Establishment of Alfalfa Intercropped under Corn in Response to Varying Rates of Prohexadione with or without Fungicide Plus Insecticide

Establishment of interseeded alfalfa (Medicago sativa L.) under corn (Zea mays L.) silage is enhanced with foliar applications of prohexadione (PHD) followed by fungicide plus insecticide (FI), but the lowest effective rates must be determined. We evaluated stand characteristics of alfalfa interseeded into corn at Arlington, Wisconsin, USA in response to PHD applied at 0 to 0.423 kg a.e. ha⁻¹ followed two weeks later with FI (none vs. 0.147 kg a.i. ha⁻¹ fluxapyroxad-pyraclostrobin plus 0.018 kg a.i. ha⁻¹ lambda-cyhalothrin). Application of PHD reduced etiolation, while FI treatment increased plant health and vigor. Following corn harvest, non-treated alfalfa stands averaged 4.2 plants m⁻² and 1.2% groundcover under wet growing conditions in 2019 compared with 71.3 plants m⁻² and 15.9% groundcover under normal growing conditions in 2020. Stand density in 2019 reached 130 plants m⁻² but failed to plateau with combined PHD-FI treatments, while in 2020, stand density averaged 177 plants m⁻² with FI regardless of the PHD rate. Alfalfa groundcover plateaued at 63% in 2019 and 71% in 2020 when 0.16 to 0.30 kg a.e. PHD ha⁻¹ was applied prior to FI. The results indicate that FI enables excellent alfalfa establishment under normal conditions, but both PHD and FI should be applied during wet growing conditions.