Pre-Emergent Herbicide Programs for Interseeded Cover Crops in Mid-Atlantic Corn Production.

Integrating cover crops into annual crop rotations can improve soil quality, nutrient cycling, pest regulation and crop productivity.  Cover crop establishment in no-till corn production is often constrained by a short cover crop growing season following corn harvest within the Mid-Atlantic region. Relay cropping, or interseeding, can facilitate the establishment of cover crops by sowing cover crops into standing corn. Recent field studies suggest that annual ryegrass and several leguminous species can be established into corn at the V5 growth stage, approximately 35 days after corn planting. At this timing, cover crop establishment may be exposed to soil residual herbicides applied at corn planting.  We evaluated the effects of various pre-emergent herbicides on interseeded cover crops in no-till corn production. We conducted field trials near Rock Springs, PA from 2013 to 2015 and Landisville, PA from 2014 to 2015. Pre-emergent herbicides were applied to four replicate plots (3 x 8 m) following planting of glyphosate-resistant corn in late-May each year. Glyphosate was applied across all plots approximately 28 days after planting and cover crops were interseeded 35 days after planting. In each site-year, herbicide trials were conducted in plots interseeded to annual ryegrass at 22.4 kg ha^-1 and medium red clover at 11.2 kg ha^-1. Herbicide treatments included s-metolachlor, dimethenamid-P, acetochlor, pendimethalin, saflufenacil, rimsulfuron, atrazine and mesotrione applied at standard label rates.  In addition, we evaluated foundation rates (½ X) of several grass-broadleaf herbicide mixtures: acetochlor + atrazine, pendimethalin + atrazine, dimethenamid-P + saflufenacil, and s-metolachlor + mesotrione + atrazine. Treatments were visually evaluated (% stand reduction) 30 days after interseeding and aboveground biomass was harvested in late fall. Mesotrione reduced red clover stand establishment 45 to 99% and s-metolachlor reduced annual ryegrass stand establishment 80 to 85%.  Other standard rate herbicides were comparable to the untreated check. Variable stand reductions of red clover (62-93%) and annual ryegrass (11-36%) were observed using the ½ X rate of s-metolachlor + mesotrione + atrazine, but other herbicide mixtures were comparable to the untreated check. Our field trials suggest that pre-emergent residual herbicide programs can be designed for corn production systems that limit injury to interseeded grass or leguminous cover crops.  However, soil texture, pH and seasonal rainfall will influence herbicide persistence and the potential for cover crop injury.