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Title:   Author  item
                                            Shipitalo, Martinitem
                                            Malone, Robert

                                                         - *Rob*item
                                            Ma, Liwangitem
                                            NOLAN, BERNARD - Us Geological Survey (USGS)item
                                            KANWAR, RAMESHWAR - Iowa State Universityitem
                                            SHANER, DALE - Retired ARS Employeeitem
                                            PEDERSON, CARL - Iowa State University
  Submitted to: Pest Management Science
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 7/29/2015
                                    Publication  Date: 8/14/2015
                                    Citation: Shipitalo, M.J., Malone, R.W., Ma, L., Nolan, B.T., Kanwar, R.S., Shaner, D.L., Pederson, C.H. 2015. Corn stover harvest increases herbicide movement to subsurface drains – Root Zone Water Quality Model simulations. Pest Management Science. 72:1124-1132. doi: 10.1002/ps.4087.Interpretive Summary: The removal of crop residues for use in the production of bioenergy and other purposes can change soil physical properties and the amount and quality of water that moves through the soil to subsurface drains. This, in turn, can alter the movement of the herbicides that are used in crop production and the chemicals that form as a result of their decomposition. The magnitude of these effects can vary from year-to-year based on the weather and are difficult to measure in the field. Fortunately, by combining actual measurements of herbicide movement and the weather with the herbicide and soil properties in a computer model known as the Root Zone Water Quality Model the effects of crop residue removal on herbicide movement to subsurface drains can be estimated. In this experiment we simulated the movement of two herbicides and one herbicide breakdown product when 50 or 100% of the crop residue from a corn crop was removed after grain harvest from a field in Iowa. The 3-year simulation indicated that residue removal decreased the amount of water leaving the field through the subsurface drains mainly by increasing the amount of precipitation lost as evaporation from the soil surface. At the 50% removal rate drainage was reduced by 31%. Most likely the removal of the crop residue will also decrease the number of pores reaching the soil surface because of increased formation of a soil crust due to greater effect of raindrops on the unprotected portions of the soil surface. Under these conditions the simulations indicated that at the 50% residue harvest level losses in subsurface drainage of the two the herbicides we investigated would increase by 4 to 5 times, but still would be in the range of the amounts typically measured in the field. On the other hand, the loss of the herbicide breakdown product would decrease by half. This research is important to farmers that harvest crop residues for bioenergy or other bio-products because it indicates that they may need to use additional conservation practices to limit herbicide losses in tile drainage water, particularly if more than 50% of the corn crop residue is removed.
                                Technical Abstract: BACKGROUND: Removal of crop residues for bioenergy production can alter soil hydrologic properties, but there is little information on its impact on transport of herbicides and their degradation products to subsurface drains. The Root Zone Water Quality Model, previously calibrated using measured flow volumes and atrazine concentrations in subsurface drains in a 0.4 ha chisel-tilled plot, was used to investigate the effects of 50 and 100% corn (Zea mays L.) stover removal with and without reductions in soil crust hydraulic conductivity and total macroporosity on the movement of atrazine, metolachlor, and metolachlor oxanilic acid (OXA). 
RESULTS: A 3-year simulation indicated that 50% residue removal decreased subsurface drainage by 31%, primarily due to increased evaporation. Reducing surface crust hydraulic conductivity and total macroporosity resulted in minor (0.4%) further reductions in drainage volume. Residue removal greatly reduced herbicide transport in drainage when the hydraulic properties of the soil were not altered, but herbicide losses increased 4 to 5-fold when surface crust conductivity and macroporosity were reduced by 25% at the 50% residue removal rate. Based on its measured sorption coefficient, however, ~ 2-fold reductions in OXA losses were noted with residue removal, even when the hydraulic properties were reduced. CONCLUSION: The most likely scenario in which reductions in surface hydraulic conductivity and macroporosity accompanied residue removal resulted in increased atrazine and metolachlor losses, but reduced losses of the metolachlor degradation product OXA in subsurface drainage.


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