Iowa State University

Evaluating risks with total post programs  
by Bob Hartzler

December 9, 2004 - In an earlier article (Managing weeds to protect crop yields) the cost of delaying postemergence applications beyond the critical period was discussed.  Data from a regional project was used to illustrate the rapid rate that yield losses can accumulate when weed removal is delayed beyond the point when weeds begin to impact yields (Figure 1).  The yield response curve represents the mean of 35 experiments conducted across the Midwest over a two year period; however, due to the huge variation in weed communities found in fields an average response does not provide a reasonable estimate of what will happen in individual fields.  The rate of yield loss would be much greater in fields with heavy weed pressure than in fields with light infestations.  This article will provide information on the range of responses observed in the regional project.

The fields included in the project had weed infestations ranging from 2 to 325 plants / ft², with a mean weed density of 70 plants / ft².  One way of evaluating risk is to  look at the number of locations that had a specific response (Table 1).  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 2 or 3 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 2" weed height minimized the risk of significant yield losses.  For example, with this application timing 75% of the locations had a yield loss of 5% or less.  Yield losses at 90% of the locations were less than 13%, indicating that 10%  of the locations (4 sites) 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, 25% of the locations had a yield loss greater than 10%.  I suspect most farmers would be unwilling to accept this level of risk.

Table 1.  Range of responses due to early-season competition between corn and weeds.

Adapted from Gower et al.  2003.  Weed Technol.  17:821-828.

The range of weed densities in the fields used in this study probably is fairly representative of those found in production fields.  Knowledge of the effectiveness of past weed control programs in a field can be used to determine the appropriateness of using a total postemergence corn program.  Fields with a good history of weed control should have low weed densities, and thus the risk of significant yield losses occurring early in the season are probably acceptable.  However, if a field has experienced control failures in recent history it is likely that high populations of weeds will emerge with the crop.  Under these conditions, it is likely that relying solely on postemergence herbicides will result in yield losses that reduce economic return.  A better option would be to utilize a preemergence herbicide to delay establishment of weeds and provide the crop with a competitive advantage.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University