Prohexadione-Calcium Improves Stand Density and Yield of Alfalfa Interseeded into Silage Corn

Interseeded alfalfa (Medicago sativa L.) could serve as a dual-purpose crop to provide groundcover for silage corn (Zea mays L.) and forage during subsequent years of production, but interspecific competition oft en leads to poor stands of alfalfa and unsatisfactory yields of corn. Four experiments evaluated whether interspecific competition could be alleviated by application of plant growth retardants (PGRs) on alfalfa interseeded at corn planting. In three experiments, foliar application of prohexadione-calcium (PHD) on seedlings approximately doubled stand density of interseeded alfalfa during establishment in corn and following 1 yr of alfalfa forage production. Untimely PHD application in one experiment, however, failed to prevent stand failure of alfalfa growing under warm and damp conditions that favored vigorous growth of corn. When successfully established without PHD, average dry matter yield (DMY) of interseeded alfalfa the year following corn was 85% greater than conventionally spring-seeded alfalfa; prior year PHD application further increased DMY of interseeded alfalfa by 15%. When tested with PHD in two experiments, seed soaking or coating with uniconazole (UCZ) or paclobutrazol (PBZ) reduced stand density and had neutral or adverse impacts on DMY of alfalfa following corn. Interseeding reduced silage corn DMY by an average of 10% compared to monoculture corn and PGR treatment of alfalfa usually failed to alleviate yield depression in corn. Overall, PHD shows promise for enhancing the establishment and subsequent yield of interseeded alfalfa, but further work is needed to refine this production system for farm use.