Some Midwestern soybean growers are moving to soybean planting dates that sound more appropriate for corn — in April. And there appears to be good reason for it.

Soybean planting-date research by Jim Specht at the University of Nebraska showed a yield loss of about ¼-⅔ bu./acre a day for every day that planting was delayed beyond May 1 in eastern Nebraska.

Improved seed quality in recent years, and fungicide and insecticide seed treatments, make such early plantings more feasible than just several years ago.

But, how does the earlier planting translate into higher yield?

The explanation relates to the number of nodes produced by the plant. Specht's studies, sponsored by the Nebraska Soybean Board in 2003 and 2004, show that soybeans produce a new node approximately every four days following the V1 (unifoliolate leaf) stage, regardless of the planting date or the cultivar. If earlier planting allows soybeans to reach that V1 stage sooner than later plantings, the earlier planting will produce more nodes. And number of nodes — where flowers and pods are produced — is a major yield factor.

In Specht's trials, beans planted on May 1 took longer to geminate and emerge than those planted in mid- to late-May. But they still emerged before those in the May 15 planting, reaching the V1 stage sooner and thus producing nodes sooner than the May 15 plantings.

Later plantings may grow taller than earlier plantings, appearing to catch up with their early bird counterparts. But, they can't catch up with the early planted beans in node production, says Specht. If the later-planted beans are taller, it's because of longer internodes, not more nodes, he says.

In Specht's early planting studies, each day's delay in planting after May 1 resulted in 0.24 bu./acre loss in 2003, and a 0.64 bu./acre loss in 2004. Or, stated another way, for every four days of delay in planting after May 1, there was a 1-bu./acre loss in 2003 and a 2.6-bu./acre loss in 2004.

Plantings in late April/early May have yielded as much as 30 bu. more per acre than mid-June plantings in Specht's studies, e.g., 45 bu./acre vs. 75 bu./acre.

Maturity group for soybeans in his trials ranged from 2.5 to 3.5, the range adapted to the geography in the studies. He planted in 30-in. rows, at a depth of 1-1¼ in. — deep enough to place seed in moist soil.

His trials were irrigated, allowing him to count on adequate moisture during the critical July-August flowering/pod-filling period. Under non-irrigated conditions, moisture during that critical July-August period may be limiting, making the advantage of earlier planting less predictable.

Results from soybean planting-date comparisons in long-term, non-irrigated, no-till plots at the University of Nebraska-Rogers Memorial Farm in eastern Nebraska bear that out. University of Nebraska Extension Engineer Paul Jasa has conducted early planting comparisons for soybeans there since 1999. March 28 plantings in the first year began emerging April 16, escaped spring frost and bested May 10 plantings by 6 bu./acre — 56 bu. vs. 50 bu.

Early plantings — mid-April to early May — haven't consistently beaten more traditional mid- and late-May soybean planting dates under non-irrigated production in Jasa's trials. One reason: More pods resulting from earlier planting dates don't always mean more yield under dryland production. Timing between rainfall and critical flowering/pod-filling is a key part of the equation.

That's borne out in results last year at the Rogers Memorial Farm plots, planted 15 days apart — April 17, May 2 and May 17. Even though the April 17 plantings had more pods than the May 17 plantings, the later plantings turned in the higher yields last year (see table below).

In Jasa's trials, the extra pods didn't translate into more yield last year. With higher-than-average temperatures and drought last summer, Jasa suspects that a number of pods on the earlier plantings were aborted. And, mid-August rains came at the right time to help the May 17 plantings more than the earlier plantings, he adds. “Dryland plots always make this more interesting, because they don't always come out the way they should.”

Nevertheless, extremely early plantings could be one way of machinery management in, say, a corn-soybean rotation where only one operator is available to plant, or where the same planter is used for both crops. But that would have to be weighed against the risk of running the gauntlet of spring freeze risk and seed diseases.

Jasa plants as deep as 2 in., which may delay soybean emergence enough to overcome some concerns about killing frost and floral initiation triggered too early.

One observation in Jasa's plots is that beans seed-treated with the systemic insecticide Cruiser tended to be taller than those without the treatment.

It's an observation farmer and crop consultant Jerry Mulliken, Nickerson, NE, has noted in comparisons between early and late-planted soybeans on his eastern Nebraska farm. The comparisons were part of an on-farm research project under the Nebraska Soybean and Feed Grains Profitability Project, where participants compare a variety of production practices on their farms and share their results.

Mulliken's early planting (mid-April) outyielded normal plantings (mid-May) in 2000, yielded less in 2001 and yielded the same in 2002. All plantings were non-irrigated.

“I'm dryland. I think early planting soybeans are better suited to irrigation. If you plant early (under non-irrigated conditions), beans tend to be too far along to take advantage of those September rains.”

Through his crop consulting work, Mulliken has observed a number of growers planting earlier than they used to. He now starts planting about May 1, and finds a systemic insecticide treatment such as Cruiser for bean leaf beetle feeding is “worth the money” on early plantings.

Along with Jasa, Mulliken has noted that beans seed-treated with Cruiser grow taller than non-treated, regardless of planting date. However, Jasa hasn't determined what's behind that growth effect and whether it makes a yield difference.

Planting Pointers

Here are tips to keep in mind with early planted soybeans, according to Jim Specht at the University of Nebraska.

Colder soils for early planted soybeans mean that germination and emergence may take up to 14 days — much longer than beans planted in the warmer soils of more traditional soybean planting dates.

Specht also suggests not planting earlier than 14 days before the latest spring freeze date in your area, so that the odds favor emergence after a spring freeze. In eastern Nebraska where his trials were conducted, May 11 is the latest historic spring freeze. So, he works backward from there, targeting planting no earlier than April 27.

Planting too early also prompts early flowering, which can restrict the final number of nodes, Specht says.

Flowering too early may not happen in late March-early April plantings, if soybean seeds germinate slowly and stay in the cold ground long enough to emerge close to the historical last spring frost date, Specht says. But, he considers planting that early a risky strategy.

Since beans may lie in cold soil for some time, a fungicide seed treatment is advised. And, since early planted beans are attractive to over-wintered bean leaf beetles, treating seed with a systemic insecticide should also be considered.

It's not the leaf feeding by the beetle that is important. Rather, it's the bean pod mottle virus that is frequently carried by the beetle and transmitted to the seedlings that's more of a threat to yield in early-planted beans, Specht warns.