Prohexadione-Calcium Rate and Timing Effects on Alfalfa Interseeded into Silage Corn

Prohexadione-calcium (PHD) can enhance establishment of alfalfa (Medicago sativa L.) interseeded into silage corn (Zea mays L.), but optimal application rates and timing for this growth regulator are unknown. Two experiments examined how single or split applications of 0.25 to 1.0 kg a.i.ha(-1) of PHD applied to 15 or 25 cm tall glyphosate-resistant alfalfa seedlings influenced growth, stand density, and yield of alfalfa and corn compared to untreated controls. The PHD treatments reduced seedling top growth by 13 to 32% and increased stand density of alfalfa by 19 to 81% in three out of four locations and alfalfa dry matter yield by 4 to 23% in two out of four locations the following year. Alfalfa responses were not consistently influenced by the rate and timing of PHD application, but higher rates proved advantageous under conditions that impaired alfalfa seedling survival under corn. Application of PHD on alfalfa had little or no effect on corn heights and yields. Corn population and alfalfa seeding rates influenced corn silage yield and alfalfa stand counts in at least one location, but these factors had little effect on crop responses to PHD. Combined 2-yr yields of corn followed by alfalfa were 12% greater for the interseeding system than for solo-seeded corn followed by spring seeded alfalfa. Overall, a single application of 0.5 to 1.0 kg a.i.ha(-1) of PHD on 15 to 25 cm tall alfalfa proved effective for improving stand density and occasionally first year yield of glyphosate-resistant alfalfa.