Effect of Foliar Verses Seed-Applied Gibberellin Inhibitors on the Establishment and Forage Yield of Alfalfa Interseeded into Silage Corn.

Prohexadione-calcium (PHD), uniconazole (UCZ), and paclobutrazol (PBZ) are inhibitors of gibberellin biosynthesis that are widely used in ornamental crops and in some fruit and field crops for limiting top growth and for enhancing root growth and plant tolerance to stress from factors such as drought, disease, and insects. Such properties should be ideal for enhancing the survival of alfalfa interseeded into silage corn for use as a dual-purpose cover and forage crop. In replicated field studies at Prairie du Sac Wisconsin, conventional or glyphosate-tolerant alfalfa was interseeded at corn planting in mid May of 2011 and 2013. In both years, June foliar applications of PHD on alfalfa in June reduced herbage mass per plant by an average of 24% in July and increased stand density by an average of 1.8-fold in October compared non-treated controls. While not affecting initial germination, seed soaking in 2011 or coating in 2013 with UCZ or PBZ severely reduced field emergence of alfalfa seedlings. Compared to untreated controls, seed treatments reduced stand density of alfalfa in July by 30% in 2011 and by 51% in 2013, while herbage mass per plant was not affected in 2011 and reduced by 40% in 2013. Average first-year yields of alfalfa established the previous year by interseeding were two-fold greater than alfalfa conventionally spring-seeded after corn. Prior year foliar applications of PHD increased first year alfalfa yields by about 12% and increased fall stand densities by up to two-fold compared to untreated controls. By contrast, prior year seed treatment with UCS or PBZ either depressed or had no effect on first year yields of alfalfa and decreased fall stand densities by about 50% compared to untreated controls.  Overall, foliar-applied PHD shows considerable promise for enhancing the establishment and subsequent yield of alfalfa interseeded into silage corn.