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Estimating early-season competition losses in corn

Agriculture.com Staff 06/05/2007 @ 7:58am

Weather conditions this spring prevented many corn fields from being treated with planned preemergence herbicide programs. Continued wet and windy conditions prevented timely postemergence applications, allowing the weeds to remain longer than desired.

A concern with delayed applications is how much, if any, of the corn yield potential will be lost due to the competition between crops and weeds before the weeds are controlled.

Early-season competition is highly variable, dependent upon weed density, weed species, environmental conditions and cultural practices. However, data from a regional project illustrated yield losses can accumulate rapidly when weed removal is delayed. The yield response curve represents the mean of 35 experiments conducted across the Midwest over a two year period.

Due to the large variation in weed communities found in fields, an average response does not provide a reasonable estimate of what will happen in individual fields. The yield loss would be much greater in fields with heavy weed pressure than in fields with light infestations.

The fields included in the project had weed infestations ranging from two to 325 plants per square foot with a mean weed density of 70 plants per square foot. One way of evaluating risk is to look at the number of locations that had a specific response.

The yield responses are from a sequential program where the initial application of glyphosate was made at the indicated size of giant foxtail and then a second application was made two or three weeks after the first application to control late emerging weeds. Thus, the yield losses are due to early-season competition between the time of emergence and the initial application.

Applying the initial application of glyphosate at a two-inch weed height minimized the risk of significant yield losses. For example, with this application timing 75% of the locations had a yield loss of five percent or less, whereas 50% of the fields had a yield loss of two percent or less. Yield losses at 90% of the locations were less than 13% when weeds were controlled at the two-inch height, indicating that 10% of the locations (four) had a yield loss greater than 13%.

A four-inch height probably is more realistic as to the target height for application used by farmers and custom applicators. With this timing, 75% of the locations had a yield less greater than 10%.

Weather conditions this spring prevented many corn fields from being treated with planned preemergence herbicide programs. Continued wet and windy conditions prevented timely postemergence applications, allowing the weeds to remain longer than desired.

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