Drought stress reduces late-maturing soybean [Glycine max (L.) Merr.] yields in the Mid-southern USA. Irrigated, early-maturing soybean cultivars have similar yields to late-maturing soybean cultivars, but they require less irrigation when grown in narrow rows and at high populations. However, there is little information on how early-maturing cultivars respond to drought compared with late-maturing cultivars. We hypothesized that yields of early-maturing soybean would be decreased more than late-maturing soybean when drought occurred. The objectives of this study were to (i) compare responses of early-maturing and late-maturing soybean to drought, and (ii) assess methods for scheduling irrigation of early-maturing and late-maturing soybean. Field studies were conducted in AR in 2007 (one location) and 2008 (two locations). Two cultivars from each of the maturity groups (MG) I, II, Ill, IV, and V were used. A line source sprinkler irrigation system was used to apply an irrigation gradient from full to no irrigation. At Fayetteville in 2007 and 2008, cultivars and MGs were used as covariates, and yield was regressed as a function of total seasonal water (TSW), which was defined as the amount of irrigation plus rainfall from emergence to R6 (full seed development). There was no difference between cultivars within each MG in yield response to TSW in both years. Yields of MG I, II, Ill, IV, and V all decreased linearly as TSW decreased from optimal to no irrigation. The slopes of the regression equations between yield and TSW became less steep as the MGs increased in 2007. In 2008, there was plentiful rainfall resulting in minimal drought stress, and early-maturing soybean and late-maturing soybean had similar yield responses to TSW. At Keiser in 2008, MG V cultivars produced higher yields than MG I and II in the suboptimal irrigation experiments. These results demonstrated that drought had a greater effect on early-maturing soybean than late-maturing soybean. Stomatal conductance and canopy temperature measurements indicated that crop transpiration began to decline at the beginning of R6, which would decrease irrigation requirement. The University of Arkansas' irrigation scheduling program and the Excel spreadsheet program proposed by Purcell et al. (2007) agreed well during August and September after the soybean canopy was completely closed. The Excel spreadsheet program provides an effective, alternative method for scheduling irrigation for soybean associated with narrow rows and high populations.
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