Mehmet Sincik

How Growth Dynamics Affect Soybean Development across Cultural Practices

Determination of the most suitable planting date, plant population and cultivar for optimal yield is an important agronomic goal in soybean production. However, soybean yield is determined by interactions with environmental conditions as well as genetic yield potential. Compared to earlier planting, delayed planting reduces yields (Beatty et al., 1982; Carter and Boerma, 1979; Parker et al., 1981; Egli and Bruening, 2000). Yield reduction in late-planted, double-crop soybeans has been attributed to a lack of sufficient vegetative growth (Ball et al., 2000 b; Herbert and Litchfield, 1984). Increasing the leaf area to maximize LI is the primary reason that increased biomass is associated with higher yields in late-planted soybean (Wells, 1991; Board et al., 1992). Previous studies have indicated that optimal plant populations vary from 30,000 to 500,000 plants ha-i (Costa et al., 1980; Parks et al., 1982; Egli, 1988; Ennin and Clegg, 2001). In general, optimal plant population is greater under poor growing conditions than good growing conditions (Wells, 1993). Also, row spacing, which determines plant population in a unit area, is a major agronomic factor affecting soybean yield. In previous studies on planting date and row spacing, yield increases associated with narrow rows appear to be greater from late planting dates than from optimum dates (Board et al., 1990; Boerma and Ashley, 1982; Boquet et al., 1982). There is interest in planting soybeans in narrow rows to increase the LI for higher yields (Board and Harville, 1993). However, Purcell et al. (2002) stated that yield does increase at high population densities because of decreased radiation use efficiency. A previous study reported that recommended populations for optimum planting dates were insufficient for late-planted soybean because of the failure of these populations to achieve maximum LI, especially in years of low rainfall (Ball et al., 2000 a). Early-maturity groups have not been used late in the season because inadequate canopy development generally occurs in the recommended populations (Kane and Grabau, 1992). Early-maturing cultivars have a shorter period of vegetative development than full-season cultivars, but the length of the seed-fill phase is about the same as for conventional cultivars (Egli et al., 1978; Egli, 1993). Growth dynamics such as LAI, LI, LIE, TDM, and CGR are major predictors of soybean yield. The relationships between seed yield and growth dynamics vary with environmental conditions and cultural practices (planting date, plant population, and cultivar, etc.). Duncan (1986) detected that greater TDM results in greater seed yield if the TDM is produced before seed initiation. In contrast, Weber et al. (1966) reported that both TDM and