A COMPARISON OF METHODS TO PREDICT WEED SEEDLING POPULATIONS FROM THESOIL SEEDBANK

Accurate prediction of potential weed seedling density would allow gro wers to implement control measures more effectively and could help avo id inappropriate and over application of preemergence herbicides. We c ompared three methods for handling soil samples to predict potential w eed seedling emergence in plow-disk and no-tillage corn: seedling emer gence from greenhouse trays, emergence from intact cores, and seed ext raction following sieving. Seedbank numbers were highest for common la mbsquarters followed by annual grasses and redroot pigweed, and seed n umbers were higher in no-tillage than plow-disk plots. Coefficients of variation typically exceeded 60%. Density of seedling emergence from cores was more similar to field density than was emergence from trays. The percent of seeds in the seedbank that emerged was commonly more t han 90% for annual grasses and usually less than 20% for common lambsq uarters. All methods gave equivalent and relatively poor predictions o f field population density. Spearman rank correlation between predicte d and actual populations ranged from low negative values for common la mbsquarters in no-tillage to 0.60 for annual grass emergence from tray s in 1991. No method consistently gave highest correlations in both ye ars and both tillage systems. Seedling emergence from intact cores, ev aluated for 4 yr in plow-disk and no-tillage soybean fields, was signi ficantly correlated (r(s) = 0.15 to 0.68) with emergence in the field. Pooling data from three to five neighboring sample sites increased th e correlation between core and field emergence densities.