Soybean Planting Date and Small Grain Residue Effects On Soybean Yield and Yield Components.

Full-season soybean [Glycine max (L.) Merr.] and double-cropped soybean following wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.) are three soybean cropping systems used in the Mid-Atlantic USA.  Research comparing these systems is limited; therefore, field studies were conducted to determine the effect of planting date and winter grain on soybean yield and yield components. Soybean yields declined with planting date at two of four locations in 2009, a year that late-season rainfall enabled later-planted soybean to yield more than is expected.  When averaged across winter grain crop, yield declined from 1680 to 910 kg ha^-1 or from 3890 to 3170 kg ha^-1 for the May and early June planting dates to the mid-July planting dates, respectively.  In 2010, drought resulted in the loss of two locations.  At the remaining two locations, yield decreased as a cubic function of planting date from 2700 kg ha^-1 to 1800 kg ha^-1 or from 1800 to 1600 kg ha^-1.  In 2011, yield decreased quadratically from 3400 to 2700 or 3700 to 2750 at two locations as planting date was delayed from late-May until mid-July.  Winter grain did not affect soybean yield in either year.  These data indicate that planting date has a greater effect on yield than the small grain residue.  The yield component data reinforced these results and indicated that the seed yield decline with later planting dates is due largely to a decrease in the number of pods.