Evaluating Chemical Suppression Treatments to Alter the Red: Far-Red Ratio in Perennial Groundcovers for Maize Production

Perennial groundcover (PGC) has great potential to deliver ecosystem service benefits and control weeds in annual row crop systems. Inadequately suppressed PGC, however, acts as an early-season weed, causing a shade avoidance response (SAR) in maize (Zea mays L.) before the critical period for weed control (CPWC) even with resource abundance. The SAR results from a low red to far-red light shift, impairing early season plant growth and decreasing yield. A field study was conducted in Ames, IA, USA to assess application timing of groundcover suppression chemicals on maize growth and development. Two suppression chemical treatments (paraquat or paraquat + glufosinate) were each applied to "Midnight" Kentucky bluegrass (Poa pratensis L.) PGC once on the day of maize planting (DOP) or consecutive maize stages from VE to V6 in a randomized complete block design with unsuppressed groundcover control. Response variables included maize plant height, maize phenological development stage, reflected red:far-red (R:FR) ratio above the PGC canopy, early vegetative and final maize plant density, maize yield and components, and weed communities. Suppression increased reflected R:FR ratio from the groundcover immediately after application. Where suppression was applied at later stages, low R:FR ratio during early vegetative growth stages triggered a maize SAR and maize plant etiolation. Final maize plant height and yield were greater in PGC suppressed at earlier maize stages, although no suppression treatment provided adequate suppression duration. Paraquat + glufosinate more effectively suppressed groundcover overall and limited groundcover competition, producing 8% greater maize grain yield than paraquat alone in year 1, and 13% greater stover and 8% greater total aboveground biomass (TAB) in year 2. Weather conditions influenced chemical suppression efficacy in year 2, emphasizing the importance of identifying reliable chemical suppression to support grain yield from the day of maize planting.