Corbin, A. T. 2008. Transitional dynamics in converting conventional field cropping systems to certified organic. Ph.D. Dissertation, Michigan State University, East Lansing, MI, USA.
Transitional management strategies for certified organic field crop production are of great concern for Midwestern U.S. producers. Rotational tactics during the three-year transition period set the stage for sustainable organic production through improved soil quality, a manageable weed seedbank and acceptable revenues. This study was conducted over four years in order to compare two separate transitional organic systems: a four-year annual crop rotation of corn, soybean, wheat/alfalfa and corn (C-S-W/A-C), which incorporated dairy manure, cover, and interseeded crops, and one year of conventional corn followed by two years of continuous alfalfa (no manure or cover crops), followed again by corn (C-A-A-C). The C-S-W/A-C treatment was split in year three to investigate two separate wheat harvest methods, as grain or as forage.
Soil quality characteristics which include aggregate size distribution, bulk density, and water filled pore space were determined after the first year and at the end of the transition period. We quantified weed seedbank populations through two seasons in the greenhouse and observed weed surface density and above-ground weed biomass in the field. Soil bulk density showed an overall decrease over the transition period for both systems, with the C-S-W/A-C system decreasing at a higher rate than the C-A-A-C system. We saw no significant treatment differences in water filled pore space at the end of the transition period.
An economic analysis was performed over the three year transition period as well as for the first fully certified organic season. Results show both systems to be profitable, with the C-S-W/A-C system generating the highest rates of return during the study when wheat interseeded with alfalfa was harvested as forage. In today’s market the C-S-W/A-C system with wheat harvested as grain shows the highest profitability.
Results of the greenhouse assay, field density and biomass estimates show a significant increase in total weed seeds germinated in the greenhouse, with a decreased response in the field for the more complicated C-S-W/A-C system.
There was an overall increase in percent macroaggregates at the 0-7 cm depth in the > 2000 μm size class over the transition period for both systems, with the C-S-W/A-C system generating significantly higher aggregates in this class when wheat interseeded with alfalfa was harvested as forage. We conclude from this study that either strategy will allow producers interested in making the transition to organic viable options to sustain profitability, cultivate a manageable weed seedbank and improve soil quality characteristics.
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