John R. Teasdale,* Ruth W. Mangum, Jay Radhakrishnan, and Michel A. Cavigelli

USDA-ARS Sustainable Agricultural Systems Lab., Building 001Room 245, Beltsville,MD20705.

Abstract

Weed management is a primary concern of organic farmers. Croprotation is an important potential management approach for regulating weed seed populations in the soil of organic farming systems. Thisresearch was conducted to determine the effect of three organic croprotations on the weed seedbank during the first 6 yr of a long-termcropping systems experiment at Beltsville, MD. The rotations consisted of (i) a 2-yr corn (Zea mays L.)–soybean [Glycine max (L.)Merr.] rotation, (ii) a 3-yr corn–soybean–wheat (Triticum aestivumL.) rotation, and (iii) a 4-yr corn–soybean–wheat–red clover (Trifolium pratense L.)/orchardgrass (Dactylis glomerata L.) hay rotation.Weed seed populations were determined by a greenhouse emergenceassay using soil samples taken in the early spring of each year. Theseedbanks of smooth pigweed (Amaranthus hybridus L.) and commonlambsquarters (Chenopodium album L.) preceding corn were usuallylower following the hay years of the 4-yr rotation or the wheat year ofthe 3-yr rotation than following the soybean year of the 2-yr rotation.However, annual grass seedbanks preceding corn tended to be higherfollowing the hay years of the 4-yr rotation than following the wheatyear of the 3-yr rotation or the soybean year of the 2-yr rotation.Seedbanks in the 3- and 4-yr rotations were similar to those of the 2-yr corn–soybean rotation (higher smooth pigweed and commonlambsquarters and lower annual grass) when these longer rotationsbegan with a corn–soybean sequence than with other sequences. Sequences beginning with hay had lower smooth pigweed and commonlambsquarters seedbank populations than all other sequences. Theseedbank in spring significantly predicted weed abundance at maturityin corn in at least 2 of 4 yr for all species. Results show that longerrotations with more phenologically diverse crops can reduce seedbankpopulations and abundance of important annual broadleaf weed species in organic production systems.