INTERRUPTING YIELD DEPRESSION IN MONOCULTURE CORN - COMPARATIVE EFFECTIVENESS OF GRASSES AND DICOTS

It has been well documented that monoculture corn (Zea mays L.) yields less than corn grown in certain rotations. This field study conducted from 1992 through 1995 investigated several agronomic crops, rotated with corn, for their relative effectiveness in interrupting the yield depression associated with corn monoculture. The study was conducted o n a Webster clay loam (fine-loamy, mixed, mesic Typic Endoaquolls) nea r Lamberton, MN. Corn grain yields were increased by a single-year int erruption with either alfalfa (Medicago sativa L.) or sunflower (Helia nthus annuus L.) for each of 4 yr, with the exception of sunflower in 1992. Averaged across 4 yr, corn yields were increased from 7.81 Mg ha (-1) for continuous corn by 19 and 17% following a single year of alfa lfa and sunflower, respectively. A single year of sorghum [Sorghum bic olor (L.) Moench], sorghum x sudangrass (intraspecific S. bicolor hybr id), or fallow did not improve corn yields compared with corn monocult ure. This was true for each year, and when averaged across 4 yr, with the exception of the corn-fallow rotation in 1995. Averaged across 4 y r, a 2-yr interruption with sunflower followed by alfalfa increased co rn yield by 22%. A 2-yr interruption of sorghum followed by sorghum x sudangrass increased corn yield by 6% over that of monoculture. We con clude that the closely related grasses were relatively ineffective rot ation crops for corn. Leguminous alfalfa and nonleguminous sunflower w ere equally effective in alleviating the corn monoculture yield depres sion.