According to the USDA NASS May 19 crop report, soybean planting was 60% complete compared to the five-year average of 66% and last year’s 74%. Unfortunately, some areas of the state have been hit with heavy rains and flooding that have limited planting progress this week and will result in replanting. Looking forward, some fields are likely not going to be fit to plant until June. If that is the case, what agronomic practices do we need to look at? Depending on the situation, soybean relative maturity, row spacing, and seeding rate likely don’t need to be adjusted until planting is delayed past June 15 or even later.

Relative Maturity

Compared to corn, we don’t need to make as drastic a change in soybean maturity groups (MG) compared to the normal for your area. Late-planted soybeans typically require fewer days to reach maturity than earlier planting dates, partly because of quicker emergence with warmer soil temperatures but also because soybeans are photoperiod sensitive. Soybean flowering and development is triggered by day length, resulting in similar maturity among planting dates, although earlier plantings will have more nodes and yield potential.

This effect is shown in Table 1, where the results of a 2003-2004 planting date study at Lincoln are shown (average response of 14 varieties — 3.0 to 3.9 MG). Note the date that plants reached growth stage R8 or full maturity. In 2003, the mid-June planting date reached maturity seven days later than early May planting date. In 2004, the mid-June planting reached maturity 22 days later, even though the planting date was 50 days later.

Results from a 2013 South Dakota State University variety trial reinforces these findings. Maturity ratings on 55 varieties from 10 companies, ranging from 1.8 to 2.9 MG, was conducted at the Southeast Research Station in Beresford, South Dakota. Study results showed the equation for days to maturity was:

Days to maturity from planting = 5.4*MG + 113 days.

In 2013, switching from a 2.8 to a 1.8 MG reduced the time to maturity by only five days, when planting in mid-June as compared to late April. Changing maturity groups during this planting period did not make a large difference in maturity — something to consider if you were thinking about changing to a much earlier maturity group.

However, if planting is delayed after June 15, you may want to go with an earlier maturity group number recommended for your given area compared to a full season for your area. This might mean reducing your MG number by 0.5-1.0, but don’t try using a maturity group much shorter than that or you will sacrifice yield potential. Frost before maturity becomes a concern with late June or July plantings.

Row Spacing and Custom Planting

The next consideration is row spacing. With late planting, narrower row spacing is generally recommended. Because the longest day of the year occurs on June 21, and all days get shorter after that, soybeans need as much sunlight as possible to make pods, seed and yield. To close the canopy sooner, you may want to consider planting narrower than 30 inches. UNL research has shown that up to 5/8 bu/ac can be lost for every day after May 1 that planting is delayed. Thus, there is now a need to mitigate — to the degree still possible — the loss in the crop’s ability to capture all incoming sunlight from now on.

While narrowing rows can help close the canopy quicker at this point, there are a few cautions to consider. First, if an operation has a wider 30-inch planter versus a narrower 15-inch planter or 7.5-inch drill, less acres are going to be covered per hour with the 15-inch planter or drill, so it still may make sense to plant some acres to 30-inch beans in order to get done sooner. In addition, if using a drill — in general — the non-uniformity of seed depth placement and of seed-to-seed placement within the row is more of a concern with drills versus 15-inch or 30-inch planter units. When using a drill, increasing seeding rates by 10% (potentially up to 20%) may be necessary to fill in gaps that occur, resulting in additional seed costs. This may not be as much of a concern with newer precision planting drills. Also, narrowing rows can favor diseases such as sclerotinia stem rot (white mold of soybean) that like a humid, moist canopy; thus you may wish to use wider rows if you have experienced a problem with white mold in your fields.

If your operation does not own a narrow row planter or drill, there are now two strong considerations for custom planting. First, if there can be two or more planters operating at a time, your last acres will be planted sooner and take less of a yield hit, especially with additional rain delays in June (e.g., 5/8 bu/acre per day at 7 days = 4.3 bu/ac advantage). Second, you can capture the yield advantage of narrow rows. Regional studies have shown a 3-4 bushel/acre yield advantage to narrow row spacing (20 inches or less).

Seeding Rate

Many sources recommend increasing seeding rates by 10% after early June for both drilled and planted beans. Compared to our normal recommendation of 140,000 seeds per acre, this would result in a target population of 154,000 seeds per acre. We understand this line of thinking to attempt to improve canopy closure for yield and weed control by having more plants per acre, but there is some debate around this practice. An Iowa State study published by DeBruin and Pederson in 2008 did not find a seeding rate (75K, 125K, 175K, 225K) by planting date (late April, early May, late May and early June) interaction for yield, indicating no need for increased seeding rates at later planting dates. In addition, given that there is a wide range of seeding rates planted across the state, a blanket statement of a 10% increase may not be appropriate as some growers may already be planting considerably higher populations than 140,000 seeds per acre. Growers will need to evaluate this recommendation based on their normal seeding rates and their planting equipment.

Field Conditions

While delayed planting is frustrating to deal with, remember that mudding in a crop into poor soil conditions is never a recipe for good yields either. Try to stay out of fields until they are fit, or plant as much of the field as you can and fill in wet spots later when possible.

Also, it is important to be aware of crop insurance considerations and your options, so be sure to talk with your crop insurance if delays continue.

In summary, soybean producers that are looking at planting delays stretching past mid-June should be considering reducing their maturity group no more than 0.5-1.0 compared to normal, narrowing their row spacing, considering custom planting, and evaluating their seeding rates based on their situation. Please see this CropWatch article for additional replant considerations.