Mechanisms of Weed Suppression in Rolled Cover Crop Systems.

Across multiple studies, the mechanism for weed suppression with rolled cover crops was investigated.  The major hypotheses include allelopathy, shading, and in the rolled rye/ soybean system, nitrogen immobilization.  While all of these factors likely play a role, dissecting the size of each effect is considered.  The effect of shading versus allelopathy plus shading was investigated by varying the rates of wheat straw and rye mulch.  Wheat straw is known to contain allelochemicals, however, most are several orders of magnitude lower than in a fresh green rye residue.  Redroot pigweed (Amaranthus retroflexus L.)  densities suggest  that allelopathy plays a role at intermediate levels of biomass, from 5,000 kg ha^-1 to 9,000 kg ha^-1 but is unnecessary at higher biomass levels.  In a separate study, available soil nitrogen was monitored in rolled rye plots versus conventionally tilled ones.  Conventionally tilled plots had up to 6.8 times the amount of available nitrogen as rolled rye over the first two weeks with differences still evident 6 weeks after planting.  In a third study, six cultivars of rye were evaluated for biomass production and weed suppression.  At zero and two weeks after rolling, soil cores to a depth of 5 cm were taken to evaluate allelochemical concentrations.  Water extracts will be applied to petri dishes containing redroot pigweed and large crabgrass (Digitaria sanguinal  (L.) Scop.) to detect differences amongst cultivars.