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 on a Webster clay loam (fine-loamy, mixed, mesic Typic Endoaquolls) near Lamberton, MN. Corn grain yields were increased by a single-year interruption with either alfalfa (Medicago sativa L.) or sunflower (Helianthus 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 alfalfa and sunflower, respectively. A single year of sorghum [Sorghum bicolor (L.) Moench], sorghum x sudangrass (intraspecific S. bicolor hybrid), or fallow did not improve corn yields compared with corn monoculture. 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 yr, a 2-yr interruption with sunflower followed by alfalfa increased corn yield by 22%. A 2-yr interruption of sorghum followed by sorghum x sudangrass increased corn yield by 6% over that of monoculture. We conclude that the closely related grasses were relatively ineffective rotation crops for corn. Leguminous alfalfa and nonleguminous sunflower were equally effective in alleviating the corn monoculture yield depression.