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Production Paper

Weed Seedbank Dynamics in Three Organic Farming Crop Rotations

USDA-ARS Sustainable Agricultural Systems Lab., Building 001 Room 245, Beltsville, MD 20705

^* Corresponding author (teasdale{at}ba.ars.usda.gov)

Received for publication October 23, 2003. Weed management is a primary concern of organic farmers. Crop rotation is an important potential management approach for regulating weed seed populations in the soil of organic farming systems. This research was conducted to determine the effect of three organic crop rotations on the weed seedbank during the first 6 yr of a long-term cropping 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 aestivum L.) 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 emergence assay using soil samples taken in the early spring of each year. The seedbanks of smooth pigweed (Amaranthus hybridus L.) and common lambsquarters (Chenopodium album L.) preceding corn were usually lower following the hay years of the 4-yr rotation or the wheat year of the 3-yr rotation than following the soybean year of the 2-yr rotation. However, annual grass seedbanks preceding corn tended to be higher following the hay years of the 4-yr rotation than following the wheat year 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 common lambsquarters and lower annual grass) when these longer rotations began with a corn–soybean sequence than with other sequences. Sequences beginning with hay had lower smooth pigweed and common lambsquarters seedbank populations than all other sequences. The seedbank in spring significantly predicted weed abundance at maturity in corn in at least 2 of 4 yr for all species. Results show that longer rotations with more phenologically diverse crops can reduce seedbank populations and abundance of important annual broadleaf weed species in organic production systems.

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