1989 - 1994
Reduced Chemical Input Systems
M.A.McGovern*, J.L.B. Culley*,
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In recent years there has been increasing concern raised over concentrations of nitrates and pesticides in both surface and ground water within the Great Lakes Basin. Increases of concentration may possibly be associated with the migration of these materials from agricultural lands (Agriculture Canada 1992). While a soil nitrogen test for corn has been developed, most producers determine their rates of nitrogen fertilizer based solely on economic factors, expected yield and the price ratio of nitrogen to corn. Successful farming practices, as measured by high economic yields, are also important environmentally. Crops take up nitrogen derived both from mineralization of soil organic matter and fertilizer sources. Nutrients not taken up by the crop remain potentially available to be lost by leaching or volatilization. Our hypothesis is that matching nitrogen application rates more to crop utilization will result in less opportunity for leaching inorganic nitrogen, no matter what its source to ground water. Eventually, this should lead to reduced nitrate concentrations in ground and surface water.
Certain weed species have developed triazine pesticide tolerance in Ontario. There is also evidence that selective soil bacteria adapt to use applied pesticides as convenient energy sources; such soils are said to have enhanced degradation capability. Annual application of the same pesticide can induce such weed adaptations and changes in soil behaviour. For many years, atrazine has been very widely used across the Great Lakes basin to control broadleaf weeds in corn and recent unpublished research indicates that there exist soil and water bacteria capable of preferentially degrading atrazine. Enhanced atrazine degradation potential in a soil can be identified by monitoring atrazine degradation while monitoring soil thermal and water regimes. A significantly reduced time for disappearance (usually expressed as its half life (t1/2), or the time required for the pesticide concentration to decrease to 50% of its initial value) may be indicative of enhanced degradation. In this report, we assess disappearance rates of atrazine, metolachlor and metribuzin at sites representative of the extremes in soils and climates in the lower Great Lakes basin.
Many farmers are using or experimenting with combined chemical and mechanical means of weed control. This study was established to determine whether such combined management practices can reduce the use of herbicides while maintaining the current level of corn yields.
The following management practices were evaluated:
Atrazine, metolachlor and metribuzin, used at minimum label rates, provided reasonable weed control, and appeared to be degraded to very low soil concentrations by harvest time. There was some indication of more rapid degradation of atrazine in Southwestern Ontario, than in Eastern Ontario. Increased (enhanced) degradation, was not a factor in these experiments. Replacement of herbicides with interrow cultivation depressed yields on coarse textured soils. There was no agronomic weed control benefit associated with mixing herbicides into the 28% N fertilizer solution but neither was it detrimental.
The agronomically beneficial corn-soybean rotation yield effect (about 10%) was confirmed. Application of 100 kg-N/ha at planting increased soil inorganic nitrogen content in mid-June less in Southwestern Ontario than in Eastern Ontario. After-harvest residual soil nitrogen content, due to nitrogen fertilizer applied at 100 kg-N/ha, was negligible in Southwestern Ontario. At the Eastern Ontario sites, about 20% of the applied N remained available for loss after harvest. These results imply that higher rates of nitrogen may be agronomically appropriate in Southwestern Ontario, and they would not, materially affect ground water contamination risk. Manure and legume derived nitrogen sources of nitrogen can materially affect soil inorganic N levels; care should be exercised in determining economic and environmentally responsible rates of nitrogen fertilizer in fields with such histories. Use of the nitrogen soil test may be very appropriate for the management of manured fields.
New Technologies and Benefits
There is a substantial potential for the use of combined methods (cultivation plus herbicide) of weed control on fine textured soils. Detrimental economic risks, probably due to reduced weed control or root damage or possibly loss of available water, appear to be greater on coarse textured soils, where competition for soil water is often more severe.
There appears to be both economic and environmental justification for greater nitrogen application rates in Southwestern Ontario. While our results indicated that there may be somewhat more rapid degradation of atrazine in Southwestern Ontario than in Eastern Ontario, it was not a concern. The field-measured degradation rates of atrazine, metolachlor and metribuzin, while in line with literature values, were quite different from those measured in the laboratory using similar disturbed soils.
Gaps/needs for Future Research
The uptake and utilization of soil nitrogen is still a poorly understood process; in particular when manure is used as a nitrogen source and forage crops are part of the crop rotation.
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