Submitted to: Field Crop Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 26, 2002
Publication Date: N/A
Interpretive Summary: Average soybean yields on growers fields are in the range of 40 to 50 bushels/acre. Yields in the 60 to 70 bushels per acre are occasionally obtained on the more productive soils in years of favorable rainfall or under irrigation. Yet in national soybean yield contests in the late 1960's, contest winning yields of 90 to 100 bushels per acre were reported. In effort to verify such yields were possible, a soybean maximum yield experiment, where all manageable yield limiting factors were minimized. In 1982, the entire test averaged 89 bu/a with 4 cultivars yielding over 100 bushels per acre. However, using the same managing techniques, these high yields were repeatable only three times in the next 17 years of the experiment, 1985, 1998 and 1999. In the intervening years the highest yields possible were in the 70 to 80 bu/a range compared to the 90 to 100 bu/a yields the four high yield years. In searching for an explanation for why the yields were so much higher in 1982, 1985, 1989 and 1999, it was noted that these years there was an unusually early warm spring that triggered the soybeans to flower almost 2 weeks earlier than normal ( June 15 vs July 1). These results indicate there is a major interaction between day length and early spring temperatures, called a photo/thermal effect, that can trigger earlier flowering in soybeans, resulting in higher soybean yields. These results suggest there is a major opportunity for breeders to increase the yield potential of soybeans by breeding for full season varieties that will flower earlier under normal spring temperatures.
Technical Abstract: Results from 18 years of maximum yield research on soybeans at Wooster, OH (40 North, 82 West) indicate there is a major photo/thermal barrier to higher soybean yields in temperate latitudes where light intensity is highest and day length is longest early in the growing season, declining as the growing season progresses. Under normal spring temperatures in May, soybeans planted at Wooster, OH the first week of May normally bloom the first week of July. However in 1982 and 1985 and again in 1998 and 1999, unusually early warm spring temperatures in May resulted in the soybeans flowering around June 15, two weeks earlier than normal. In 1982, 1985, 1998 and 1999, test average soybean yields obtained were 5963, 5549, 5383 and 5416 kg/ha, respectively, with individual lines producing replicated yields in the 6000 to 7000 kg/ha range. In the intervening years, 1983 and 1984 and from 1986 to 1997, with more normal spring temperatures, test average yields in the maximum yield environment ranged from 3575 to 4862 kg/ha, with highest yielding individual lines producing yields in the 4200 to 5500 kg/ha range. These results indicate there is a temperature by photoperiod interaction in soybeans, a photo/thermal effect, that results in soybeans flowering up to 2 weeks earlier than normal in response to above normal temperatures in early spring. This results in the soybeans entering the reproductive cycle earlier in the growing season when days are longer and light intensity is higher, resulting in a significant increase in yield potential of soybeans. These results indicate if breeders can develop full season soybean cultivars that will bloom earlier under more normal spring temperatures, yield potential of soybeans in the temperate latitudes could be significantly increased.