weeks to early June.

Planting dates were converted to days after April 1, and yields within each trial to percent of maximum yield for that trial. We saw little yield decrease when planting was done by May 1 (day 31), about 7 percent lower yield if planting was on May 15 (day 46), and 14 percent lower yield if planted on June 1 (day 62).

While we did not plant before April 10 in any of these trials, the fact that yields were no higher from planting on April 15 than on April 30 shows that the “early planting” advantage is generally maximized if planting can be done by the end of April.

It’s not clear what advantage there might be in planting soybeans in March, or even, as some did in 2017, in February. Emerged soybean plants can tolerate low temperatures, with the exception of the few days when the “hypocotyl hook” appears above ground but before it straightens. If frost hits at this point, the exposed hook can be killed, which kills the seedling.

Seedlings are usually in slightly different stages down the row, so frost at this stage will seldom kill all of the seedlings, but it can certainly thin them out.

Besides stand loss, soybean plants exposed to low temperatures early in the season typically stay short and often do not yield as much as later-planted soybeans.

The goal of planting early is not to have the crop survive, but to have it yield more. Low stands and short plants aren’t generally conducive to the highest yields, and issues with crop insurance coverage may be another disincentive.

There certainly seems to be little reward for taking the risk of planting very early.

Planting maturity

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 it 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 seven-tenths 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 (see chart).

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.

Planted on April 26, the earliest variety reached first flower on June 9 and matured on Aug. 28, compared to June 15 and Sept. 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 Sept. 12 and Sept. 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.