THE NATURE AND CONSEQUENCE OF WEED SPREAD IN CROPPING SYSTEMS

Weeds spread through movement of seeds and vegetative reproductive pro pagules. Pollen movement can spread weedy traits, such as herbicide re sistance, between related weed and crop species. Weed seeds can spread short or long distances by natural plant dehiscence mechanisms, wind, water, animals, and man's activities. This symposium paper is a pract ical review of short-distance spread of weed seeds in and between near by arable fields and noncrop lands, examining some of the causes of sp read and subsequent effects on crop production. Pollen movement, as it affects the spread of herbicide resistance, also is considered a comp onent of short-distance weed spread. Specific weed management options can be used to reduce man-caused weed seed spread within and between n earby fields, thus reducing potential crop yield losses. Long-term man agement will be more difficult for weed seed spread by natural dispers al mechanisms.