Soybean Variety Selection
Soybean variety selection is the first step in producing a high-yielding soybean crop. Each variety has specific strengths that can make it highly suited for a certain environment but less suited for another. Soybean maturity and disease tolerance are 2 of the most important traits to consider when selecting a variety. However, other traits may be just as critical for local environments such as high pH soils.
Selecting the appropriate maturity is essential to maximizing soybean yield. Each variety has a relatively narrow geographical range in which it will perform as a “full-season” crop, utilizing all of the available growing season but still reaching physiological maturity before frost. This geographical range is usually no wider than 100 to 150 miles north to south. This is because soybean development depends on summer day length, which is shorter in the South than in the North.
When the days begin to shorten in the summer, a soybean variety in its ideal geographical range is triggered to flower. When a variety is grown north of its ideal geography, it will flower later in the season when the days shorten. Thus, the variety will mature later than normal and may be at risk of a killing frost. Conversely, if a northern variety is moved south, it will flower earlier than it would in its ideal geography. If flowering occurs before the variety reaches adequate height, yields may be reduced.
Ideal maturity is also influenced by planting date, as illustrated by a 2013 DuPont Pioneer study that evaluated the response of different soybean maturities planted at 3 timings. Maturity group 2.8 and 3.0 varieties had the greatest yield with mid-May planting (Figure 1). Longer maturity varieties yielded well with either late April or mid-May planting.
Figure 1. Soybean maturity group and planting date effects on yield in a 2013 study conducted near Owensboro, Ky.
Another major consideration with soybean variety selection is genetic disease tolerance. This is especially true for hard-to-manage diseases such as white mold and sudden death syndrome (SDS).
There is no absolute resistance available to white mold, but differences in tolerance exist between varieties. The DuPont Pioneer rating system rates varieties on a scale of 1 to 9 (9 = tolerant). Varieties are rated from 3 to 6 for this disease. Ratings reflect varietal differences in the speed at which infection develops and the extent of damage it causes, and they are based on data from multiple locations and years.
Soybean varieties can also show dramatic differences in tolerance to SDS. A 2011-2013 DuPont Pioneer/Michigan State University study demonstrated the extent to which yield can be influenced by varietal tolerance. An SDS-susceptible variety (Pioneer® variety 92M82 (RR) ) averaged only 10 bu/acre under severe SDS pressure, whereas a more tolerant variety with Peking SCN resistance(Pioneer® variety 92Y53 (RR) ) averaged 50 bu/acre.
Figure 2. Differences in SDS symptoms between susceptible (left) and tolerant (right) soybean varieties in a DuPont Pioneer/Michigan State University research study near Decatur, Mich., in August 2012.
Iron deficiency chlorosis (IDC) tolerance is a critical trait for soybean varieties grown on high pH soils. Varieties vary widely in tolerance to IDC, and variety selection is the first and most important step in managing this problem (Figure 3).
Figure 3. Differences in iron deficiency chlorosis symptoms between susceptible (left) and tolerant (right) varieties.
In a 2-year DuPont Pioneer study conducted at several high pH sites in Minnesota, IDC-tolerant varieties outyielded susceptible varieties by an average of 18 bu/acre (Figure 4).
Figure 4. Average yield of soybean varieties susceptible (IDC score 1-2) and tolerant (IDC score 5-8) to iron deficiency chlorosis in a 2-year DuPont Pioneer study conducted at 4 locations with high soil pH in Minnesota.