A long term maximum yield soybean [Glycine max (L.) Merr.] research project was initiated at Wooster, OH (40°N latitude) in 1977 with the specific objectives of determining the yield potential of soybeans and identifying yield limiting factors. Results from this research suggest there is a delayed flowering barrier to higher soybean yields in the higher latitudes where the light intensity (sun angle) is highest and the day length is longest early in the growing season, declining as the growing season progresses. At Wooster, OH, the average 24 h total solar radiation declines from 474 Langleys (cal/cm2) in June to 351 Langleys in September. The maximum daily solar energy declines from 680 Langleys (15-30 June) to 444 Langleys 15-30 September. Under normal spring temperatures in May, soybeans planted during the first week of May normally bloom during the first week of July. However, in 1982, 1985, 1998, and 1999, unusually early warm spring temperatures in May resulted in the soybeans flowering around 15 June, 2 weeks earlier than normal. In a maximum yield environment, where all manageable yield limiting factors were minimized, test average yields were 5963 kg/ha in 1982, 5549 kg/ha in 1985, 5383 kg/ha in 1998, and 5416 kg/ha in 1999, with individual lines producing replicated yields in the 6000-7000 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 4200-5500 kg/ha range. These results indicate there is a temperature by photoperiod interaction in soybeans that results in soybeans flowering up to 2 weeks earlier than normal in response to above normal temperatures in early spring (in May at Wooster, OH). This results in the soybeans entering the reproductive cycle earlier in the growing season when the days are longer and the light intensity is higher (greater total solar radiation is available). Also the length of the reproductive cycle was increased since maturity was similar to years of more normal spring temperatures. This resulted in a significant increase in the yield potential of soybeans in years of unusually early warm spring temperatures. These results suggest if breeders can develop full season soybean cultivars that will bloom earlier under more normal spring temperatures, the yield potential of soybeans in the higher latitudes could be significantly increased.
Cooper, R. L. (2003). A delayed flowering barrier to higher soybean yields. Field Crops Research, 82(1), 27–35. https://doi.org/10.1016/S0378-4290(03)00003-0