Mid-June Notes on the 2021 Soybean Crop

The 2021 Illinois soybean crop was planted relatively early, with the 50%-planted date of May 5, more than two weeks ahead of normal. As in most recent years, some producers began planting soybeans before they started planting corn, although the first official record of soybeans planted (5% by April 19) was a week later than that for corn (5% by April 12). Planting conditions for soybeans were fair to good, but weather after planting was not consistently good. The main problem was low temperatures, and we found that soybeans can be killed by frost that occurs at or soon after emergence. Even with some replanting (or repair-planting), established stands aren’t great in all fields.

Crop Concerns

In addition to lower stand establishment due to frost damage and cool (and sometimes wet) soils after planting, the soybean crop has not grown as vigorously as hoped once temperatures turned warmer. Due perhaps to cool soil temperatures coupled with dry surface soils that may have limited root growth and water uptake in shallow-rooted plants, plants in many fields remained short well into June, and some are still short: we planted a trial on May 7 that now, six weeks later, is only 5-6 inches tall and at stage V2. We do not have a clear idea of how this might affect early-planted soybean yields, but the lower internodes appear to have been shortened, and plants may end up shorter than usual. That may not be a problem as long as the canopy can develop quickly. But these plants are not intercepting nearly as much light as they could if they were larger with more leaf area, and this continues to limit growth rates.

Concerns about dryness persist in some areas, and if soybean root systems are unable to grow out into the soil quickly, plants may continue to lag behind in their development. Adding to this concern is the lowered plant stands in some fields, which may end up lowering the completeness of canopy cover, which could in turn lower yields. We have seen in rare cases early-stressed soybeans that ended up not yielding very well, even in good seasons. We don’t expect this in 2021, but it may take a return of more consistent rainfall to overcome the slow start that the 2021 crop has experienced. In fields that have “broken through” and are now at stage V5 or higher with good growth rates, a few flowers have appeared in the last week, especially following warm nights. But as usual, most flowers will make their appearance after the summer solstice on June 20.

Nutrients

Nitrogen: We have seen some suggestions recently that adding fertilizer N to soybean at planting has increased yield before, and may do so again this year, with increased profits due to higher soybean prices. It is, of course, too late to apply N at planting, and applying N after emergence is more likely to limit nodulation and N fixation than to help the plant. But some may be inclined to apply N during the season in hopes of adding yield. While we have found some large responses to planting time N on light-textured, irrigated soil where adding N boosted early growth, applying in-season N has not only failed to boost yield, but has occasionally lowered yield. Although some research has shown that dry soils may delay or diminish soybean N fixation, the soil N supply from mineralization is probably higher than normal at this stage of soybean growth, and this will help supplement fixed N as growth accelerates.

Potassium: Dry soils that limit root development can limit K uptake, and if soils stay dry we might begin to see K deficiency symptoms as plants grow and need more K. Symptoms of K deficiency in soybean—loss of color moving in from the leaf margins—is easily recognizable. This is more commonly seen in no-till fields, due to root restrictions and drier surface soils, but can occur in tilled fields as well, regardless of soil test K levels. Rainfall relieves K deficiency quickly, affected leaf margins may die. There have been suggestions to apply K in-season including as a foliar spray, but a number of studies have failed to show higher yields from such applications.

Sulfur: Sulfur deficiency has been found in soybeans, and Shaun Casteel at Purdue and John Sawyer at Iowa State have reported some yield increases from adding S in studies. This response is inconsistent, and tends to happen more often in lighter-textured soils, where lower soil organic matter means less mineralized S, and where sulfate can leach more easily. The symptom of S deficiency is lighter green color of newer leaves. Cool, cloudy conditions can produce a similar effect, and failure of leaves to darken in the sunshine may help confirm S deficiency. If newer leaves of soybeans growing in lighter-textured soil remain pale green, then applying S as dribbled ammonium thiosulfate (ATS) or in some other way might be considered. We have had reports of S deficiency in corn this year (Nafziger, June 16, 2021), but those seem to be diminishing as soil mineralization continues and root systems develop. Soybeans that have reached V4-V5 and are growing rapidly do not seem to be showing S deficiency, so this may not be a problem in most fields this year.

Micronutrients: Like all crops, soybeans need micronutrients in very small quantities. Although many products are sold to supply micronutrients to soybeans, there is very little data showing crop response to individual micronutrients or to mixtures. That doesn’t prove that such products have no effect, but it does indicate that most soils provide enough of these nutrients to support full yields.

Cover crop

Although soybeans can usually be planted into cover crop cereal rye without difficulty, cereal rye, as well as weeds, that grow and extract water before they are killed can affect crop establishment and early growth. They also take up N, and although we don’t often think about the N needs of small soybean plants, the fact that N fertilizer at planting can occasionally stimulate soybean growth suggests that N availability could be a limitation to early soybean growth in some cases. Whether or not that has an effect on yield isn’t known, but we have seen a few cases of yield reduction from planting soybeans into cereal rye. In an experiment in place on South Farm to look at effects of cover crop and tillage, we saw slightly less growth of soybeans planted no-till into cover crop residue, but that difference has diminished as soybeans have grown. We included strip-tillage treatment in that study, and will be interested to see if keeping cereal rye residue away from the soybean row might provide a benefit.

Foliar Fungicide

Despite the fact that pressure on soybeans from fungal diseases that are controlled by foliar fungicides is typically low in Illinois, foliar fungicides continue to be promoted, with emphasis on their ability to help the crop withstand stress. As I discussed in the article on corn this week (Nafziger, June 16, 2021), there is some physiological basis for such claims for strobilurin fungicides. The extended flowering and podsetting in soybeans, though, means that a temporary boost in the photosynthate supply after applying fungicide may not always happen when the crop can benefit from it. Several years ago we applied fungicide to soybeans at different timings in order to prolong this effect, and were unable to increase yield. So while there may be such an effect in some cases, it is not very consistent.

Double-Crop Soybeans

Wheat harvest is getting underway, and will be followed by double-crop soybeans in most fields, at least in the southern half of Illinois, where most of the wheat is grown. Some have promoted early wheat harvest to help get DC soybeans planted earlier, but it is not clear how much time is gained by harvesting at 20-22% instead of 12-13% moisture. Wheat grain moisture drops very rapidly after maturity, losing as much as 6 to 8 percentage points from morning to mid-afternoon on a warm, sunny, breezy day with low humidity. Most rotary combines can handle wheat moisture in the low 20% range without damage, and grain at such moisture can be bin-dried in the summer. Still, waiting until grain moisture drops in the afternoon may be preferable to harvesting wetter grain, as long as the weather and number of acres to harvest and plant allow. If soils are dry enough that soybean seeds won’t germinate until it rains, early wheat harvest and soybean planting won’t provide much benefit.

The yield penalty from planting delays for doublecrop soybeans has been suggested to be a bushel per acre per day after mid-June. This response is highly variable depending on the weather the rest of the season, but data from Eric Alinger (retired from Pioneer) from more than 230 fields planted between July 1 and July 20 in southern Illinois in 2019 (when planting was delayed by wet soils in June) showed a yield loss of only 0.3 bushels per acre per day. And in 2015, with even longer delays to the start to planting, some 80 soybean fields planted during the last half of July averaged 36 bushels per acre, with no clear planting date effect until planting was delayed into August. So yield loss as planting of DC soybeans is delayed may not be as high as it once was, at least when the weather cooperates. Doublecrop soybeans should generally be of about the same maturity as full-season soybeans, and though there have been suggestions to raise seeding rates, planting more than 150,000 to 160,000 seeds is not necessary when planting conditions and seed quality are good.

Nafziger, E. “Mid-June Notes on the 2021 Corn Crop.” Department of Crop Sciences, University of Illinois, June 16, 2021.