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Selecting a Soybean Variety for Late Planting

Soybean is a photosensitive plant. In other words, day length signals the plant to cease vegetative growth and begin flowering and setting pods. Indeterminate varieties, generally speaking, will continue to simultaneously grow vegetatively and bloom. In contrast, determinate varieties have a distinct vegetative stage and blooming stage. Since day length can vary with latitude, soybeans are divided into maturity groups representing their days to physiological maturity. Smaller or lower numbers represent maturity groups that mature early while the groups with larger numbers represent soybeans that mature later (There are 13 groups). This allows the producer to manage planting date, typical climatic variances and maturity groups to optimize potential yield. Within Eastern North Carolina, the earliest maturity group commonly planted is Group 4. Typically, this group is planted from mid-April through early May and matures about mid-September. The latest maturity group commonly planted is Group 7. Group 7 soybeans are typically planted between late May through June and mature between mid-October and early November. Having thus said, Group III through Group VIII are appropriate for the area.

Choosing differing maturity groups by anticipated planting dates allows ample time for soybean vegetative growth to reach about three feet tall prior to onset of the bloom period. This plant height affords enough leaf area to prevent weeds from becoming established and obtain optimum yield potential. Planting multiple maturity groups provides a reasonable risk management tool to protect against potential periods of drought, damage from tropical storm systems, early frost or water saturated soils that may prevent fall harvest. Thus, proper management of soybean production includes matching planting dates with the appropriate soybean maturity.

Often, weather related planting delays reduce or eliminate this risk management strategy. Choosing a proper maturity group for a later than desired planting date requires examination of personal risk and knowledge of soybean growth. As a general example, Group 5 soybeans planted in mid-May usually begin to bloom in about 60 days (late-June or early July). These same Group 5 soybeans planted mid-June will also bloom about the same time period since the soybean is photosensitive. However, the later planted soybean plant will be very short and thus limit yield potential. In this example, choosing a later maturing variety is a better choice for a later planting. However, planting a later maturity (Group 7 or 8) group assumes that one can plant early enough to allow ample time for development of the soybean prior to the first killing frost. (Keep in mind that it is not the first frost but the first killing frost date that is critical. Soybeans can tolerate a light frost). Thus, soybean producers must weigh two risks:  1) Potential reduction of yield from planting an early maturing group that may bloom when the plant is short; or, 2) Risk of early frost that may reduce yield of later maturing soybean groups.

Either decision may result in less than optimum yield and may even result in disastrous yield depending upon environmental circumstances. The key relies upon the actual planting date and climatic conditions following planting. Regrettably, variance in annual climatic conditions prevents providing any strong, consistent data to support or refute either choice. Until weather can be more precisely predicted, the final decision will fall upon the individual grower. No magical date can be supplied that will verify when it is appropriate to switch to a later maturating soybean group.

One of the first considerations is to examine the intended planting date. Generally, data supports that planting soybeans much past June 10th to be late. Thus, one must adjust thinking and anticipated target planting date to accept that any soybean planted much after June 10th is not just “late” but these soybeans may be considered to be very late. Fortunately, there is not always a tremendous yield reduction for plantings that occurs between June 10th through the end of June if maturity Groups VI- Groups VIII are chosen. This does not imply that there is no yield reduction for late planting. Rather, it simply states that within any maturity group, there is not always a huge yield reduction that will likely result from a 10 -15 days planting difference. In other words, under normal circumstances, planting the same soybean in May usually yields higher than the same soybean variety planted in June. However, there is not always a dramatic difference between soybeans of the same maturity group (and variety) planted June 10th and June 20th. As such, assuming a soybean variety is selected from Groups VI-VIII for the later planting, there is a great chance to achieve good soybean yield. (Realize that research data for very late planted soybeans varies according to climatic conditions. In some cases, there is no significant yield reduction. In other cases, the reduction is severe).

Secondly, examine the specific growth habits of the varieties considered. Later planting soybeans are more likely to grow with higher temperatures, lower rainfall, increased disease risks and greater insect pressure within Eastern NC. Too, varieties that are inherently smaller are more likely to develop fewer leaves. Thus smaller plants and less leaves (less leaf area index actually) are less likely to provide adequate competition to prevent germinating weeds from becoming a problem nor have optimum yield. Thus, when planting later than desired and given a choice, choose a variety that has a tendency to grow taller.

Thirdly, consider the row spacing and planting population. Planting into narrow rows increases the chance that the soybean plants will canopy the ground. This not only reduces potential weed pressure, it ensures that maximum sunlight is intercepted by the plant to produce energy for soybean production. If the plant does not fully canopy the ground, potential yield is loss. Likewise, the planting population should consider the row width, potential of the soybean plants to reach a full canopy, as well as historical production. NCSU data shows that 75,000-90,000 harvestable plants to be optimum for yield with the higher rates appropriate for later planting dates. Thus, some slight increase in plant population may be necessary but avoid excessively high planting populations. Higher plant populations simply decrease profitability.

Next consider the later maturating groups more closely. Consider that soybeans often delay maturation by as much as 10 days due to the later planting. Thus, later maturity groups (Group VI throuh GroupVIII) are still a viable consideration for late planting. Assuming a soybean variety from these maturity groups is chosen, then risk management must now consider the potential damage from extreme climatic variances.

While climatic variations are important factor for consideration, do not place too much emphasis on extreme variances. Yes, it is possible to have an untimely tropical storm system that decreases yield or delays harvest. It is also possible that an early frost occurs. However, these are factors that one cannot control. Thus, it is more feasible to concentrate on making the most yield.

Some key points to consider regarding varietal choices and late planted soybeans along with supporting web links for additional information is provided below.

  • When planting late, data shows that planting a later maturing soybean variety more consistently produces high yield than earlier maturing varieties planted the same date.
  • Varietal resistance to diseases (such as frog eye leaf spot) may be an advantage for later planted soybeans within Eastern NC since late climatic conditions may favor development of multiple diseases. Click HERE for a list of soybean varietal disease resistance.
  • The potential performance for soybeans among the top yielding varieties within differing yield environments and planting dates is available from NCSU. Choosing  a variety that is versatile and top yielding under more stressful conditions reduces risk of loss. Click HERE to examine this information.
  • Generally speaking, one loses about 0.5 bu/ac per day when soybeans are planted much past June 10th. While there is tremendous variance in this date due to climatic variation and soil types, expect some reduction in yield when planting late.
  • A 10% loss of 40 bu/ac soybean yield due to early frost is still better than a 5% reduction of 35 bu/ac soybeans that might mature earlier. Thus, don’t place too much emphasis on the potential of an early frost damage. The first typical killing frost date for New Bern, NC is November 9th and for Morehead, NC November 19th. Moving inland, first typical killing frost for Kinston, NC is October 29th. (For more frost dates and lcoations within NC, click HERE). All of these dates may vary by 10-13 days so variance does occur. Putting the variance in perspective, there is only a 10% chance that a frost will occur within Craven County NC prior to October 22nd and only a 20% chance of a frost/freeze occurring before October 27th. There is a 50% chance this will occur by November 4th. (Source of frost probability data from the Soil Survey of Craven County, North Carolina, March 1989)
  • Tropical storms systems may delay or even prevent harvest. However, just as with frost, these storm systems vary in occurrence. Too, if one does not have a good yield in the first place, then the impact of the storm is not significant.
  • When faced with a limited selection of varieties and given a choice between a lower yielding variety in the preferred maturity group or a higher yielding variety with a slightly later maturity, unless early harvest is a priority or one has distinct varietal resistance needed, strongly favor the later maturing variety.