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The Bulletin

Wheat and Double-Crop Soybeans

Emerson Nafziger

and Giovani Preza Fontes
Department of Crop Sciences
University of Illinois

May 15, 2023
Recommended citation format: Nafziger, E.. "Wheat and Double-Crop Soybeans." and Giovani Preza Fontes, Department of Crop Sciences, University of Illinois, May 15, 2023. Permalink

Planted wheat acreage in Illinois increased by 35%, from 650,000 acres in 2022 to 880,000 for the crop to be harvested in 2023. Wheat acreage by county or crop reporting district is not available, but indications are that some of the additional acreage is in parts of central Illinois where wheat acreage has been limited in recent decades. Wheat yields in Illinois were record-high (79 bushels per acre) in both 2021 and 2022, which along with high wheat prices added to expectations that wheat would be a profitable crop in 2023.

Wheat yields tend to be high in central Illinois, but so are yields of corn and soybean, against which wheat competes for acres. Along with high wheat prices last fall and prospects for high wheat yields, the prospect of planting soybeans after wheat harvest also provided some incentive to add acres. Doublecropping is not a novelty in central Illinois, but doublecrop soybean yields have not generally been consistent enough to make this a sure source of added income.

Some producers in central Illinois practice relay cropping, with soybeans planted between (wide) wheat rows to allow the soybean crop to get established before wheat harvest. But much of the doublecropping in this region has been attempted mostly in those years when wheat harvest is early (before the end of June) and soil moisture is good. When the conditions the rest of the season remain good and fall frost is delayed, yields of DC soybeans can be half to two-thirds those of full-season soybeans. On the other hand, a dry spell or early frost can lower soybean yields and quality to the point where it doesn’t pay to harvest the crop.

A study underway in Piatt County has provided useful data on doublecrop soybean yield in central Illinois. The producer Eric Miller has been working with Lowell Gentry (NRES) and Dan Schaefer (IFCA) to see how wheat plus doublecropped soybeans yield compared to corn and full-season soybean, and how these crops affect nitrate loss in tile lines. The trial is set up in three portions of a field, each with a different phase (crop) of the corn-soy-wheat/DC soy sequence each year for the past seven years: each crop has cycled through the three fields twice. Table 1 shows yields of each crop by year, and averaged over years.

Table 1. Yields of crop yields (bushels per acre) of corn, soybean, and wheat in a rotation study in Piatt County, Illinois. The first year (2016) was a setup year without doublecrop soybeans, and in 2019, corn planting was prevented by wet soils.

All three primary crops produced high yields, with full-season soybean yielding (in bushels) about one-third the yield of corn, and wheat yielding about 40% as much as corn. Doublecropped soybean produced about half the yield of full-season soybeans; at 40 bushels per acre, DC soybeans have been a profitable crop. We think these are reasonable yield expectations for productive soils central Illinois, including the variability in DC soybean yields over years. The exact reasons why DC soybean yields were low in several of the years are not clear, but research has shown that the amount of time the crop spends between podsetting and the end of podfilling is strongly correlated with yield. Soybeans planted only about 100-110 days before first frost simply don’t have time to develop sizeable plants, large numbers of pods, and then to fill those pods to produce high yields. Anything (dry or cool weather) that nibbles away at the number of days the crop has available can have a large effect on yield.

If 20 bushels per acre (the lowest DC soybean yield among the six years) covers the expenses of doublecropping, and if the wheat crop provides as much net income as the 42 bushels of soybean (the difference between full-season and DC soybean yields) lost when wheat is planted, then planting wheat and DC soybeans can be a profitable enterprise. Wheat price in comparison to corn and soybean prices will be a major factor in how profitable wheat/DC soybeans will be in central Illinois going forward.

Getting doublecropping done

The success of doublecropped soybeans in Illinois has increased in recent years, due to genetic improvements, better planting equipment, and rotary combine technology that has enables wheat harvest at high moisture. Breeding for somewhat earlier-maturing wheat varieties, including at the University of Illinois, has helped as well. These all take on added importance as doublecropping is moved northward into central Illinois. Following is a summary of practices that can help increase chances of success when doublecropping soybeans:

  1. Harvest wheat as early as practicable: Early harvest at wheat grain moistures up to or slightly above 20% moisture does two things: it lowers chances of repeated cycles of wetting and drying in the field that diminish grain quality; and it moves doublecrop planting earlier. The development of rotary combines largely solved the problem of physical damage to (soft) grain at high moisture. Wheat reaches physiological maturity (maximum dry weight) at moistures above 30%, so there’s no danger of losing yield when harvesting wheat at 20% moisture. Very early-maturing varieties may reach 20% moisture a few days earlier than most varieties used in Illinois, but these tend to break dormancy earlier in the spring, which can increase chances of freeze injury, and they may not yield quite as well as medium-early varieties.
  2. Dry high-moisture wheat quickly: Because it’s usually warm when wheat is harvested, drying wheat from around 20% down to 13 % moisture can often be done with unheated air, as long as air can be moved through wheat grain fast enough to prevent spoilage. That usually means using high-capacity fans and limiting depth of the grain layer in the bin so that air can get through more easily. Flow-through dryers may work well, and heating the air will lower the relative humidity and speed the drying process.
  3. Handle straw to prevent planting difficulties: If there is no plan to harvest the straw, cutting a foot or more above the ground will lessen the amount of straw moving through the combine and make planting soybeans easier. Straw can be dropped behind the combine and baled off if there is a market for it, but this adds a soil-compacting and time-consuming operation. If straw is not harvested, it should be chopped and spread uniformly across the entire combine pass width.
  4. Make sound variety and seed treatment choices: Soybean planted in late June or early July will experience nights long enough cause plants to flower as soon as they reach stage V3, regardless of whether they’re in maturity group 3 or 4 in central Illinois. Later-maturing varieties might set pods for a few days longer than early-maturing ones, but whether or not that helps yields depends on how well seeds fill and when the first frost arrives. Some seed treatment might be helpful, although soybeans planted into warm soils usually emerge and grow quickly. Treated soybean seed generally cannot be carried over, so treated seed that was returned to the dealer may be available for a lower price, and might be a choice worth considering.
  5. Plant without delay: Most doublecrop soybeans are planted without tillage, usually in 15-inch rows. When soils are dry, the perennial question is whether to wait until it rains to plant or to plant then hopes it rains. If rain is in the forecast, it makes sense to plant, keeping planting depth shallow (an inch or so) for quick emergence. If it’s dry with no rain is in the forecast, it might make sense not to plant at all, especially if it’s already after July 1.
  6. Keep costs low: The decision whether or not to plant wheat is more important than decisions regarding doublecropping soybeans after wheat harvest. Once wheat harvest approaches and doublecropping soybeans looks favorable, it still pays to minimize costs, given the variability in DC soybean yields. Getting lower-cost seed (with good germination) can help. Waiting to apply herbicides until the crop has gotten established can also help. If at any point during the season it appears that the DC soybean crop won’t return enough to pay the additional expenses, it can be treated as a cover crop in preparation for the next year.
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