The year 2008 is one for the record books. Frequent rains delayed corn planting in Missouri and are continuing to aff ect soybean planting. Numerous streams and rivers have fl ooded and water levels remain above fl ood stage in parts of Missouri. Th is means that some soybean fi elds have yet to be planted or were planted and severely damaged by fl oods. In the past these fi elds would have been abandoned and no crop planted. However, with soybean prices at near record highs, it may make sense to plant soybean on dates as late as the end of July.
Our planting date studies have never included this late date, but we may be able to make some estimates about yield potential and maturity. One of the reasons that delayed planting reduces soybean yield is that the critical stage of development (podding, R3 and R4) is delayed into August – a month with less than optimal rain. Less rain means more pod abortion and less yield. Th e good news for very late planted soybean is that September often experiences higher rainfall than August. Two other reasons for reduced yield from delayed planting is shortened seed-fi lling period (photoperiod controlled) and less light available for photosynthesis (shorter days and lower sun angle). Th ese problems will be very much in play with very late planted soybean.
In 2005 and 2006 we conducted a project funded by the Missouri Soybean Merchandising Council. Th e primary purpose of the project was to evaluate soybean management strategies related to soybean rust. Planting dates and Maturity Groups were two factors studied in the project. We planted three varieties from each Maturity Group 1, 2, 3 and 4. These varieties were planted on three dates at Albany (NW MO) and Columbia (Central MO). Among the data collected were stages of development dates. Although the latest planting date was June 16, we can use this study to try to predict maturity dates and the potential need for switching varieties for ultra late planted soybean.
Planting varieties that mature before frost is an important consideration. Soybean plants killed before maturity will result in green seeds that are often heavily docked. These seeds do not lose their green color during storage. Freeze probabilities do not follow straight lines across Missouri so it is best to use a database that includes individual communities like the one provided by NOAA at http://cdo.ncdc.noaa.gov/climatenormals/clim20supp1/states/MO.pdf. Many datasets include probability percentages, i.e., 10 percent or 50 percent and a temperature, i.e., 28 or 32 degrees Fahrenheit. For spring freeze damage we often use 28 degrees Fahrenheit for four hours to represent a killing frost. For soybean in the fall, I think 32 degrees Fahrenheit is a more realistic temperature. The difference between 10 percent and 50 percent probability of a freeze is about 10 days to two weeks. I believe that a 50 percent chance of damaging temperatures is too risky so I chose 10 percent probability as an appropriate target date for completing soybean maturity.
Figure 1 illustrates the effect of planting date on dates for physiological maturity for soybean planted near Albany in 2006. Physiological maturity is when soybean leaves and pods begin to turn yellow and the plant has reached the end of its life cycle. Yield cannot be reduced by frost or other stresses after this date, but the grain moisture may be as high as 60 percent. Cautious interpretation is in order because this is one-year data. For Maturity Groups 2 and 3, a 50 day delay in planting resulted in a 5 to 8 day delay in maturity. Soybean maturity is strongly regulated by day length, but this delay in maturity is less than expected and may be related local conditions. Physiological maturity occurred about 5 days earlier for Maturity Group 2 varieties than Maturity Group 3 varieties. Normally, the difference between Maturity Groups averages about 10 days.
In north Missouri where flooding has yet to end, soil may not be fit to plant until late July. The 10 percent chance of an air temperature of 32 degrees is September 25 for several communities in north Missouri. Although difficult to estimate from our limited data set, I believe that north of Highway 36 farmers may want to consider using a Maturity Group 2 variety if planting after July 19. This switch in maturity would help ensure mature soybeans if frost occurs earlier than normal this autumn.
Figure 2 illustrates the effect of planting date on dates for physiological maturity for soybean planted near Columbia in 2005 and 2006. Because this dataset represents two years and the two years were nearly identical for response, we have greater confidence in interpretation. For Maturity Groups 2 and 3, a 60 day delay in planting resulted in a 20 day delay in maturity. At Columbia, physiological maturity occurred about 10 days earlier for Maturity Group 2 varieties than Maturity Group 3 varieties.
The 10 percent chance of an air temperature of 32F is October 2 for several communities in central Missouri. Unfortunately, freeze probability lines do not extend in straight line west to east. So, some areas may experience frost earlier than in other areas even if the two areas are relatively close in distance. Also, night temperatures in fields are often colder than temperatures where agencies monitor temperatures. Given these caveats, if soybean is planted in late July in central Missouri, it appears that Maturity Group 3 varieties should mature before frost.
REVISED: October 2, 2015