Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/19/2006
Publication Date: 7/1/2007
Citation: Krutz, L.J., Koger III, C.H., Locke, M.A., Steinriede Jr, R.W. 2007. Reduced Surface Runoff Losses of Metolachlor in Narrow Row Compared to Wide-Row Soybean. Journal of Environmental Quality 36:1331-1337. doi:10.2134/jeq2006.0548. Interpretive Summary: Planting crops in narrow rows rather than wide rows may reduce the movement of pesticides applied over-the-top of the crop’s canopy thereby protecting surface water quality. The effect of row spacing on the movement of the herbicide metolachlor was evaluated with simulated rainfall in Stoneville, MS. Planting soybean in narrow rows reduced the loss of metolachlor by 40% compared to planting soybean in wide rows. Results from this study demonstrate the potential for using narrow-row cropping systems as a best management practice for improving surface water quality.
Technical Abstract: Cultural management practices that reduce the off-site transport of herbicides applied to row crops are needed to protect surface water quality. Greater water use and canopy coverage in narrow-row soybean systems may reduce surface runoff losses of foliar applied, non-polar compounds. The effect of row spacing on the transport of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(methoxy-1-methylethyl) acetamide] applied post-emergence, over-the-top of soybean in the four-leaf growth stage was evaluated with simulated rainfall in Stoneville, MS on Sharkey Clay (very-fine, smectitic, thermic chromic Epiaquert). The calculated mass of metolachlor intercepted by the soybean canopy was 0.39 kg ha-1 for the narrow-row system (50-cm-wide rows) and 0.23 kg ha-1 for the wide-row system (100-cm-wide rows). One day after application, the mass of metolachlor available for foliar washoff was reduced by 98% for both systems. Antecedent soil moisture at three depth intervals was lower in narrow-row soybean indicating greater water use relative to the wide-row system. Greater water use in narrow-row soybean altered the hydrology by increasing the time-to-runoff by 69% relative to wide-row soybean. The concentration of metolachlor in runoff at all sampling points was greater in the wide-row system. Cumulative losses of metolachlor from the wide-row system were 47.5 g ha-1 compared to 22.6 g ha-1 for narrow-row soybean. These values correspond to fractional losses of 3.7% and 2.2% for wide- and narrow-row soybean, respectively. Results from this study demonstrate that narrow-row soybean reduce surface runoff losses of metolachlor when the herbicide is applied post-emergence over-the-top of V-4 soybean by reducing the mass available for transport in the mixing zone through foliar interception, foliar absorption, and leaching below the mixing zone.