Few resources are available to producers and agricultural professionals relative to yield losses from late- season frost injury to soybean plants. A study investigating the risks and benefits of long-season soybean varieties was established in 2008. This work was carried out by the Naeve Soybean Production Project, and was funded by the Minnesota Soybean Research & Promotion Council. While we don't have all of the answers that folks search for after a late-season frost, a small piece of this research effort is described below.
In 2009, 2010, and 2011 soybean plots were established to investigate the yield effects of early frost on a range of soybean maturities. Three varieties with maturities of RM 0.8, 2.0, and 2.8 were examined in 2009. These were planted at a normal seeding date (around May 1) and at a late planting date (around May 21). Frost was simulated with applications of Liberty herbicide at a rate of 32oz per acre in 10 gallons of water on September 7 (early) or September 21 (late). In 2010 and 2011, five varieties ranging from MG 0.8 to 2.8 were planted at a single planting date (early May) and treated to simulate frost on three dates (approximately September 7, 14, and 21).
Photo of a soybean plot 'frosted' with Liberty approximately 10 days prior. In the left of the photo is a soybean plot 'frosted' 3 days prior.
Among the early-planted soybeans in 2009, simulated frost on September 7 reduced seed yields of the adapted variety (AG2108) by 10 bushels per acre. Yields of the adapted variety were not significantly reduced by frost on September 21. In this case, the short-season soybean variety had a lower yield potential, but was not affected by frost at any date. Alternatively, the very long-season variety (AG2802) showed a 20 bushel per acre yield reduction when 'frosted' on September 7. Again, the late September frost did not reduce yields of this variety. When soybeans of all three maturities were planted late, early frost had a large yield effect. Frost in late September affected both the long-season variety as well as the adapted one. As expected, frost had a much larger effect on yields of late-planted soybeans.
This research highlights an important soybean management issue. Early planting did not benefit soybean yields in the absence of frost, except in the case where the soybean variety planted was of a much later than normally adapted maturity. Late planting primarily affected yield through increased losses due to early frost.
Figure 1. Soybean yield as affected by planting date, maturity, and early frost simulated with Liberty herbicide.
Although fall conditions in 2010 were very different from those in 2009, yield responses to early frost were very similar. Yields of the adapted variety were reduced by about 10 bushels per acre by a simulated frost on September 7. Yield losses were much smaller due to a September 14 simulated frost, and nonexistent for a September 21 simulated frost. Again, the very long-season variety (here DKB27-52) had yield reductions of about 20 bushels per acre when 'frosted' early in September. Frost effects from mid- and late-September events were relatively small.
Note, while short-season soybeans are virtually 'frost-proof', only the very earliest frost date tested reduced yields of the full- and long-season varieties below those of the very short-season variety. Short-season soybeans are not a good risk-management strategy unless planting dates get pushed back to the end of May.
While the data described here will not help predict yield effects of the frost of September 15, 2011, it should provide some insight into how soybean maturity and the timing of frost events interact to affect soybean yields.