Early High‐moisture Wheat Harvest Improves Double‐crop System: II. Soybean Growth and Yield

Double cropping soybean [Glycine max (L.) Merr.] after winter wheat (Triticum aestivum L.) increases total food production without additional land. However, double-crop soybean usually yields less than full-season soybean, mainly due to late planting. We evaluated double-crop soybean growth and yield as affected by early planting immediately after high-moisture wheat harvest across 20 site-years in five Mid-Atlantic states during 2015-2017. At each site, six soybean cultivars from relative maturity group (rMG) 3.1-5.9 were planted at three to five dates in a 4- to 14-d interval. Soybean growth, measured by normalized difference vegetation index (NDVI) across the growing season, was affected only by planting date. Although NDVI peaked near the R5 stage, it took 9-27 more days to reach the peak NDVI (0.84-0.98) for early-planted soybean than for late-planted soybean. Relative yield declined with planting dates, which explained 41-81% of the relative yield variability. The yield loss from delayed planting was greater in the north (33-80%; Pennsylvania, Maryland, and Delaware) than in the south (20-27%; Virginia, North Carolina) due to longer delay in planting and shorter growing season in the north. Soybean NDVI from the R1-R6 stages was associated with yield, with the strongest association (R-2 = .55-.57) at the R2 and R3 stages. The area under the NDVI curve (AUNDVIC) was also strongly associated (R-2 = .77) with relative yield, indicating an excellent tool for explaining double-crop soybean yield loss due to poor growth. High-moisture wheat harvest facilitated soybean planting 4-21 d earlier, which increased growth and yield.