Delineating Soybean Maturity Groups across the United States

TLDR: Time to fl owering may be changing as soybeans’ genetics change due to breeding eff orts and genetic engineering and there have been signifi cant changes in soybean germplasm and management practices during the past 12 yr which could affect the environmental adaptivity of the current available varieties.

Abstract: 1397 Soybean is the most important oilseed crop in the United States and its cultivated area is the second largest in the country aft er corn (Zea mays L.) (USDA, 2016). Th e cultivated areas include a wide diversity of environments from the Great Plains to the East Coast (Fig. 1). Maturity group zones represent defi ned areas where a cultivar is best adapted without implying that MG-specifi c cultivars cannot be grown elsewhere (Boerma and Specht, 2004). Maturity groups range from 000 for the very early maturing varieties to 10 (Boerma and Specht, 2004) for the latest maturing varieties. Gradations within MGs are also commonly noted by adding a decimal to the MG number. Maturity group is determined by two abiotic factors, photoperiod and temperature (Cober et al., 2001; Garner and Allard, 1930; Major et al., 1975; Summerfi eld et al., 1998; Wilkerson et al., 1989). It has been reported that short days promote development but when daylength is greater than a critical value the rate slows (Summerfi eld et al., 1998, 1993). Several studies concluded that elevated spring temperatures promoted earlier fl owering; however, fl owering timing diff ered signifi cantly due to photoperiod (Cooper, 2003; Hartwig, 1973; Major et al., 1975; Upadhyay et al., 1994). It has also been reported that soybeans’ growth is aff ected by a photoperiod and temperature interactive eff ect with genotype (Cober et al., 2001; Heatherly and Elmore, 2004; Summerfi eld et al., 1993). Four major genes (E1, E3, E4, and E7) have been identifi ed and characterized for their responses to long day conditions (Cober and Voldeng, 2001a, 2001b; Saindon et al., 1989a, 1989b). Additionally, major genes and quantitative trait loci (QTLs) for fl owering oft en interact with one another to determine time to fl owering (Watanabe et al., 2012). Th erefore, time to fl owering may be changing as soybeans’ genetics change due to breeding eff orts and genetic engineering. Photoperiod is a constant value for specifi c time period and location and due to the strong relationship with soybean development, more than 45 yr ago, Scott and Aldrich, delineated hypothetical optimum MG zones across the United States based on photoperiod (Scott and Aldrich, 1970). More recently, Zhang et al. (2007) redefi ned the optimum MG zones using yield variety trial data from 1998 to 2003. Th ese authors found that adaptation regions for varieties with MG = 0 to 3 have not changed whereas, for varieties MG = 4 to 6 adaptation zones are much broader than those proposed by Scott and Aldrich. Nevertheless, there have been signifi cant changes in soybean germplasm and management practices during the past 12 yr which could aff ect the environmental adaptivity of the current available varieties. For example, soybean’s value Delineating Soybean Maturity Groups across the United States