Weed seedbanks and corn growth following continuous corn or alfalfa

Specific crop rotations may reduce weed problems and supplement agrichemical inputs. This study, conducted from 1991 through 1994, compared weed densities and corn (Zea mays L.) growth in continuous corn rotation (C/C) and in an alfalfa (Medicago saliva L.)/alfalfa/corn rotation (A/C) among different input levels (tillage, herbicide, and fertilizer rates varied to achieve high, intermediate, and low treatments). Alfalfa suppressed weeds. During establishment, grass and broadleaf weeds made up about 36 and 10% of the forage biomass, respectively. During the second year, grass and broadleaf weeds made up no more than 17 and 4% of the forage biomass, respectively. Weed seed density in both A/C and C/C corn low input plots averaged about 15 000 seeds m(-2) in 1993; however, grass seed comprised 73% of the C/C seedbank, but only 26% of the A/C seedbank. In 1994, weed seed density increased to 38 000 seeds m(-2) in the low input A/C corn, and grass species made up 55% of the seedbank. However, grass densities in A/C and C/C low-input corn averaged about 300 and 4000 shoots m(-2), respectively, each year, No grain was harvested in C/C low-input corn, due to intense weed competition. Grain yield in A/C low-input corn was 37% of the A/C high-input treatment in 1993, 66% of the yield in A/C high-input treatment in 1994, and similar to yield in the C/C medium-input treatment both gears. Alfalfa in rotation with corn may be used to reduce weed competition while sustaining yield.