Emerging soybeans

Editor’s note: The following was written by Emerson Nafziger, University of Illinois Extension crop production specialist, for the university’s Crop Development Bulletin May 6.

Evaluating soybean stands is a little more “complicated” than evaluating corn stands, although the same problem of uniformity occurs in both crops.

An adage that we might apply to soybean is, “When plants are easy to count without bending over, there aren’t enough of them.”

One concern is that those who advocate for lower seeding rates and early planting may encourage stands to be kept even when they will result in a yield loss. We have over the years learned that stands that may look inadequate when plants are small usually fill in nicely and produce high yields. This has taught us to be patient when growth starts slow and not to let emotions guide replant decisions.

Still, staying with stands that are too low to produce maximum yields should not be done just to “prove a point.”

Instead of laboriously counting the number of plants in a hoop or 3 feet of row, it might be faster and accurate enough to use a scale of 0 to 4, with each number the approximate number of plants per square foot. On this scale: 1 (43,560 plants per acre) would be too low; 2 (about 87,000/acre) would be probably be acceptable if plants are healthy; 3 (131,000) is a full stand; and 4 (174,000) is more than enough.

One plant per square foot is a plant every 4.8 inches in 30-inch rows, and every 9.6 inches in 15-inch rows.

With a little practice, it should be possible to get close-enough stand counts with a relatively quick glance at the ground. Getting quick counts in more places is usually preferable to getting counts that are more accurate in fewer spots.

I took the same approach to estimating expected outcomes from replanting soybeans as for corn. One caveat is that we do not have much data from recent studies that included both planting date and seeding rate, so our assumption that soybeans respond similarly (on a percentage basis) to seeding rate at different planting dates is not well supported.

Joshua Vonk did some studies that included both seeding rate and planting date in 2010 and 2011 and found no interaction between these two factors across sites. But as others have found, he didn’t see much response to seeding rate, so not finding such an interaction was expected.

Those studies also didn’t include planting quite as early or quite as late as the table includes, and so the indication that 50,000 plants will yield only 10% less than 130,000 plants when planted in mid-June isn’t rock solid.

Soybean yields table

Soybean yields (percent of maximum) for different combinations of planting date and plant population. Derived from the results of 29 planting date and 25 seeding rate trials in Illinois.

According to the table, as much yield will be lost from planting after May 20 as from losing nearly half the stand from April-planted soybeans.

Experience and judgment will play a role in this decision, as will the cost of replant seed and planting.

Unlike corn, there’s no need to destroy existing soybean plants when replanting. In a study we did some years ago, supplementing low plant stands with a reduced amount of seed or destroying the low stand and planting a full rate of seed produced the same yield.