Sometimes it is difficult to find silver linings on clouds, and the clouds we’ve seen in Missouri for the past six weeks have hidden those linings quite well. Missouri farmers are faced with soggy soil and over 2 million acres of bare fields where soybean plants should be flowering. So, what I offer is more like tin than silver. Because Missouri lies further south than many of its Midwestern neighbors, we have a longer growing season. Later fall freeze occurrences gives us options if we decide to plant soybean on dates that I’ll call ultra-late – after July 1.
Many Missouri farmers have experience with double cropped soybean after wheat harvest, so we know that plants will often mature before fall frost and profitable soybean yields are possible. But, as planting continues to be delayed, risks from freezing temperatures and less than profitable soybean yields increase.
For several years after the wet spring of 2008, I conducted an experiment that focused on yield potential of soybean planted in July. The experiment also included seeding rate and row width components. I’ve discussed those two components in a previous article. Figure 1 presents the yield data collected in central Missouri. The data have been averaged over two years and two row spacings (15-inch and 30-inch). Because seeding rate affects yield response to planting date I selected data from only the 150,000 seeds/acre rate. The latest planting dates in the study were July 16 and July 20 (2009 and 2010). The variety I used was late MG III for maturity.
In both years, plants matured before frost. The Columbia area did not experience a hard freeze (under 28°F) until Oct 18 in 2009 and Oct 29 in 2010. Yield potential averaged over the two years decreased by about 55% from June 1 through the July 20. But, average yield was still about 30 bushels/acre when planted after mid-July. Remember I’m an agronomist and not an economist, so you need to determine profit or benefits from your insurance program. And, my yield information is from a small data set. Finally, weather in late August and early September will greatly affect ultra-late planted soybean yields, and weather is difficult to predict.
University of Missouri Extension has produced a new website that will help farmers determine chances for fall freezes. Information can be found at http://ipm.missouri.edu/FrostFreezeGuide/index.cfm . I’ve presented some of the available information for six weather stations in Table 1. Please remember than weather stations often measure temperature four feet above soil level and cold air will drain to lower parts of fields. I normally use 28°F as a killing frost. Plant leaves do not freeze at 32°F, but temperatures near 32°F will affect plant growth and may negatively impact tender plant and cell parts. Note that the 30-year average for first occurrence for 32°F is about two weeks earlier than for 28°F. Use the probability number to match your acceptable risk. Most average freeze dates use 50%, but that means that in half of the years a freeze event would occur before the date. Using dates associated with 10% or 30% would decrease the risk of freeze injury to plants.
Soybean yield is protected from frost if the plants have reached R7 or physiological maturity. At R7, seed moisture is about 60% and some green color is still present in seeds. If a killing frost occurs before R7, leaves will remain on the plant making harvest more difficult. Soybean seeds will not change from green to normal yellow color, and green beans are often docked at point of sale. This green color may fade with storage, but a change in color is surely not guaranteed.
I have a limited data set in which I determined R7 dates for soybean plants that differ for planting date. Rules of thumb can be misleading, but in general, a three day delay in planting will delay R7 by one day. This is because photoperiod strongly influences soybean maturity. Maturity groups differ for R7 date about 8 to 10 days. Please be careful how you apply these rules of thumb.
In north Missouri, I determined R7 for three mid MGIII variety planted on June 6 at Albany for just one year. Average R7 date was September 22. I’m reluctant to extrapolate one-year data or use my rule of thumb for later plantings. But, this is at least some indication that soybean could mature before frost if planted in late June. In central Missouri, I have two years of data from a study in which I planted four MG III and four MG IV varieties in late June (approximating double cropped date). Averaged R7 date was October 4 and Oct 11 for MG III and MG IV.
In central Missouri, I do not recommend switching soybean variety maturity unless planting is delayed past July 12. Exchanging soybean seed at seed dealers is difficult and carrying over soybean seed to next spring while maintaining vigor is nearly impossible. Varieties that mature earlier than varieties adapted to a location usually yield less than adapted varieties because they are shorter and produce less leaf area. For ultra-late planted soybean, quickly developing leaf area to capture available sunlight is key to success. But, any decision on what variety to plant or if planting is warranted must be made in consideration of the plants having enough time to mature before killing frost this fall. To improve yield potential, plant in rows as narrow as possible and increase seeding rate at least 30,000 seeds/acre.