WEED SEEDBANK DYNAMICS IN THREE ORGANIC FARMING CROP ROTATIONS

Authors: Teasdale, John; Mangum, Ruth; Radhakrishnan, Jayakumar; Cavigelli, Michel

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2004
Publication Date: 9/1/2004
Citation: Teasdale, J.R., Mangum, R.A., Radhakrishnan, J., Cavigelli, M.A. 2004. Weed seedbank dynamics in three organic farming crop rotations. Agronomy J. 96:1429-1435.

Interpretive Summary: Organic farming is rapidly expanding in the United States. Weed management has been identified by organic farmers as their primary research priority. Crop rotation has the potential to reduce weed seed populations in soil that can lead to additional weed problems in future years. Three organic farming rotations were compared in a long-term Farming Systems Project at the USDA-ARS Beltsville Agricultural Research Center to assess how populations of weed seeds changed over a six year period since the comparison began in 1996. Rotations composed of only the summer annual crops corn and soybeans had high soil seed populations of summer annual weeds. Adding a winter annual wheat crop or a perennial hay crop reduced seed populations of the most troublesome summer annual weeds. Although substantial increases in weed seeds in the soil could occur following years of poor weed control in corn or soybean, substantial decreases could occur if weeds were controlled using diverse rotational crops. Sequences that began with corn and soybeans had higher weed seed populations than sequences that began with hay. Lower weed seed populations in soil often led to better weed control in the succeeding corn crop reinforcing that maintaining low weed seed populations is important for organic farming. This information will be useful to organic farmers and technology transfer professionals to design optimum organic farming rotations as well as to weed and crop ecologists to understand crop rotational effects on weed population dynamics.

Technical Abstract: Weed management is a primary concern of organic farmers. Crop rotation is an important potential management approach for maintaining low soil weed seed populations in organic farming systems. This research was conducted to determine the effect of three organic crop rotations on the weed seedbank during the first six years of a long-term farming systems experiment at Beltsville, Maryland. The rotations consisted of 1) a two-year corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation, 2) a three-year corn-soybean-wheat (Triticum aestivum L.)/fallow rotation, and 3) a four-year 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 four-year rotation or the wheat/fallow year of the three-year rotation than following the soybean year of the two-year rotation. However, annual grass seedbanks preceding corn tended to be higher following the hay years of the four-year rotation than following the wheat-fallow year of the three-year rotation or the soybean year of the two-year rotation. Seedbanks were similar to those of the two-year corn-soybean rotation (higher smooth pigweed and common lambsquarters and lower annual grass) when three- and four-year 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 and rotations. The seedbank in spring significantly predicted weed abundance at maturity in corn in at least 2 of 4 years for all species. Results show that longer rotations with more phonologically diverse crops can reduce seedbank populations and abundance of important annual broadleaf weed species in organic production systems.