Influence of Tillage Systems and Winter Cover on Off-Site Losses of Sediment, Nutrients and Atrazine in Silage Corn Production

Nonpoint source pollution has been recognized as a primary cause of water pollution in the United States. Agricultural activities have been cited as the leading contributor of nonpoint source pollutants. Runoff and eroded sediment are the primary transport agents for agrichemical losses from agricultural fields. Conservation tillage has been promoted over the past two decades as a cost-effective agronomic practice that can reduce runoff and erosion from agricultural fields. The goal of this study was to compare the edge-of-field losses of waterborne substances from conservation tillage and conventional tillage plots both with and without the use of a winter cover crop under the corn-for-silage management program. Corn for silage is a prevailing practice in New England and comprises about 20 percent of the total acres harvested. Twelve field plots measuring 3.4 meters wide by 22.1 meters long with a 2.5 percent slope were equipped with an overland flow collection system. Runoff was monitored during the 1985 and 1986 growing seasons (June through November). Runoff samples were analyzed for sediment, nitrogen, and atrazine content. Runoff occurred on 22 out of 51 rainfall events that occurred during the study period. In all treatments, 57 to 62 percent and 70 to 77 percent of the runoff and soil loss, respectively, were associated with excessive rate storms. Runoff and soil loss were considerably higher on plots with less than 30 percent residue cover. Surface residue from the winter cover crop reduced runoff and soil loss by 29 and 54 percent, respectively, compared to plots without the winter cover crop. Total nitrogen losses through overland flow during the 1986 growing season ranged from 0.33 to 3.42 kg/ha or 0.1 to 1.3 percent of the applied nitrogen. Nitrogen losses were highest on plots without a winter cover crop. Total Kjeldahl nitrogen accounted for 89.5 to 94 percent of the total nitrogen loss. The greatest losses of total Kjeldahl nitrogen were associated with the events that had the greatest sediment movement. Total atrazine losses through overland flow was less than 0.5 percent of that applied for all treatments. Atrazine losses were 74 percent lower in conservation tillage systems than in tillage systems with less than 30 percent residue cover. Tillage method had no significant effect on flow weighted atrazine concentrations in runoff. The hydrology component of the CREAMS computer model predicted runoff closest to the observed runoff values using the breakpoint method in the conventional system and the curve number method in the notill system. The breakpoint method performed better than the curve number method for small intense, storms. In order to obtain close agreement between predicted and observed runoff values, recommended parameter values describing soil properties were adjusted to reflect lower infiltration rates