Water conservation and nutrient, sediment, and herbicide movement in furrow-irrigated tillage systems
Date
2013
Authors
Driscoll, Jordan A., author
Hansen, Neil C., advisor
Bauder, Troy A., committee member
Westra, Philip, committee member
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Abstract
Due to an increase of population in the state of Colorado, as well as several years of receiving less than average precipitation, water allocation has become a state-wide concern. Agricultural, municipal, and recreational sectors demand ever-increasing volumes of water, which has caused the state to re-assess the amount and location of where water will be best economically and environmentally utilized. From an agricultural standpoint, furrow irrigation is a less effective method of irrigation than sprinkler or drip irrigation, however land suitability and socio-economic factors keep furrow irrigated acres high throughout Colorado. Therefore, there is a need to develop cropping systems that increase the irrigating efficiency of furrow irrigation in the state while decreasing sediment and nutrient contamination of water sources. Adoption of conservation tillage in furrow-irrigated cropland is limited compared to rain-fed and sprinkler irrigated systems. Residue on the soil surface impeding furrow irrigation flow and establishing a quality seed bed are the primary concerns. A two year field-based study was conducted in Fort Collins, Colorado during 2011 and 2012 to compare (a) soil moisture and irrigation requirements, (b) water outflow, infiltration, and advance in furrows and (c) sediment and nutrients in runoff for minimum till (MT) and strip till (ST) systems to a conventional till, plow-based system (CT). The MT and ST systems included a modified row-cleaning operation to move residue from irrigated furrows to adjacent non-irrigated furrows. Crop residue was greater on the soil surface in MT and ST than in CT, which resulted in higher soil moisture content at planting yet still allowed for successful irrigation. Average advance of water through furrows in 2011 was faster in MT (79 min) than CT (101 min) and ST (108 min), and in the order of ST (109 min) > MT (99 min) > CT (88 min) during 2012. Penetration resistance measurements showed that CT (567 kPa) and ST (275 kPa) created good seedbeds, but hard soil on MT (848 kPa) beds caused poor seed placement. Within individual irrigation events, tillage practice had little effect on the concentrations of sediment or nutrients in runoff, except MT had higher concentration and load of nitrate (NO3-) than CT and ST. Sediment concentrations and loads were similar for all tillage systems and average annual sediment loss in irrigation runoff was 4.9 Mg ha-1. Conservation tillage systems can be successfully modified for application to furrow irrigation systems and can decrease the dependence on irrigation at planting by maintaining crop residue at the soil surface, although high amounts of residue on the seedbed can be of concern for effective planting in MT ST was a better approach than MT because it created better seedbed conditions. An additional concern for growers in regards to conservation tillage is decreased efficacy of herbicides due to interception of herbicides by residue on the soil surface. To address this concern, an herbicide dissipation study was included in this tillage study to compare the fate and movement of atrazine, s-metolachlor, and pyroxasulfone in the three tillage systems. Pyroxasulfone is a newly released herbicide that is applied pre- or post-emergence to corn and has a similar weed control spectrum to atrazine and s-metolachlor, but with significantly lower application rates. The molecule has low water solubility and the potential for longer persistence in the soil than atrazine and s-metolachlor. More information is needed about the behavior of pyroxasulfone in the environment and about interaction with varying management systems. This field study was performed at the same site as the tillage study during 2011 and 2012 to 1) compare sorption of pyroxasulfone to that of atrazine and s-metolachlor for an alkaline, loam soil, and 2) to evaluate and compare the persistence and movement of pyroxasulfone, atrazine, and s-metolachlor in conventional (CT), minimum (MT), and strip (ST) tillage systems under furrow irrigated grain corn. In each year, labeled rates of 0.28 kg ai ha-1 for pyroxasulfone, 0.74 kg ai ha-1 for atrazine, and 1.71 kg ai ha-1 for s-metolachlor were applied pre-emergence to corn. Four depth increments of soil samples were taken over the top 30 cm in each tillage system at five time intervals over 60 days. Herbicides were extracted and analyzed by GC/MS to determine the dissipation and movement in soil. All three herbicides had low to moderate sorption and the rank order of sorption coefficients (Kd) was s-metolachlor (0.96 L kg-1) > pyroxasulfone (0.56 L kg-1) > atrazine (0.45 L kg-1). Pyroxasulfone had a much longer half life in all tillage systems when compared to atrazine and s-metolachlor. For pyroxasulfone, DT50 was longest in ST both years, and were not quantifiable because its persistence was longer than the 60 day sample period. Tillage practice affected DT50 of all herbicides, mainly due to residue coverage differences, with herbicides persisting longer in the conservation tillage systems than in conventional tillage.
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Subject
nutrient runoff
strip tillage
sediment runoff
pyroxasulfone
conservation tillage
minimum tillage