CHARACTERIZING SPATIAL STABILITY OF WEED POPULATIONS USING INTERPOLATED MAPS

Intensive surveys were conducted in 2 fields in eastern Nebraska to de termine the spatial stability of common sunflower, velvetleaf, green a nd yellow foxtail, and hemp dogbane over 4 yr (1992 to 1995). The 1st field was planted to soybean in 1992 and corn in 1993, 1994, and 1995. The 2nd field was planted to corn in 1992 and 1994 and soybean in 199 3 and 1995. Weed density was sampled prior to post-emergence herbicide application at approximately 800 locations per year in each field on a regular 7 m grid. The same locations were sampled every year. Weed d ensity at locations between the sample sires was determined by linear triangulation interpolation. Weed seedling distribution was significan tly aggregated, with large weed-free areas in both fields. Common sunf lower, velvetleaf, and hemp dogbane patches were very persistent in di ameter in the east-west and north-south directions and in location and area over 4 yr in the Ist field. Foxtail distribution and density con tinuously increased in each of the 4 yr in the first field and decreas ed in the 2nd field. A geographic information system was used to overl ay maps from each year for a species. This showed that 36% of the samp led area was continuously free of common sunflower, 62.5% was free of hemp dogbane, and 11.5% was free of velvetleaf in the Ist field, but o nly 1% was free of velvetleaf in the 2nd field. The persistence of bro adleaf weed patches suggests that. weed seedling distributions mapped in one year are good predictors of future seedling distributions. Impr oved and more efficient sampling methods are needed.