Corn Response to Nitrogen After Alfalfa As Affected by Tillage and Regrowth.

Current N guidelines for corn following alfalfa in Minnesota suggest that when compared to corn following corn, N rates for first-year corn after alfalfa can be reduced by 168 kg N ha^-1 when ≥43 alfalfa plants m^-2 are present at termination. Two unanswered questions regarding N availability to first-year corn after alfalfa relate to the timing of primary tillage for alfalfa termination and the amount of alfalfa regrowth incorporated. In 2010, experiments were conducted in first-year corn after alfalfa on six farms in southern and central Minnesota with medium- to fine-textured soils and ≥43 alfalfa plants m^-2 at termination. The response of grain and silage yields to N fertilizer was evaluated within four combinations of alfalfa regrowth management (regrowth after early September left in place or harvested) and tillage timing for alfalfa termination (disc-chiseling in the fall vs. spring) on each farm. Corn grain and silage yields were not affected by regrowth management or tillage timing on any farm. There was above-normal potential for N loss at these farms, as precipitation from 1 Oct. 2009 to 31 Sep. 2010 was 20 to 57% above the 30-yr average. Corn grain yield ranged from 11.3 to 14.5 Mg ha^-1 among farms, and grain and silage yields responded to N fertilizer on just one of six farms. At this farm with above-normal rainfall and poor draininage, net return to N fertilizer within $2.50 ha^-1 of the maximum for grain yield occurred with 76 to 90 kg N ha^-1, assuming $0.99 kg^-1 N and $206.71 Mg^-1 grain. Similarly, silage yield on this farm was increased by 13% with 110 kg N ha^-1. 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 and silage yields are often maximized without N fertilizer, regardless of alfalfa regrowth management or the timing of primary tillage for stand termination.