Weed Suppression and Organic Soybean Performance As Influenced by Rye Cultivars in the "Roll-Killed" Cover-Crop System.

The “roll-killed” cover-crop system has been successful in reducing weed pressures in organic soybeans.  This study examined six-rye cultivars as cover crops to determine which were most suitable for use in the rye roll-killed organic soybean system for the Southern US.   Six rye cultivars, three facultative, non-winter hardy Southern cultivars, and three non-facultative, winter hardy Northern cultivars, were planted at three locations over the 2009 and 2010 growing seasons.  Non-facultative winter hardy rye cultivars require vernalization offered by a fall planting in order to complete their life cycle.  However, facultative rye cultivars do not require vernalization to complete their lifecycle and can be planted in spring or fall (Reberg-Horton et al., 2003).  Each rye cultivar was roll-killed at soft-dough and soybeans (Glycine Max L. ‘NC Roy’) of maturity group VI were planted on two separate dates (i.e. early and late roll-kill).  Favorable weather through out the 2009 growing season combined with early rye planting resulted in all rye cultivars being roll-killed with 100% kill rating for both dates. All rye cultivars produced greater than 9000 kg ha^-1 rye biomass with exception of Rymin at Tidewater.  Weed control during the 2009 growing season was excellent and there was no rye cultivar effect on soybean yield.    Due to an unseasonably cold and wet winter, after the early roll-kill date in 2010, only the Southern cultivars were rated at 100% kill.  Northern cultivar Aroostook, a Northern cultivar, along with all the Southern cultivars were the only rye cultivars rated at 100% kill by the late roll-kill date.  Weed pressures were more variable during 2010. Wheeler, despite producing sub 9000 kg ha^-1 biomass, had the lowest broadleaf densities, which could be explained by Wheeler’s reported allelopathic activity.  Soybean yield in 2010, was modeled with rye biomass (kg ha^-1) and soybean stand count (counts m^-1) where rye biomass and soybean stand count were used as covariates.  Due to a dry and hot summer, soybean stand count negatively effected soybean yield during the late roll-kill date.  Overall, Wheeler was found to be the most suitable rye cultivar for organic soybean production using the roll-killed system in the Southeast US based on its good biomass production, allelopathic properties, and a termination date that closely synchronizes with soybean planting dates (Mid May-June).