U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


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Date of this Version



Published in Ecological Effects of Pest Resistance Genes in Managed Ecosystems.


The grains working group identified three crops which have sexually compatible weedy relatives likely to be subject to gene flow in US agricultural systems (Table 1). The ease of cross pollination and the successful production of a fertile hybrid vary with each case. If the selective advantage of an introduced trait is positive, however, introgression of the new trait into an existing weed population is possible. The risk of ecological harm is then dependent upon the habitat of the weed. In the crop-weed complexes considered here, in which the habitat of the weedy relative is limited to agricultural systems, the chance that a new trait may threaten natural ecosystems is not likely.

Crop-companion weed complexes often have a common progenitor and a parallel evolution (Harlan 1992). None of our three examples are native to North America; the cultivated crop and the weed relative were introduced into US agriculture at the same time. Rice and wheat seed imported for planting also contained seed of the weed. Sorghum, johnsongrass, and sudan grass were independently introduced into US agriculture as forage crops. See Annex 1 for more detailed background information on each of the crop and weed complexes.