Alfalfa nitrogen credit to first-year corn: Potassium, regrowth, and tillage timing effects

Compared with corn (Zea mays L.) following corn, N guidelines for first-year corn following alfalfa (Medicago sativa L.) in the U.S. Corn Belt suggest that N rates can be reduced by about 168 kg N ha^-1 when ≥43 or 53 alfalfa plants m^-2 are present at termination. These guidelines have been questioned by practitioners, however, as corn grain yields have increased. We conducted experiments at 16 locations in Minnesota to address questions regarding N availability to first-year corn after alfalfa relating to the effect of carryover fertilizer K from alfalfa and the amount and timing of alfalfa regrowth incorporation. Corn grain yield, silage yield, and fertilizer N uptake were not affected by carryover K or amount or timing of regrowth incorporation. Maximum corn grain yield ranged from 12.0 to 16.1 Mg ha^-1 among locations but responded to fertilizer N at only one. At that location, which had inadequate soil drainage, the economically optimum N rate (EONR) was 85 kg N ha^-1, assuming prices of US$0.87 kg^-1 N and US$132 Mg^-1 grain. The EONR for silage yield across 6 of 15 locations where it was measured was 40 kg N ha^-1, assuming US$39 Mg^-1 silage. These results demonstrate that on highly productive medium- to fine-textured soils in the Upper Midwest with ≥43 alfalfa plants m^-2 at termination, first-year corn grain yield is often maximized without fertilizer N, regardless of alfalfa regrowth management or timing of incorporation, but that small N applications may be needed to optimize silage yield.