May 29, 2015

With parts of the state experiencing record rainfall amounts and frequent rainfall events in May, soybean planting is behind normal this year. According to the USDA NASS crop report on May 26, soybean planting was 59% complete compared to the five-year average of 73% and last year's 85%. Areas of the state will need considerable drying before fields are ready to plant. Many producers may be wondering if they need to rethink their agronomic practices such as relative maturity, row spacing, and seeding rate. The following are considerations when planting soybean in June or July.

Relative Maturity

At this time we would recommend sticking with normal maturity groups (MG) for your area. Changing to shorter maturity groups is not needed at this time because late planted soybeans will typically require fewer days to reach maturity than earlier planting dates. Table 1 shows the soybean growth and development model based on results from a 2003-2004 planting date study conducted at Lincoln (average response of 14 varieties — 3.0 to 3.9 MG). Note the date that plants reached R8 or full maturity. In 2003, the mid-June planting date delayed maturity by 7 days compared to the early May planting date.  In 2004, mid-June planting delayed maturity by 22 days even though the planting date was 50 days later. Because soybeans are photoperiod sensitive, flowering and development will be triggered by day length, resulting in similar maturity among planting dates, although earlier plantings will have more nodes and yield potential.

Results from a 2013 South Dakota State University variety trial reinforce these findings. Maturity ratings on 55 varieties from 10 companies, ranging from 1.8 to 2.9 MG, were conducted at the Southeast Research Station in Beresford, S.D. 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 5 days.  Therefore, changing maturity groups will not make a large difference in maturity so keep this in mind if you are considering changing to an earlier maturity group.

If planting is delayed past June 15, you may want to go with the earliest maturity group number recommended for your area, such as reducing your MG number by 0.5-1.0. Frost before maturity becomes a concern with late June or July plantings, but don't try using a maturity group much shorter than that or you will sacrifice yield potential.

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. We like to see that canopy "green to the eye by the fourth of July" which may or may not be possible at this point. 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. In general, 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. Increasing seeding rates by 10% (potentially up to 20%) may be necessary to fill in gaps. 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. While sclerotinia has not been a major issue in Nebraska, it has been observed in some fields; we would not recommend narrow 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 if additional rain delays occur in June (e.g., 5/8 bu/acre per day for 7 days = 4.3 bu/ac advantage). Second, you can capture the yield advantage of narrow rows. Regional studies have shown a 3-4 bu/ac yield advantage to narrow row spacing (20 inches or less).

Seeding Rate

Many sources recommend increasing seeding rates by 10% after early June for drilled and planted beans. We understand this line of thinking to attempt to improve canopy closure by having more plants per acre, but there is some debate around this practice. An Iowa State University 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. Growers will need to evaluate this recommendation based on their normal seeding rates and planting equipment. 

It's also important to be aware of crop insurance considerations and your options. For more on this, please see the CropWatch article, Late Planting Provisions for Crop Insurance.

In summary, right now soybean producers should be considering

• maintaining their current maturity group (or reducing it no more than 0.5-1.0);
• narrowing their row spacing;
• considering custom planting to finish planting sooner; and
• possibly increasing their seeding rates by 10%

Aaron Nygren, Nebraska Extension Educator
Nathan Mueller, Nebraska Extension Educator
Jenny Rees, Nebraska Extension Educator
Jim Specht, UNL Professor Emeritus Agronomy and Horticulture