Weed Seedbank Responses in Two Transitional Field Crop Rotations.

Weed management during the required three-year transition period significantly influences the weed seedbank for the initial years in certified organic systems.  We studied two transitional field cropping systems during the three-year transition period and the first fully certified organic season.  Previous experimental evidence has suggested that longer crop rotations with diverse crops can reduce seed bank populations of important weed species in organic production systems, however, seed bank responses in the field often show insignificant correlation with seed bank germination assays.  Prediction of weed seedling densities from buried seed reserves has been enhanced by considering specific sampling size, date, number and technique. For example, some investigators have found the greenhouse germination assay to be more reliable with a smaller sample size taken in early spring than seed extraction by elutriation when correlating field seedling densities. Germination techniques as predictive tools have been supported as more accurate methods of seedling density estimation. We quantified weed seedbank populations through two seasons (year three and four) in the greenhouse and observed weed surface density (year three) and above-ground weed biomass in the field. Using a modified sampling technique designed to capture spatial variability by increasing the number of soil cores of a reduced core diameter, we attempted to show predictability of weed seedling densities using a greenhouse germination assay.  A four-year rotation of corn, soybean, wheat/alfalfa, corn (C-S-W/A-C) produced with organic sources of nutrients (manure and cover crop residue) was compared to a corn, alfalfa, alfalfa, corn (C-A-A-C) rotation for the transition to a certified organic system.  Results of the greenhouse assay, field density and biomass estimations show a sixty to nearly three hundred percent increase in total weed seeds germinated in the greenhouse, with a sixty to over five hundred percent decreased response in the field for the more complicated C-S-W/A-C system.