kberg@purdue.edu

Little is known of the mechanisms by which P and K enhance forage yield and persistence. Our objective was to determine how P and K nutrition affects forage yield, yield components, and physiological and biochemical characteristics associated with high yield and altered plant persistence. A factorial design of five K (0, 100, 200, 300,and 400 kg/ha/yr) and four P (0, 25, 50, and 75 kg/ha/yr) treatments were arranged in a random block design replicated four times. Plots are fertilized after the initial harvest in spring (late May) and after the last forage harvest in mid-September, with one half the specified amount per year in each application. Herbage was harvested with a flail-type chopper and forage yield determined. Shoots are removed at random from the plot area to determine yield per shoot and shoots per area. Roots are dug and plants counted in May and Dec. in order to determine the influence of P and K nutrition on plant persistence, and whether plants die during summer and fall (May to Dec.) or during winter and early spring (Dec. to May). Root tissues are analyzed for biochemical constituents known to be associated with winter survival and plant regrowth including: buffer-soluble protein; sugar; starch; and total amino nitrogen. Yield increased with addition of P and K fertilizer. In 1998, forage yield increased from 13.8 Mg/ha with no P and K added to 16.1 Mg/ha with 75 kg P/ha and 400 kg K/ha. In 1999, forage yield increased from 14.0 to 17.9 Mg/ha with no fertilizer applied and 75 kg P/ha and 400 kg K/ha respectively. Shoots per plant and plants per area were unaffected by P and K fertility. Yield per shoot increased with P and K application and was most closely associated with enhanced yield (Fig. 1). Plant populations declined from 400 to 200 plants/m² between Dec. 1997 and December 1998 but remained unchanged overwinter 1998-1999 (Fig. 2). From May 1999 to Dec. 1999, plant populations decreased from 200 to 154 plants/m², and again overwinter 1999-2000 populations remained unchanged. There was no effect of P and K fertilization on plant populations in the first four root samplings, but beginning in Dec. 1999, plants/m² decreased with increased P nutrition. We expected P and K deficiency to limit root reserve accumulation.Root protein concentrations increased with additions of P but were unaffected by K. Starch concentrations in May declined with P application, while starch concentrations in Dec. increased with additional P. Potassium increased root sugar concentrations in May, but had no effect in Dec. Amino nitrogen concentration was increased with P additions. Although this study has not shown increased plant population with increased fertilization, yield has increased with lower stand counts produced by increased P fertilization.