Weed seedbanks and corn growth following continuous corn or alfalfa

Specific crop rotations may reduce weed problems and supplement agrichemica l inputs. This study, conducted from 1991 through 1994, compared weed densi ties 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 differen t input levels (tillage, herbicide, and fertilizer rates varied to achieve high, intermediate, and low treatments). Alfalfa suppressed weeds. During e stablishment, grass and broadleaf weeds made up about 36 and 10% of the for age biomass, respectively. During the second year, grass and broadleaf weed s 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 a veraged 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 199 3, 66% of the yield in A/C high-input treatment in 1994, and similar to yie ld in the C/C medium-input treatment both gears. Alfalfa in rotation with c orn may be used to reduce weed competition while sustaining yield.